TBI Fuel Pressure

[biggsteve97]

I have an 87 Silverado with a built 350 TBI. And I am trying to find information concerning a possible issue with fuel pressure. I have been unable to find any info online or thru gm and gm service manuals. With my pressure tester connected key on engine off the fuel pressure jumps strong to 18 psi but the second the pump shuts off the fuel pressure drops like a rock to 0 psi. I know that these throttle bodies have a bleed off to allow pressure to drop when the engine isnt running but I am unable to confirm how fast or slow the pressure is suppose to drop. No tests I have found are concerned with this they are only concerned with the pressure itself. I am questioning this because I have a hard start and an intermittent no start at operating temp only. Once it starts whether I wait the 3-5 mins or shoot a very short burst of either in it it runs great as if nothing is wrong. The fuel pressure drop after pump prime or engine shut off is the only thing I cannot find info on or confirm. I dont believe the drop should be instantaneous but unable to find out. Any thoughts would be appreciated.

[rfmaster]

Originally Posted by biggsteve97

I have an 87 Silverado with a built 350 TBI. And I am trying to find information concerning a possible issue with fuel pressure. I have been unable to find any info online or thru gm and gm service manuals. With my pressure tester connected key on engine off the fuel pressure jumps strong to 18 psi but the second the pump shuts off the fuel pressure drops like a rock to 0 psi. I know that these throttle bodies have a bleed off to allow pressure to drop when the engine isnt running but I am unable to confirm how fast or slow the pressure is suppose to drop. No tests I have found are concerned with this they are only concerned with the pressure itself. I am questioning this because I have a hard start and an intermittent no start at operating temp only. Once it starts whether I wait the 3-5 mins or shoot a very short burst of either in it it runs great as if nothing is wrong. The fuel pressure drop after pump prime or engine shut off is the only thing I cannot find info on or confirm. I dont believe the drop should be instantaneous but unable to find out. Any thoughts would be appreciated.

During POST ECM will energize FP relay for about 2 seconds. You may observe 9 to 12 PSI readings during this time, but this reading is unreliable. To verify fuel pump operation with engine off you can apply +12volts to the red pig tail attached to the FP relay socket from a fused jumper wire (use 10 amp inline fuse). Once FP builds pressure you can verify FPR is doing job maintaining pressure between 9 to 12 PSI (higher is better). Quick drop-off in fuel pressure after pump is shutdown is normal, but there is no factory spec.

With regard to your issue with difficult warm re-starts. Make sure that FP is energized during cranking! It is common for FP relay to develop bad contacts over time. The oil pressure switch takes over and powers-up FP after oil pressure builds up above 4 to 6 PSI. When engine is cold oil pressure build up is very quick. Once engine oil is hot oil pressure build-up takes time to exceed 4 to 6 PSI threshold of the oil pressure switch. You can verify if FP relay is functional by temporarily disconnecting oil pressure switch. If you get no start with engine cold you've found root cause - FP relay circuit.

Pull FP relay - take a close look at socket contacts, verify relay contact resistance with DVM or simply replace it with known good unit. On a very rare occasions ECM may blow switching transistor used to apply +12volts to FP relay coil. However this failure mode is rare and happens if harness is damaged and relay power circuit has been grounded.


//RF

[biggsteve97]

Originally Posted by rfmaster

During POST ECM will energize FP relay for about 2 seconds. You may observe 9 to 12 PSI readings during this time, but this reading is unreliable. To verify fuel pump operation with engine off you can apply +12volts to the red pig tail attached to the FP relay socket from a fused jumper wire (use 10 amp inline fuse). Once FP builds pressure you can verify FPR is doing job maintaining pressure between 9 to 12 PSI (higher is better). Quick drop-off in fuel pressure after pump is shutdown is normal, but there is no factory spec.

With regard to your issue with difficult warm re-starts. Make sure that FP is energized during cranking! It is common for FP relay to develop bad contacts over time. The oil pressure switch takes over and powers-up FP after oil pressure builds up above 4 to 6 PSI. When engine is cold oil pressure build up is very quick. Once engine oil is hot oil pressure build-up takes time to exceed 4 to 6 PSI threshold of the oil pressure switch. You can verify if FP relay is functional by temporarily disconnecting oil pressure switch. If you get no start with engine cold you've found root cause - FP relay circuit.

Pull FP relay - take a close look at socket contacts, verify relay contact resistance with DVM or simply replace it with known good unit. On a very rare occasions ECM may blow switching transistor used to apply +12volts to FP relay coil. However this failure mode is rare and happens if harness is damaged and relay power circuit has been grounded.


//RF

I have dual tanks and doesnt matter which tank Im running I can hear the pump run every time I turn the key during the hard start and even with the intermittent no start. And that is with both just turning the key on not cranking the engine as well as when cranking. The injectors are spraying fuel while cranking. Though on the turn of the key before even letting go of the key the engine will fire, imediately die and then just crank. I have watched the injectors when this happens, they spray briefly, pause then slowly start spraying again, with each pulse spray getting courser until it usually starts. Ironically, whether its the hard start condition or the intermittent no start condition, if I shoot a very short burst of starting fluid into just one of the throttle bores, the engine will fire right up. And the injectors are spraying in all situations.
My chip guy burnt a new chip with longer perameters for injector pulse width while cranking and it has helped alot both cold and hot starts. So far since I put the new chip in the no start condition hasnt happend but it will still fire, die then crank for about 2-3 seconds then fire at operation temp. So I am installing a permanent fuel pressure gauge tomorrow so I can see exactly whats happening at the moment when the hard start happens.

[rfmaster]

Originally Posted by biggsteve97

I have dual tanks and doesnt matter which tank Im running I can hear the pump run every time I turn the key during the hard start and even with the intermittent no start. And that is with both just turning the key on not cranking the engine as well as when cranking. The injectors are spraying fuel while cranking. Though on the turn of the key before even letting go of the key the engine will fire, imediately die and then just crank. I have watched the injectors when this happens, they spray briefly, pause then slowly start spraying again, with each pulse spray getting courser until it usually starts. Ironically, whether its the hard start condition or the intermittent no start condition, if I shoot a very short burst of starting fluid into just one of the throttle bores, the engine will fire right up. And the injectors are spraying in all situations.
My chip guy burnt a new chip with longer perameters for injector pulse width while cranking and it has helped alot both cold and hot starts. So far since I put the new chip in the no start condition hasnt happend but it will still fire, die then crank for about 2-3 seconds then fire at operation temp. So I am installing a permanent fuel pressure gauge tomorrow so I can see exactly whats happening at the moment when the hard start happens.

So it looks like it is not an issue with FP relay powering up fuel pump. Before making any further diagnostic recommendations install FP gauge and see what is going on with fuel pressure during cranking.

Another reason for hard hot restart can be attributed to ICM, but that's unlikely since injectors are pulsing during hot cranking condition, but not spraying sufficient fuel.

//RF

[biggsteve97]

Originally Posted by rfmaster

So it looks like it is not an issue with FP relay powering up fuel pump. Before making any further diagnostic recommendations install FP gauge and see what is going on with fuel pressure during cranking.

Another reason for hard hot restart can be attributed to ICM, but that's unlikely since injectors are pulsing during hot cranking condition, but not spraying sufficient fuel.

//RF

I thought that first and replaced the ICM and the pickup about 4 weeks ago and made no change. Its prolly a good thing I replaced them though, the wires were rusted pretty good at the module and the pickup was thoroughly rusted as well. Though when I was in there I did find that the tension spring inside the rotor had failed allowing the rotor to become severely loose and to rotate back and forth on the distributor shaft. Causing my timing to bounce around irratically at 1500 rpm. Guess just have to see what the fp gauge has to say and go from there.

[biggsteve97]

Update. I have not installed the fuel pressure gauge as of yet because the wiring harness did not arrive until last night. However, my truck started a brand new issue on Saturday after we unloaded at the scrap yard. As we were heading to the scale to weigh out, because of traffic I pushed in the clutch and while coasting the motor stumbled and died. Then it did not re-fire for almost 30 mins. I got it restarted finally and it revved fine and sounded good so we got in line for the scale and after about 2-3 mins of sitting there idling it stumbled and died. It did not start again until after I towed it home. about an hour and half later. It was idling low again barely 500rpm. I have noticed the last few days I drove it it was idling round 500 occasionally and not around 900 like it always had. I was able to work on it some on Sunday. I decided to try different computers and all four gave the same result so I no longer thing it might be computer related. However now it doesnt want to idle sometimes and its never done this before. It fired right up on each computer but after about a 3-4 mins of idling it would stumble and die then it wouldn't start even though fuel was spraying out of the injectors and I had spark. If I sprayed some carb cleaner in the throttle body it would fire then die unless I put my foot in it and got the rpm above 1000 above that it runs great and smooth like it always has. but below 1000 rpms it will not run. Nor start on its own once it stumbles and dies. When it was idling and I touched the gas pedal to bring rpm up the engine would stumble almost miss as it came off idle. Also engine temp is no longer a factor cause now it will stumble and die only mins after start up and the temp gauge hasn't even moved yet. Once it dies then it does the no start issue it use to only do at operating temp. This is new as of Saturday as well. I feel that whatever was causing my intermittent no start is directly related to it dying and not starting on its own now. I suspected it might be a faulty idle control motor since it has been idling so low recently so I ordered one. It is still original to the throttle body so I though id give it a shot. Ive tried everything else. I am hoping to have the pressure gauge installed soon. I feel like this will give me a better idea as to what might be happening. The only code that has come up in the computer at all other than the normal code 12 was code 23 vss failure and that was after I unplugged the idle control motor to see what the engine would do. When i got it started the rpm immediately jumped to 2200 rpm ran like that for 3-4 mins without issue until i plugged the iac back in, the engine immediately died. Ive never I had that happen before when I have unplugged and plugged in the iac years ago setting the base idle. I guess I am back to the beginning on my diag again with this new issue. Hopefully I can get this figured out soon.

[biggsteve97]

Well, I finally got my new permanent fuel pressure sensor and electric 30psi gauge installed over the weekend and I did it after I installed the new idle control motor and it had run for about 10 mins. When it suddenly just died. No warning. Just died though i heard the idle control motor function to raise the idle so the system seems to be functioning atleast. After installing the gauge setup. Upon turning the key to on engine off, fuel pressure was only 13 psi. Which is a little low since I have the 18psi spring installed. It was very hard to start but it did start. Fuel pressure dropped to 10psi while cranking and it was only 12psi while idling and it would flucuate while idling between 11-13psi. This not where the pressure was two weeks ago when i checked it with my fuel pressure tester. It was a solid 18psi then, but that was before things changed when it decided it wanted to die suddenly and not idle below 1000rpm after it died if it even started. And it doesnt matter which tank im running. So could this be the pressure regulator or could the fuel pumps be fatigued enough since they both are 16 years old and i have been running them at almost there limit for the last 6 years? I was lucky that my pumps are the 80lph instead of the more common 70-75lph fuel pumps. I know its not much difference but it was enough that the engine could run fine at wide open throttle and i didnt have to find special pumps when i installed the engine 6 years ago.

[rfmaster]

Most likely the rubber hose between FP outlet and feed through has developed a hole. It takes a while for a fuel to warm up, but once it does the hole in hose bleeds off more fuel. As a result FP drops off and once below 6 PSI that were things shutdown. With 18 PSI FPR spring you need to R&R pumps and switch to TPI style pumps that can deliver 65PSI dead head pressure. A common upgrade is to go with EP241 pump. You'll need to source 5/16" SAE 30R10 submersible hose. TBI pumps use 3/8" hose.

//RF

[biggsteve97]

Originally Posted by rfmaster

Most likely the rubber hose between FP outlet and feed through has developed a hole. It takes a while for a fuel to warm up, but once it does the hole in hose bleeds off more fuel. As a result FP drops off and once below 6 PSI that were things shutdown. With 18 PSI FPR spring you need to R&R pumps and switch to TPI style pumps that can deliver 65PSI dead head pressure. A common upgrade is to go with EP241 pump. You'll need to source 5/16" SAE 30R10 submersible hose. TBI pumps use 3/8" hose.

//RF

You are not the first to suggest a possible hose failure inside the tank. I had dissmissed it before since the fuel system always held 18 psi but since it things have changed, im no longer dissmissing the possiblity of anything. Its odd since it is the same on both tanks. I will say this truck has always run better on the left tank than it ever has on the right tank. Been that way since it had 30k miles on it. Changed both fuel pumps with the intank hoses, the switch valve, the sending units assemblies in both tanks, and all the rubber hoses on top of both tanks atleast twice thru the years and it still always ran better on the left tank. I considered changing to the different pumps 6 years ago but after talking with Brian at TBICHIPS.com we decided not to go that route over concern with the switch valve. I have since figured out the switch valve has a 65 psi maximum pressure rating so I will most likely go with the 255lph pumps that he shows as a direct replacement for the tbi pump.
I have read that if I restrict the return line and the pressure goes up then the issue is pressure regulator, if it does not change then it is the pump and or the hose on top of the pump. Does this sound correct? I was thinkin of trying that, if I ever get more than 15-30 mins a day to work on the truck.

[biggsteve97]

Well i got my fuel pressure gauge installed saturday and figured out a couple things. First even cold the pressure was barely hitting 10psi while cranking and even less while crankin when warm. Once started it was onky holding 13psi on both tanks. When i bolcked the return hose pressure went to 17psi and while it stayed there i deemed weak pumps so i pulled left tank. Found out it was newer and of a much higher pressure and gph than i had remembered it being but still not as good as it could be though more than enough to support the engine. I got a new fuel filter just because it been a few years. Good thing. Turns out it was atleast 80% or more restricted and i believe may have been most of the root of my problem. Fuel pressure is now 15psi cold and warm running and 12-13 psi while cranking and thats just on the right tank and inly from changing the filter. Only had about 10,000 miles on it and i run premium so didnt think that would be as clogged as it was. Im gonna install much stronger pumps this week anyway just incase they are getting weak but im thinkin my problems are gone. It idled strong for almost 25mins after the filter change until i shut it off. No longer did it die or struggle to start. Im hopin its good again.

[rfmaster]

It is good that you're able to find a fuel flow obstruction in your truck. Well, as I said FP and volume must be there to support engine requirements. These horses love to drink!!

//RF

[biggsteve97]

Well unfortunately the truck isnt fixed. It died on me tonight and wouldnt start. Things i noticed since replacing the fuel filter both before and after one of the pumps. There was residual pressure in the line with key on engine off and slow bleed down unlike before were as soon as pump stopped, pressure would drop like a rock to zero. Second pressure was 12-13psi while cranking and 15psi running. Tonight it started right up and psi was 15psi while running. Engine sounded great smooth and ran good. After about 5 mins it stumbled hard but did not die. After that pressure crept upto 17psi. Ran smooth, strong. Shut it off started right back up. Psi still 17psi. Ran for about 30mins, no stumble, miss or anything. Suddenly it just died and wouldnt fire. From that point on. Key on engine off, psi dropped like a rock to zero. There is no longer residual pressure in the line again. Cranking it is still 12-13psi. It will not spit sputter or nothing just cranks. Now, if i spray starting fluid or carb cleaner in one of the throttle bores it will start and imediately die, unless I put my foot in the throttle. As long as i keep the rpms above 1000rpm it runs strong and sounds great but below 1000 it dies. Even if you put ur foot back into it. It will struggle but ultimately die. When i get it started with starting fluid and cram my foot in it, i noticed fuel pressure was back down to 15psi with a slight fluctuation in the needle. It runs perfect though just wouldnt idle again. Im lost but considering possible regulator problem or blockage of some sort in injector pod assembly. As before it still sprays fuel from the injectors and has spark just wont start on its own and idle once it died suddenly. Just odd it runs perfectly for almost 25mins. It used to only run for 5mins when the dying started. Wish i had another throttle body just to try. I will add once i get it started and cram my foot in it and keep above 1000rpm it ran perfect for well over 5 mins and i just shut it off. I did try unplugging the coolant temp sensor for the computer and tried to start it and it fired and died once but thats all. Even tried unplugging the map as well and fired and died once. Swapped computers and nothing changed either.

[rfmaster]

Check your fuel pump relay and relay socket. The fuel pump power is controlled by ECM. During start, with engine cranking once ECM sees DRP pulses from dizzy it will energize pump relay. If FP relay contacts are bad or socket contact are oxidized FP will not turn. However, GM installed oil pressure switch as a back-up, which will close its contacts once engine oil pressure is over 6 PSI. This will turn on FP - as long as engine oil pressure is above 4 to 6 PSI.

Based on your last post you see 12 -13 PSI during cranking, but injectors are not firing, but the moment you spray engine fires right up. This leads me to believe that ICM in the dizzy is functioning since you have spark (and it has power +12V). However, I would make sure that ECM power circuit is hot (+12 volts) when ignition key is in cranking and run position. Check Pin A6 pink-blk wire - you can back probe it with DVM test leads - it should read +12 to 13.8 volts.

Over time Ignition cylinder contacts may have worn-out and make intermittent contact during cranking. If ECM is not powered during cranking injectors will never pulse.

//RF

[biggsteve97]

No, the injectors are spraying. Thats what has me baffled. The injectors are pulsing and i have spark while cranking. Ever since the intial no start when hot condition first arose almost two years ago now the injectors wer always pulsing/spraying fuel. Its like it goes extremely lean for absolutely no reason. Ive had my laptop connected to it and recorded all the tables while the no start was happening and sent them to my chip guy and he said he didnt find anything out of the norm in the readings but that was before it started the dying suddenly thing after running for awhile.

[biggsteve97]

I have inline spark tester that i put on number one cylinder while cranking during the nostart before i sprayed the fluid in the throttle body and spark is strong while cranking even thoug it wont start. And the injectors appear to be sprayin strong. I did move the spark tester to 3,5,&7 while cranking but i did not put the tester on the passenger side. Since it starts with the spray i didnt see a reason to do so. I went out about 20mins ago and it fired right up. Pressure was 17psi when running again. I couldnt let it run long since its after 10pm, my truck is a little loud and my neighborhood is ridicously quiet.

[biggsteve97]

The relay and connector are clean and surprisingly in good shape despsite the trucks almost 440k miles. Nothings loose or crispy.

[rfmaster]

In view of post #14 to 16 I would check the following:

1) FP voltage at the tank interface during cranking. This should be solid 11.4 to 12.0 volts
2) Check fuel supply lines for any restrictions, kinks etc. FF was replaced, correct?
3) I have suspicion that you may have a bad in tank hose between fuel pump and tank feed through assembly. The 12 PSI that you see during cranking might be compressed air in the fuel line. Since you have 18 PSI spring you should see 17 to 18 PSI during cranking (I use 18 PSI springs in my TB's as well).
4) fuel pump internal pressure regulator is bad.

You can manual force FP to operate by applying +12 volt to the red pig tail dangling from FP relay socket - no engine cranking is required. Run a fused (10 AMP inline fuse) jumper wire from +BAT to this pig tail. While FP is running perform a deadhead test by pinching return line - find rubber section and pinch it gently with pliers. Fuel pressure should increase well above 25 PSI if you have upgraded to TPI pump. If it does not look for a leaky fuel hose inside gas tank.

//RF

[biggsteve97]

Originally Posted by rfmaster

In view of post #14 to 16 I would check the following:

1) FP voltage at the tank interface during cranking. This should be solid 11.4 to 12.0 volts
2) Check fuel supply lines for any restrictions, kinks etc. FF was replaced, correct?
3) I have suspicion that you may have a bad in tank hose between fuel pump and tank feed through assembly. The 12 PSI that you see during cranking might be compressed air in the fuel line. Since you have 18 PSI spring you should see 17 to 18 PSI during cranking (I use 18 PSI springs in my TB's as well).
4) fuel pump internal pressure regulator is bad.

You can manual force FP to operate by applying +12 volt to the red pig tail dangling from FP relay socket - no engine cranking is required. Run a fused (10 AMP inline fuse) jumper wire from +BAT to this pig tail. While FP is running perform a deadhead test by pinching return line - find rubber section and pinch it gently with pliers. Fuel pressure should increase well above 25 PSI if you have upgraded to TPI pump. If it does not look for a leaky fuel hose inside gas tank.

//RF

I will check fuel pump voltage during cranking tomorrow since I will actually have most of the day to work on it.
Yes I replaced the fuel filter. There are no kinks in any of the lines from the switch valve to the throttle body or any visible damage. Hard to tell since they are braided stainless lines from the fuel filter upto the throttle body and the short steel piece from the braided line to the switch valve is straight and clean. I replaced the hoses from the tank to the switch valve with new and they are not kinked. I know it is possible to get a bad fuel pump right out of the box I just hope that's not it. That tank was a pain since it was almost full.
I will also do the pinch test tomorrow. And the hose from the pump to the feed line looks like they put a 55gal barrel on it instead of the wimpy little line I am used to seeing. It is over 1/2" thick on these new pumps. I would hope I didn't somehow cut it putting the new unit in the tank but i guess anything is possible.
Do you know what might have caused the system to suddenly have a slow bleed off of fuel pressure after prime or running just from putting in the new fuel filter? I noticed even after changing the pump and it did it all week until it suddenly died on me. After that, the pressure again bleeds off as soon as the pump stops. I dont know if that actually has to do with anything but just found it odd.

[biggsteve97]

Well, I got both new pumps installed saturday and the right tank was working great. After install fuel pressure during crankin was 14 psi and 15psi while running. So I pinched off the return line and fuel pressure went upto 25psi. Then i switched to the left tank and imediately noticed a problem. It was hard to start and crankin pressure was only 6 psi. I got it started and running pressure was 14 psi and it started to slowly drop over a few mins of running eventyally the engine died when pressure reached 12 psi. Shortly after that the left pump barely would push 5 psi in any test. So it appears that one was bad out of the box. When I switched back to the right tank evrything went back to 14psi crankin and 15 psi running. I couldnt figure out why though it was only producing 15psi though with the 18psi spring so i pulled the pressure regulator and recompared the postion of the regulator tang to the picture list that came with the spring and turns out with the regulator cover I have i will only reach 15psi. So i am now converting the regulator cover to make it adjustable and will do more testing once back together and the left tank is reinstalled. I did try to apply power to the little connector on the relay pigtail and it was soo corroded internally that i couldnt get full power thru it. Tried cleanin it but still couldnt get it to work right. So i pulled the relay and just jumpered the pins in the connector to get the pump to run.

[biggsteve97]

I got the regulator reinstalled tonight and pressure adjusted to 19 psi before it started raining on me. While i was adjusting the pressure it was at 20psi and i was letting it idle when suddenly it died. I was in the cab lookin at the pressure gauge checking where the pressure was at when it died. And i can say this, fuel prrssure was a solid 20psi when it died. It didnt faulter and took a second to drop after the motor died. I could still hear the pump after the engine died for a breif moment. Tried to start it and though it was hard to start it eventually started ran for a minute and died again. I noticed when tryin to rev the engine the it would hesitate slightly for a moment just as the engine came off idle. It would raise about 100rpm the feel a roughness for about 200 rpm then it would smooth out. It smooth at idle and above 1000 but from about 700-900 rpm it would have a roughness if you moved the throttle too quickly. If you move the throttle slowly its fine. I found it odd. It died on me twice more and hard to start both times but then it ran perfectly fine for about 8-10 mins then it started pouring rain and had to shut it off. Is it possible for the throttle position sensor to be bad but not show up on the min/max voltmeter test? Ive always been able to find a bad tps that way before but this one checks good. Could this be an irratic timing issue?

[Just call me Sean]

I have an '87 305 TBI, as well as two '92 vans that have the same system. Tried to start using the truck for work but it's having issues I'm still trying to figure out, seems to be a fuel issue. Then the work van started having issues too. Having more than one is kinda helping for comparison. Long story short the van had a clogged injector. Mostly clogged at idle and very light throttle, but still worked to a degree. Just because you see it spraying doesn't mean it's spraying enough, plus if you're cranking the truck while it isn't starting you're not getting a good look. Basically I'm saying your injectors may be clogged. My plan for the truck is to put a new pump in the tank that doesn't work and see if it runs better, otherwise I'm going to suspect the injectors on it too.

[biggsteve97]

I have someone else crank it for me when im watching the injectors. Both injectors have a nice fan spray that gets courser the longer it cranks with no dribbling or dripping or droplets. They both spray as they always have and they spray the same as my other tbi engines. Its just weird it will run perfect for a long time sometimes and only a minute other times but only when it dies will it become hard to start and if it does start I would have to put my foot in it and keep rpms above 1000 for it stay running and it runs perfectly fine just wont idle. Until I replaced the pump though once it died it wouldnt refire on its own after I replaced the pump it will refire on its own and die. Did it 3 times in a row before it ran perfectly fine again. If it doesnt die it runs perfect. I am no longer suspecting a fuel delivery issue but I havent completely ruled that out yet. I do have a smaller set of injectors I could try if it comes down to that.

[rfmaster]

Well, if it helps I am scratching the back of my head as well. FWIW it has signs of electrical intermittent. Assuming that ECM is not capturing any fault codes it might be subtle, heat related fault.

Check EGR - stuck or leaky EGR will effect idle.
Check MAP sensor.
Distributor reluctor coil and ICM

//RF

[biggsteve97]

Originally Posted by rfmaster

Well, if it helps I am scratching the back of my head as well. FWIW it has signs of electrical intermittent. Assuming that ECM is not capturing any fault codes it might be subtle, heat related fault.

Check EGR - stuck or leaky EGR will effect idle.
Check MAP sensor.
Distributor reluctor coil and ICM

//RF

You, me, and everyone else whose looked at it. Im about to pull my hair out. There are no fault codes in the ecm. I even tried swapping out the ecm with both my extra 87 truck ecm and my 92 buick roadmasters 350 ecm and other than cold start was quicker on the 87 truck ecm all other results were the same. Its strange that at times i can cold start it and it will run 1-2 mins then die and other times i can cold start it and it will run for 30-45 mins then die.
I will recheck the egr. I pulled it off and checked it 4 days before the dying started and the
egr was clean and seated. So i reinstalled it and plugged off the vacuum to make sure the
solenoid wasnt leakin vacuum or sticking. I forgot to reconnect it to the egr and it was still
plugged off the day the dying started. Ive reconnected it since then but that made no
change. Is there a way to check the map sensor without a scan tool? I had my laptop connected using my aldl datalogger and we didnt see anything strange in the map readings however this was before the dying started. It has the msd billet distributo in it an I had installed a new pickup and icm almost 3 months ago now because of a hard start issue. Even though the old pickup and icm were heavily rusted installing the new one made no change then for the hot hard start. Unfortunately msd soldered the wires to the icm on the replacement kit so i cant just pull the icm and have it tested. Im tempted to go get a used distributor and splice in the connector and put so i can remove the icm if needed and install replacent icms instead of the whole thing. Wish i had an extra distributor too to try just to see. I am taking thursday and friday off and the plan is to focus atleast one if not two days of the fourday weekend just on the truck. Ill be glad to put more than just 1-2 hours at a time or day to work on it and maybe get this problem solved.

[rfmaster]

Yes, MAP sensor functionality can be checked with hand held vacuum pump (MitiVac). With WinALDL running, Ignition ON you should see MAP sensor voltage.

At idle Vacuum = 18 to 16 in-Hg (30 to 40 kPa) MAP voltage 1.3 to 1.8 V range.
WOT 0 in-Hg (100kPa)MAP voltage around 4.5 to 4.7 Vdc.

Below - two typical GM 1-bar MAP sensors characterized over the expected vacuum range.




TPS can be checked with DVM - it is a variable resistor. A to C ~ 5kOhm and A to B 1 to 6kOhm varying with shaft position.

CTS is thermistor - resistor that varies vs. temperature. -40C ~103kOhm; 25C ~2.8kOhm; 85C ~338Ohm.

Assuming there are no fuel delivery issues it feel as if you are loosing power to distributor - I would put a DVM on the pink wire just to make sure that dizzy is not loosing power.

//RF