Im STUMPED

[olivedrab]

i have an 87 swb chevy 4x4 with the tbi and computer. recently my msd 6a box along with the ignition module in the distributor go out. So i eliminated the msd box and changed the ignition module in the distributor.

here is the problem:::: the truck runs great, nothing abnormal until it warms up. after it warms up, i put it in drive and it dies. its almost like it just chokes out.



things ive checked: idle air control valve, dampness in the distributor cap, ignition circuit wirring, vaccum leaks around the intake and such. still havent found anything abnormal.

for the temporary fix i just unpluged the temp sending unit that goes to the computer from the top of the intake manifold and it idles a little higher thus resulting in it running normal now.



anybody run into something like this?

thanks in advance for the help

[Captkaos]

Is it throwing any codes?

[grinch4om]

when you unplug the ECT, it tells the computer the engine is real cold, thus throws more fuel at the engine,(pulsing the injectors more rapidly and or holding them open longer) if it runs normal when you do this, look for a vacuum leak...bottom of intake? Fords say cold when unplugged (ECT), find out what GM does, if it is opposite, it would lean out a fat mixture, either way, I would check the ECT first with an ohm meter if it is ok, go after the above.

[suicide 63]

I had a similar problem on a 89 tbi thought it was ignition problem ,but it was a worn out intank fuel pump.It would run good when cold but after the pump got hot it would not pump enough pressure.Good luck

[olivedrab]

Originally Posted by Captkaos

Is it throwing any codes?

no its not throwing any codes...

[olivedrab]

Originally Posted by grinch4om

when you unplug the ECT, it tells the computer the engine is real cold, thus throws more fuel at the engine,(pulsing the injectors more rapidly and or holding them open longer) if it runs normal when you do this, look for a vacuum leak...bottom of intake? Fords say cold when unplugged (ECT), find out what GM does, if it is opposite, it would lean out a fat mixture, either way, I would check the ECT first with an ohm meter if it is ok, go after the above.

i bought the $12 sensor that tells the comp. the temp but its still doing the same thing. i also took a can of carb cleaner and sprayed around the intake to see if it would suck anything up and idle faster momentarily while burning the carb cleaner and still found nothing...

[olivedrab]

Originally Posted by suicide 63

I had a similar problem on a 89 tbi thought it was ignition problem ,but it was a worn out intank fuel pump.It would run good when cold but after the pump got hot it would not pump enough pressure.Good luck

this is a good idea but i just changed the fuel pump about 3-4 months ago.. so im pretty sure its not that in this case...

[rfmaster]

Olivedrab

Take a look at fuel spray pattern at idle. It should be a nice cone of fuel mist hitting the walls of the TB bores, not a large drops dripping from injectors falling straight down onto throttle plates.

Disconnecting CTS results in ECM seeing high input CTS voltage (about 5 volt) which corresponds to a very low coolant temperature (-40F). Furthermore, with CTS disconnected, after about 2 minutes of running ECM will set Code 15 (open CTS circuit). Based on your description it is worth while to check fuel pressure. It is not too uncommon for a 'new' fuel pumps to fail, or output hose between fuel pump and feed through to burst.

Also check fuel filter.

//RF

[suicide 63]

Originally Posted by rfmaster

Olivedrab

Take a look at fuel spray pattern at idle. It should be a nice cone of fuel mist hitting the walls of the TB bores, not a large drops dripping from injectors falling straight down onto throttle plates.

Disconnecting CTS results in ECM seeing high input CTS voltage (about 5 volt) which corresponds to a very low coolant temperature (-40F). Furthermore, with CTS disconnected, after about 2 minutes of running ECM will set Code 15 (open CTS circuit). Based on your description it is worth while to check fuel pressure. It is not too uncommon for a 'new' fuel pumps to fail, or output hose between fuel pump and feed through to burst.

Also check fuel filter.

//RF

Like he said if it is a parts store fuel pump even though it is pumping it may not be putting out the psi you need.We replaced the plugs ,wires ,then went to the module and then replaced the whole dizzy and still had the same problem.

[olivedrab]

alright i guess i have to drop the tank then. the only thing that puzzles me is why it would happen the same time my ignition module went out on me driving down the road.... somethings fishy but ill find it. ive got to get this thing back on the road so i can start on my 72 2wd to 4x4 conversion.

thanks again rfmaster and suicide 63

[BLE 'BURBAN]

Your stumped?
What happened?
Accident?
Cancer?
Birth Defect?....... Oh
uhmmmmmmmmmmm...............

wait a sec.......

Ok never mind. My colleague just explained it to me....

[grinch4om]

like I said, if you unplugg the ECT(which is confirmed as telling the computer its -40) its lean and the computer fattens it up with the ECT umplugged therefore, fuel pressure, fuel volumn, vacuum leaks, injector spray, all the things that would cause it to be lean but able to be fattenned up by the computer. diagnosing is a fine art and you have to take a step back and think...which is cause and which is effect

[olivedrab]

alright i got another new fuel pump for it yesterday. today im gona drop the tank, stick the new one in and see what happens.

[olivedrab]

ok. i changed the fuel pump yesterday with a new one. same problem.

so far this is what it isnt:
fuel pump
vaccum leak around intake
moisture in distributor
coolant temp sending unit(for comp.)
ignition wiring
idle air control valve

still need input please!!

thanks

[rfmaster]

Olivedrab

Did you actually measure fuel pressure before and after swapping pumps at the inlet to TB??? If you are able to keep it running with disconnected CTS it indicates fuel starvation.

Things to look for -

Broken fuel regulator spring inside TB - this will keep fuel pressure low and starve injectors! A simple fuel pressure check will show that. TB needs 11 to 14 PSI for happy operation. Anything below 9 PSI will be trouble. TB Rebuild kit will have a new spring and diaphragm. Pop the torx screws of the top of the regulator and take a look inside. Link below shows what it looks like.
http://members.shaw.ca/betterthanyou...to/fpr/fpr.htm


Clogged injector - take a look a the injector spray pattern at idle. Disconnect one injector at a time see if idle stumble is the same of either injector.


//RF

[87 STEPPER]

When you replaced the Ign. module did it come with a small packet of white paste like grease? and if it did! did you use it?

I know alot of people throw it away.

Cuz your suppose to paint that stuff on the under side of the module to isolate it from the heat of the dizzy. if you dont it will cause the Ign. Mod to have a dielectric break down and fail and that may be whats happening to you.

Good luck.

[olivedrab]

Originally Posted by 87 STEPPER

When you replaced the Ign. module did it come with a small packet of white paste like grease? and if it did! did you use it?

I know alot of people throw it away.

Cuz your suppose to paint that stuff on the under side of the module to isolate it from the heat of the dizzy. if you dont it will cause the Ign. Mod to have a dielectric break down and fail and that may be whats happening to you.

Good luck.

yea i used the entire packet of the special grease that came with it...

[olivedrab]

Originally Posted by rfmaster

Olivedrab

Did you actually measure fuel pressure before and after swapping pumps at the inlet to TB??? If you are able to keep it running with disconnected CTS it indicates fuel starvation.

Things to look for -

Broken fuel regulator spring inside TB - this will keep fuel pressure low and starve injectors! A simple fuel pressure check will show that. TB needs 11 to 14 PSI for happy operation. Anything below 9 PSI will be trouble. TB Rebuild kit will have a new spring and diaphragm. Pop the torx screws of the top of the regulator and take a look inside. Link below shows what it looks like.
http://members.shaw.ca/betterthanyou...to/fpr/fpr.htm


Clogged injector - take a look a the injector spray pattern at idle. Disconnect one injector at a time see if idle stumble is the same of either injector.


//RF

no i didnt check the fuel pressure before or after. the pump that was in there was still under warranty so i pulled it anyways since it wasnt costing me anything but time and effort, and it also give me the chance to double check everything as far as that goes.

quick question, is there any way to check fuel pressure without an inline fuel pressure gauge? cuz i dont have one of those... but i can get a hold of one.

thanks again for all ure help rfmaster

[rfmaster]

Yea, stock TBI system does not lend itself for ease of fuel pressure checking. Depending on which fuel pressure gauge test set you have it may have and TBI specific T adapter section. Even if you do have an adapter it is a PITA since OEM fuel lines are hard to get to and move in order to insert T section (in the back of TB).
Alternatively, you can insert a 3/8" T and appropriate sections of rated fuel hose at the fuel filter. It will not be as accurate, but it will give you a clear indication if you have low fuel pressure.

//RF