1987 gmc R1500 tbi. LOPEY IDLE/STALL/NO POWER

[jesdude7789]

I have done just about everthing I know to do to this truck and cannot figure this one out.
This is my daily driver for the last 7 years.
The engine has 130,000 miles on it and the trans was rebuilt about 50,000 miles ago.

About two months ago my truck started leaking oil from of the back of the motor and it really got bad a few weeks ago. I eventually traced the problem to the back of the intake. The engine would also run a little rough once I accelerated from idle but it wasn't bad.... YET. I R&R'ed the intake manifold and installed new fel pro gaskets and layed a nice fresh bead of rtv along the front and back edges like you're supposed to. Now there is NO oil leak. Also did a new oil pressure switch while I was back there.
The truck still idles just fine but it still has problems accelerating once i'm off idle. If I immediately stop again then the engine will get a really lopey idle and sometimes just about stall out and then if I don't touch the gas pedal and wait a few seconds or so it will almost instantly regain a smooth consistent idle.
If I go ahead and keep accelerating then it will smooth out enough once I get it going 20-30 mph and above to highway speeds. But I can still tell that the engine isn't running correctly. It was doing this to some degree before I R&R the intake manifold.

I have done:
a new TPS (adjusted to .55 volts)
a new MAP
a new CLT
rebuilt the throttle body completely
a new fuel pump, even though the fuel pressure was a steady 12 psi before.
a new fuel filter
a new oil pressure switch
checked for vacuum leaks (found none)
a new coil, plugs, wires were put on yesterday
a new spark module
entire distributor and cap and rotor are all less than 6 months old
checked the timing (with tan/black wire unplugged)

I have great voltage at 14.1 when running
It starts up just fine like it always has and actually even a little bit quicker with the fresh plugs and wires.
I replaced the MAP sensor because at one point I was getting a 34 check engine code for that. I replaced the TPS because of it's age.
As of yesterday, I do not have any check engine codes. I should also mention that I have a laptop with the ALDLcable program installed and a OBD1 cable which allows me to see the live sensor data. The OBD1 is a very old system though and so the data refresh rate is about every second or two.


The ONLY thing I haven't replaced yet is the EGR valve but I did remove and reinstall it because it looked pretty good and didn't seem to be sticking or anything. I also haven't done it because I have already spent so much money and time on this truck over the past 2 weeks. There was no heavy carbon build up.

I'm pretty mechanically inclined and have always worked on my own cars/trucks which have mostly been chevy. What do you guys think?

[jesdude7789]

I meant to say that I replaced the ignition module under the distributor cap.
I have NOT replaced the spark control module unit that is next to the map sensor.

[Desert1957]

I have seen unusual idle and stumble problems on TBI engines over the years. I actually made up a block off plate for the EGR just for this reason.

Simple to make , just use an old EGR gasket or trace the valve flange area while is unbolted. use a piece of scrap metal plate, ( I used aluminum) , Install on intake with a new gasket , doesn't matter what it looks like, Road test after installation, see if engine runs better.

I would say about 50% of the time it did trace my problems back to the EGR valve. Be aware after battery disconnect or clearing codes , the ECM does have a "Re-Learn" feature. Takes some road testing for the changes to be noticed.

Desert

[jesdude7789]

I forgot to say that I also replaced the IAC valve a few weeks ago as well before I removed the intake manifold.
I have also double checked my idle setting during the relearn procedure. It is set at 600 rpm idle when in drive just like the manual says to do.

I will be replacing the EGR valve tonight since I know that mine is the original gm part.

Is there a way to test mine? What about the egr solenoid?

[jesdude7789]

I went out at lunch and checked. My egr is an original gm part and has the numbers:
17087196 267 71N

I looked online and I cannot find a delphi replacement.
Only the acdelco and bwd are available at my local parts houses.

[crashz]

I know its a totally different engine, but I had the exact same symptoms with my Saturn when a chunk of carbon got lodged in the EGR valve.

Even if yours is fairly clean and closing completely when off the truck, the spring pressure may be overcome with exhaust gas. I'd try replacing it with the AC Delco unit.

[jesdude7789]

I wasn't able to get the EGR valve yet. BUT I did just go out to the truck and started it from cold. I looked down at the IAC pintle in the throttle body and it is all the way in so it COMPLETELY closes off that passage while the engine is idling. I goosed the throttle a little and then I can see the pintle retract just a few steps and then close the port back off quickly. I wanted to check for vacuum leaks again so I pinched off the PCV hose with a pair of pliers and the idle changed some but not enough for the IAC to move. I pinched off the hose going to the brake booster but that didn't make any difference at all. I pinched off the hose on the back of the intake that feeds the a/c controls and cruise control and that didn't make a difference either. I also now have a code 45 - rich exhaust condition.
Isn't the IAC supposed to be retracted a lot in order to actually CONTROL the idle??

[jesdude7789]

Does anybody know what my IAC count should be when the engine is started from cold?
What about when it gets to temperature?

[jesdude7789]

I went by oreilly's and picked up the BWD brand of egr valve replacement and it definitely did the trick.
Here's what I did tonight when I got home:
I had my laptop hooked up and watched the sensor data and tweaked my idle screw at first in order to try and get the IAC to open up but even with the engine warmed up and everything else looking good, the IAC counts would be at 0 and I confirmed it with my own eyes. I figured that the IAC should be open to some degree when the engine is warmed up and that this would help it "catch" my engines stumbling while driving since it would already be in the midrange of it's movement and not closed. I set the parking brake, chocked the wheels, and put the truck in drive and let it idle until it was warmed up completely. I unscrewed the throttle screw until it was all the way off the throttle arm and the tps voltage was now at about .40 volts (it was .59) and the idle rpm was about 475 but the engine just kept going and the IAC stayed at 0 counts. Then I goosed the throttle and the IAC would open a little and then close again. If I sat in the driver seat and kept the parking brake on and slowly tipped the throttle then the engine would sputter and surge and stalled once or twice. Started back up just fine and then I'd try again. I was just trying to make sure I didn't miss something before I swapped out the egr valve. I let the engine cool a little bit and then swapped the egr valve out. Hooked everything back together and this time the engine started like normal and idled like normal but when I tipped into the throttle it wouldn't surge or buck or stall anymore. I'm assuming this means that my old egr diaphragm was leaking and therefore was not able to open when the solenoid wanted it to and this caused the engine to run rich and stumble and I could smell rotten eggs sometimes.
I drove it around town for a while and I may have to back off the timing a few degrees because I heard some pinging I believe when I would go WOT with it but other than that, it ran great and never stumbled when coming to a stop or accelerating like normal.

[Desert1957]

Originally Posted by jesdude7789

I went by oreilly's and picked up the BWD brand of egr valve replacement and it definitely did the trick.
Here's what I did tonight when I got home:
I had my laptop hooked up and watched the sensor data and tweaked my idle screw at first in order to try and get the IAC to open up but even with the engine warmed up and everything else looking good, the IAC counts would be at 0 and I confirmed it with my own eyes. I figured that the IAC should be open to some degree when the engine is warmed up and that this would help it "catch" my engines stumbling while driving since it would already be in the midrange of it's movement and not closed. I set the parking brake, chocked the wheels, and put the truck in drive and let it idle until it was warmed up completely. I unscrewed the throttle screw until it was all the way off the throttle arm and the tps voltage was now at about .40 volts (it was .59) and the idle rpm was about 475 but the engine just kept going and the IAC stayed at 0 counts. Then I goosed the throttle and the IAC would open a little and then close again. If I sat in the driver seat and kept the parking brake on and slowly tipped the throttle then the engine would sputter and surge and stalled once or twice. Started back up just fine and then I'd try again. I was just trying to make sure I didn't miss something before I swapped out the egr valve. I let the engine cool a little bit and then swapped the egr valve out. Hooked everything back together and this time the engine started like normal and idled like normal but when I tipped into the throttle it wouldn't surge or buck or stall anymore. I'm assuming this means that my old egr diaphragm was leaking and therefore was not able to open when the solenoid wanted it to and this caused the engine to run rich and stumble and I could smell rotten eggs sometimes.
I drove it around town for a while and I may have to back off the timing a few degrees because I heard some pinging I believe when I would go WOT with it but other than that, it ran great and never stumbled when coming to a stop or accelerating like normal.

Its a great feeling when it all goes right, Congrats!

Desert

[crashz]

Glad you got her running smooth agian!

[anubisani]

had this same problem 3 weeks ago and it was also egr vavle being stuck I took mines out and soak the inside with pb blaster then cleaned it out with brake cleaner and reinstalled . it has worked so far. mine is 88 tbi 454

[jesdude7789]

Well here's an update. While the truck runs a lot more like it should again, now that it has had over a week to relearn, I have noticed a couple things.

The other day on my way to work I stopped at a drive-thru for breakfast, ordered my food, and then started inching forward around the curve to the window and while doing this, the engine did the same stumbling thing as before but on a much much smaller scale. Maybe 20% of what it used to do. It regained idle without me touching the throttle or anything. I thought "oh great" and got my food and went to work. That was probably a week ago and it hasn't done it since so I'm thinking the computer might have still been relearning things.

The other thing I noticed is that when I am just slowly cruising through a parking lot or something like 10 mph or less. When I let off the throttle, I can hear the engine rpm drop in a way that sounds like it was not transferring enough torque to move the truck. Kinda like I have a torque converter not working until the rpms get up higher. Or let me say it this way: My truck idles right at 600 rpm when in drive and foot on the brake, then I push the gas lightly and the rpms will raise to maybe 900-1000 or so before it actually moves the truck. Does that make sense? If I were at a show or something and cruising this thing around then I would be somewhat embarrassed because you would hear it easily since I have a flowmaster exhaust. Do I have a worn out torque converter maybe?

[crashz]

So when in Drive (or OD), idling, on flat paved ground, you take your foot off of the brake, the truck will not even roll forward? What if you shift manually down to 1st gear?

If you are talking about raising the rpms while holding the brake though, that reaction seems normal. Stock TC's lock in between 1200- 1500 rpm depending on application. Add some age and it may be a bit higher. All traditional automatics have torque converters that provide a fluid coupling that allows slip below a lock-up rpm. Until you hit that rpm, its supposed to slip.

[jesdude7789]

I know it will move when I first start it up and put it in OD but that's definitely because of the higher idle from a cold start. I will test it again on my street when I get home today but i'm pretty sure it doesn't move when warmed up and at the normal idle speed of 600 rpm. I'll try shifting to 1st as well and see if that makes a difference.
It is a stock transmission but it was rebuilt for the first time back in 2010. They probably stuck a rebuilt converter in there. I think I've put at least 60,000 miles on it since then without any service so I will look into doing a filter and fluid change. The additives are probably about used up.
No, i'm not holding the brake when doing this.

[jesdude7789]

Okay. In Drive, on my street which is flat and paved, I can take my foot of the brake and the truck will move slowly. Maybe 1-2 mph?
BUT it still does what I'm saying, the high rpm's when cruising anywhere from off idle and above. I definitely don't like it.....