I have an 1986 TPI with an A4 trans. all stock except for a Hypertech stage 2 chip and a 160° thermostat. The chip and thermostat have been in for 12 years, the car has 156,000 miles.
Problem
The problems began as an intermittent stumbling on throttle tip in off idle accompanied by a SYS light, code 34 (MAF voltage low). Over time the stumbling became more frequent until it was 90% of the time, now 100% of the time. As the car warms up the stumbling goes almost completely away. However, when warm and cruising at low speeds (i.e. low throttle openings) the engine will momentarily cut out and then recover. Increasing throttle opening eliminates cut outs.
Attempts to repair to date
Initial testing indicated MAF sensor bad. Purchased a reman. From NAPA and drivability was worse and now had both code 33 and 34 (MAF voltage high and low).
Returned MAF then purchased a new from GM. Same as original MAF, drivability issues and code 34.
Replaced burn off relay with new from GM, no change.
Replaced burn off module with new from GM, no change.
On a whim replaced IAC with new from GM, no change.
Inspected wire harness from firewall to all connectors, no visible issues where found with wires or connectors.
Replaced ECM with reman from GM, no change.
Tried original PROM, no change.
Dealership work
Took to local Chevy dealer, they spent about 6 hours on it (nice folks they were they only charged me for 2 hrs.).
What they found was that they could not get a data stream out of the ECM to the TECH 1 unit.
They checked the wire from the ALDL to the ECM and it is good.
They tried both the original ECM and new ECM, same result.
They tried both PROMs, same result.
They did not have any other ideas at this time and I did not want to spend more money on having them try to diagnose.

Any suggestion would be appreciated. I am reluctant to keep throwing money at the car on parts with no success on fixing the problem. I also do not want to go to a carb.

Thank you.

 

just because the wire is good, does not mean there is contact. I would be willing to run a test wire from the ECM to the Tech I.

I was going to suggest harness... but you have already checked through that. Have you checked the harness all the way down to the firewall? That's the real pain...

These older cars can be a real pain to track down...

What about the TPS? Has it been checked?

Are the various temp sensors working? There are 4 to consider... 1 and 2 are on the intake. One is for the ninth injector, the other, for the fans. The next one is on the head, between 1 and 3. Finally, the oil temp switch, is on the block, near the oil filter. It does nothing in relation to things.

I am curious about fuel... injectors? pump? Filter? What is the pressure like?

With all those miles, you might even be dealing with a valve train issue. If the cam lobe is down or a spring has freaked, then all bets are off, mechanically.

One last thing - thank you for the clearly written problem! It is so rare when people actually take the time to outline the problem in detail.

 

Also, when you replaced the IAC did you clean the pintel bore under the TB and the TB itself?
Then the base idle should have been reset to 450rpm and the TPS base voltage set.

To check MAF burnoff working you can look into the front of the MAF as someone turns off the engine. You should see the sensor wires glow red for 2sec if all is well there.

Since you can pull the codes I would not suspect a bad ECM just yet, though a scanner should be able to commumicate. I have an AutoXray that works on my '88 Vette and '86 IROC but have had problems hooking up to an '85 Vette ECM.

BTW, you can replace your MAF with a used one from a TPI F-body car for less $$ than getting a referb unit. My Vettte is now running the original MAF from the IROC and I have the Wells MAF in the IROC.

 

Do your dash lights flicker and go out from time to time?
Is the TPS voltage at .54?
Does the car idel down after the temp hits 170 degrees?
With the scan tool was it staying in close loop?
YOu have no cracks around the MAF on filter or TB side,where the clamps tighten down?
No outside air can get into the MAF except through the air filter or you will get a stumble.
Good post and not that long ,it just has facts and not words.

 

Thank you for your replies.

A couple of things I have tried, but forgot to mention in the original post.
I check TPS voltage, right where it should be per specs, I don't remember exact voltage.
I also did a vacuum leak check by spraying starting fluid around the various locations from the MAF to everything on the intake system, found nothing.
I did a fuel pressure check and it is right on spec, do not remember value.
I also replaced the distributor cap and rotor and verified timing was correct.

I have considered changing the two temp sensors that are in the front of the intake, one is for the ECM water temp and the other is the cold start injector temp signal.

No dash light flickering and the car idles down just fine. No scan tool data can be read even though the connection between the ALDL and the ECM is good.

Thanks again, and if there are any other thoughts I would be thankful for the input.

Brian

 

Probably a long shot but have the alternator checked. I had a wek one once and it gave me a code 34 and later a code 36. Something to check that is free.

 

Ik fuel presure is at 40 psi.
voltage is .54 on the TPS.
When you put the round pad inside the cap with the spring and carbon center did you put the spring and carbon piece under the round rubber pad or on the top ?
Have youdone a compression test and what were the results?
Do you have the right #4 PCV valve in it?
Does the pcv valve hose collaps or is the hose in good shape.
Last do you have the right throttle body gasket for the 86 year installed.

 

Did you change the ignition control module inside the distributor?

Sounds EXACTLY like my problem last summer......

I found the best price at Autozone (25 bucks) and carry a spare now.

JRC

 

shot worth taking.. stumbling on idle in mine was cause by a failed egr valve..at the time i did not know that it could simply disconnect it, however, they do fail. checking the valve will eliminate one more issue.

i would want to check, the sensor on the underside/rear of the plentum, near the vaccum line for brake booster. check for connection/termination/operation. while in the same area check the pvc hose on the intake for cracks/leaks

when you did the fuel pressure test..at idle about 40lbs?.. was it 40lbs when driving / accelaration?...was there a pressure drop when ign off? how much drop and how long did it take?(may suggest leaky injectors/ or weak fuel pump), was the fuel filter replaced recently?

in some cases, a stumbling effect could be related to a weak/failing ignition module. have you checked this? these can be checked free at some auto parts stores.

hope this helps