Hello everyone. I'm a new member. Couldn't find an existing post that addresses this problem. Hopefully someone can help me resolve it.
I have an 87 TransAm with the 305 TPI (LB9) and MT. Acquired it at 67276 miles. It was extremely well taken care of and drives and runs really well except for one little problem. When I snap open the throttle ( from gas pedal or throttle body ) from idle while in park or neutral ( regardless of engine temp ), the engine bogs or stumbles briefly. The only likely cause I could think of is a worn out throttle position sensor ( the contacts wear the most at closed throttle ). I can't find any record from the previous owner as to whether or not the throttle position sensor had ever been replaced, so I replaced it and properly adjusted the voltage specs for closed throttle ( about 0.55 volts). The voltage at WOT is around 4 volts. I also verified a smooth continuous increase in voltage from closed throttle to wot. Unfortunately, this did not correct the problem. I'm not going to replace any other parts until I know what else is likely responsible for this issue. Does anyone know what could be causing this problem? Thank you for any help you can provide. Admjgreer.

 

Tip in sag. Mine did that since new. I believe it's the electronics.
also, you say MT, then park & neutral. So WTF are you talking about ???

 

Check for vacuum leaks. On a MAF car, vacuum leaks are bad news, especially at idle where leaks can be a significant percentage of total airflow.

A small vacuum leak at idle is going to be drawing in air that the ECM isn't aware of and you're going to be running lean. Then when you snap the throttle, you're coming off a lean condition.

A small vacuum leak becomes less noticeable at higher engine speeds since it's a much smaller percentage of the total airflow.

 

Sorry about that. Must have had a brain fart. Not Park (it's a MT), in neutral.

 

Thank you. I will check for vacuum leaks

 

As @EightynineF indicated, this was a phenomenon present in a lot of the early TPIs. In very late 1986 (after the '87 model year production started) there was a factory TSB regarding programming issues, and one of the symptoms was the lean throttle tip-in condition causing some stumble. The TSB was number 86-6(GAS)-66, issued October 17, 1986.

Some of the text of the bulletin is as follows:

PONTIAC ROUGH IDLE, CHUGGLE, LOW SPEED. Page: 1
86-6(GAS)-66 Issued: 10/17/86
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
1986 FIREBIRD VEHICLES WITH 5.0L LB9
TPI ENGINE (ENGINE CODE F)

New service calibrators (Ref: Called Mem-Cal In Shop Manual) have been
released for the 1986 Firebirds with the 5.OL TPI engine. These new
calibrators will address the following customer comments:

1. "Rough Idle" on both hot and cold engine - the idle speed is raised to 650
plus or minus 50 RPM.

2. "Chuggle or Shudder" at low speeds - torque converter clutch (TCC)
engagement speed is raised to 41 MPH.

3. "Hesitation" on light acceleration with a hot engine.

4. "Cold Driveability" - New calibration eliminates light to moderate sag on
cold driveaway.

5. "Service Engine Soon” Lights - Eliminates the following false Codes:

A. Code 32 - EGR System failure
B. Code 33 - Mass Airflow Sensor Voltage High
C. Code 34 - Mass Airflow Sensor Voltage Low
D. Code 43 - Electronic Spark Control
E. Code 44 - Lean Exhaust Indication

If normal engine diagnostics and repair do not correct the above conditions,
it will be necessary to replace the calibrator in the ECM. Below is a listing
of the new calibrators and their usages.

ON AIR CONDITIONING (C-6O) FIREBIRDS

With QYZ (P215/65 R 15/N 15" Tire) Use - 1228290 Calibrator
With QAC (P235/60 VR 15N Tire) Use - 1228291 Calibrator
With QDZ (P245/50 VR 16N Tire) Use - 1228291 Calibrator

ON NON-AIR CONDITIONING (C-41) FIREBIRDS

With QYZ (P215/65 R 15/N 15" Tire) Use - 1228292 Calibrator
With QAC (P235/60 VR 15N Tire) Use - 1228293 Calibrator
With QDZ (P245/50 VR 16N Tire) Use - 1228293 Calibrator

Use Applicable Labor Time and Operation Numbers.

1228291 Calibrator available $63.64 F.O.B. factory

 

https://www.thirdgen.org/forums/diy-...um-8993-a.html

 

The problem is finding the vacuum leak. It's like finding a needle in a hay stack on a TPI motor.

 

I guess if everything mechanical and electrical checks out, it could just be inherent in the car. In which case, it could be a matter of going into the calibration and play around with the accelerator enrichment.

 

First of all, thank you for all the helpful responses from Eightyninef, ULTMEZ, Vader, & Tuned Performance. I really appreciate your help.There is a lot of information I need to think about and consider. Will respond with more questions and comments when I need to. Thanks again. admjgreer.

 

I guess I should clarify on the vacuum leak thing...

The ECM does have the ability to trim the fuel based on what it sees from the O2 sensor. So in closed loop, it can potentially correct the lean condition by automatically richening up the mixture if the O2 sensor in the exhaust is detecting a slight lean condition. So even with a slight vacuum leak, the engine isn't actually "running lean" per se...

This of course wouldn't apply to open loop (when the engine is cold and the ECM isn't paying attention to the O2 sensor signal), and you're saying that the hesitation occurs regardless of engine temp.

So that may actually rule out a small vacuum leak as the culprit... which means it could just be inherent in the car.

A scan of the operational data while the car is actually running could be useful if you have access to a scanner.

 

Thank you ULTM8Z

 

I had an issue similar to this. Car would bog down before reving up when given gas. It was my MAF sensor relay.

You can check by un-plugging your MAF sensor, and then start the car and see if the bog disappears. For me, it did. Thus = MAF sensor or relay.