Hi all! I've gone over so many threads over the same issue I've been having in the hopes of avoiding a separate post, but sadly haven't been able to track down my particular cause. Please forgive me in advance for the long post, just trying to be as thorough as possible to try to narrow down this issue. I have an '88 Trans Am with an LB9 305. It has always been a very smooth running, reliable car that has been from the Florida Keys to California. In May, I put a new A/C compressor on the car and had a local A/C shop recharge the system. Fifteen minutes after after picking it up from shop, on the drive home, it began developing stalling idle problems and ran rough. Upon slowing down at a stop light, the car would just shut off, almost as if someone just turned the key. It would always fire right back up, but upon shifting into gear, it would shut right off again unless I was giving it gas. This happens with the air conditioning on or off, so I don't believe it is tied into the compressor swap, but that roughly was when this all started. In the past few months, I have done the following:

- Replaced/reset IAC Valve (AC Delco).
- Tested TPS with multimeter (Good).
- Disassembled and thoroughly cleaned plenum and throttle body passages.
- Flushed fuel rail, replaced stock fuel injectors with Bosch III's from SouthBay and replaced fuel filter.
- Ensured good spark to all spark plugs (wires less than 5 years old), replaced spark plugs with AC Delco Platinum ones.
- Replaced MAF sensor (Delphi) and relays (AC Delco).
- Replaced coolant temperature sensor, air intake temperature sensor, and O2 sensor (all AC Delco).
- Checked all vacuum lines for leaks using starter fluid (none found, and rubber hoses are less than five years old).
- Checked timing with EST wire disconnected, set at 6 degrees.
- Replaced ignition control module (AC Delco), (Distributor completely cleaned/rebuilt 3 years ago and also replaced coil).
- Tested EGR by applying vacuum (seemingly good, EGR less than 3 years old).
- Tested pressure at fuel rail, steady ~35 psi (fuel pump less than three years old).
- Catalytic converter seems to be clear and working properly, (230 F in, 630 F out).
- Replaced ECM with new one.

After all of these things, the car seems to run smoothly but the idle problem still persists once the engine reaches operating temperature. The fuel mixture (both before and after the injector swap) seems to be a bit rich and there was a fair amount of carbon build up in the throttle body/plenum passageways prior to cleaning.

There was one small incident that occurred during the A/C upgrade. An aluminum line accidentally got pinned against the lower temperature cooling fan, preventing it from spinning. The fusible link on the orange wire burned up, along with the cooling fan motor. Both were replaced and I checked for shorts in the wires and also made sure the relays were okay. After the fix, both fans functioned normally without any trouble for several weeks before the idle trouble began. Don't necessarily believe the two incidents are connected, but wanted to include it in case someone might catch something I did not!

I love this car to pieces and am determined to figure this out. I'm very much a do-it-yourself guy but after several months of trying various things I finally caved and took it to one of the best shops in the area, yet they could not even figure it out. Most forum posts on here I've found about this seem to end in frustration or no conclusion, but I really hope to avoid that. Again, I'm sorry for typing a small novel, but wanted to include as much information as possible in the hopes that someone could point me in another direction or see something obvious I missed! Thanks much in advance!!!

 

So you have gone through the minimum idle air setting procedure?

What is the IAC step count at idle in park and is the ECU recognizing the park/neutral switch? What do the steps do when you put it in drive?

GD

 

Hi GeneralDisorder,

I went through that procedure several times with the engine warmed up (jumper ALDL, turn on ignition, unplug connector, wait 30 seconds, etc.) but sadly it did not seem to make a difference in my case.

That said, I did not check the step count nor am I sure how to? Would I need TunerPro to be able to do that? Thanks much.

 

TunerPro would be one way. There is other software or you could get an OBD-I scan tool like the SnapOn red brick 2500.

GD

 

I appreciate the suggestion! I've been incredibly close to going out and buying the cable/downloading the software. Might just take the plunge before long as I'm exhausting other options quickly, but hoping maybe some others might have some suggestions for things I missed. Thanks again.

 

As you have seen - loading up the parts cannon and troubleshooting with Visa hasn't been that effective. You need to do your troubleshooting WITH the computer, not trying to guess what it wants.

GD

 

I may still have the cable you need from my days of torture with a TPI car, pay shipping and it's yours

 

GD, you've definitely hit the nail on that one... Shame the computer won't throw me any codes! I decided last night to go the diagnostic cable/software route, as you said, the guesswork is getting old. Seems TunerPro is the most common on here, is that what you would recommend?

Tyler, I really appreciate that offer and I shot you a private message.

 

Codes aren't usually as useful as you might think. The ability of the computer to tell what's wrong is severely limited. Even modern cars have trouble with that, and the 80's was significantly worse in that respect. You can't rely on the computer to diagnose the situation, but you must know what it's inputs and outputs are or you are wasting your time.

TunerPro is free to use and I do occasionally use it for tuning when the (unfortunate) need arises.

WinALDL is another:

http://winaldl.joby.se/

The easiest way is to just get a real scan tool. The SnapOn 2500 is often on ebay for pretty cheap these days. Shops don't use them anymore - we all have newer scan tools that are backwards compatible but the price is obscene.

GD

 

TunerPro and the Moates cable is great. What brand MAF did you buy? Unfortunately I do not think there are any new replacements that are any good. The only solution IMHO is to find a working original Bosch MAF. I had running issues for a long time, even with custom tunes, before ditching the non-original MAFs.

I doubt you need to set the minimum idle if you haven't touched it before.

The Bosch III injectors are nice, but they are not an exact flow match for the stock units and a tune is very helpful in dialing in the BLMs.

I agree with GD that the error code usefulness is limited but TunerPro with a good connection into the ECM is very helpful.

It is also likely that your original timing chain (if you still have it) has too much slack. This can cause a hunting idle since the cam is not locked in to the crank (it wobbles from the slack and the valve springs pushing back). Easy way to check is to look at the base timing while idling. It should be steady and locked in. If it is bouncing around, the chain is done.

I suggest you go with a custom tune now. They are not expensive and with the age and changes to the parts you have, you will know that it is running perfectly.

 

Good morning, all!

Tyler was nice enough to send me his Red Devil River ALDL cable and I just finished following the instructions from RDR to get drivers and TunerPro set up to run diagnostics. I was really impressed with how informative the info from RDR was and am feeling confident now in moving forward. Time to move the computer outside!

Tootie Pang, I ended up settling on a Delphi one, but I'm not even convinced the original Bosch MAF was bad as I noticed no change in operation after replacing the MAF. I've refused to touch my minimum idle during this mess so it still has the cap on it and all. I haven't ruled out the timing chain, but it has not seemed to be bouncing around as the base timing holds consistent at 6 degrees every time I check it.

Will keep you all posted after I try my hand at TunerPro! Thanks again!

 

Alright, so I got everything hooked up and connected between the laptop and ALDL port, and all set up flawlessly. Started the engine and began recording data with TunerPro RT also with no problems. However, it seems with the Red Devil River ALDL cable the car behaves differently and has a boosted idle, therefore not dying when shifting gears and such. Also, when shifting from drive to neutral the idle jumps high and slowly comes back down again. There is a removable jumper on the cable that I thought may have been putting the car in a diagnostic mode, but upon removal I could not connect to the TunerPro RT setup. Does anyone have any ideas on this? Thanks.

 

I believe the jumper is the 10k resistor. You have to remove that to take it out of diagnostic mode. It seems that most people have the best success by starting the data stream and then removing the 10k resistor after the data stream has been established. The 1227165 ECM is difficult to stream data from under the best conditions so you'll have to be patient with it.

Now you are beginning to see why a LOT of us have moved away from the 30 years old stock computers to modern stand alone hardware.

GD

 

In 10k mode idle will increase. I’ve had nothing but bad luck with Red Devils cable . Get a moates xtreme aldl cable.

 

Hello all! Just wanted to post that after a couple years of on and off experimenting, the stuttering and dying was all caused by a bad mass airflow sensor. The MAF was the very first thing I replaced when this all started, and upon doing so, the check engine light went away and I assumed the problem must lie elsewhere. I had wanted to replace it with a Bosch MAF, but in 2020 when I went to source one, they were all out of stock and I settled on a Delphi unit. Needless to say, just this month, I tried three brand new MAF's on the car. Another Delphi, a Bosch, and a third brand I don't remember. The brand new Delphi caused the car to continue to run rich and stumble, while the Bosch immediately fixed the fuel mixture and idle problem. I'm not sure if this will help anyone else with this same problem, but my best advice would be to avoid Delphi MAF's! So thankful that after two years it finally runs properly again!!!

 

My idle problem was a bad distributor. Took about 10 years to resolve it. The shaft was worn beyond recognition.

 

Thank you for posting an update to how it was solved! So many dead end threads with no solutions...