So, for the past 10 months my 91Z has had multiple issues. First the speedo would randomly stop working, with SES light on. Car would rund horrible and very rich when this occured. Swapped out VSS and speedo gears. Problem continued. Then I would get hard starting, " double peddling" to keep car running at low rpms, and it would randomly hesitate/die out at highway crusing. When this happend, it would take a few times to restart. Then it would run kind of ok again. Sometimes it would run for 1 minute... sometimes 1 hour.
I have checked the injectors, map, fuel pressure and vac lines. All (seem) good. I have hooked up ALDL cable with TunerPro and it said connected but would not read any data. I have tried jumping a&b on the port, and I am getting no codes. It doesnt even seem like ECM is going into diag mode (no code 12). So now I am thinking that the ECM is shot.
Like I said the car is a 91Z, 305tpi, bone stock.

 

Most likely is the ECM. Each time as you do a key-on observe the SES light. It should turn on, blink off, then turn on solid. No blink off and the ECM isn't running from the PROM.

One of the fans is also likely to run at this time.

RBob.

 

Just checked.. SES light does not blink, and the one fan does come on. Any idea where to get a QUALITY replacement for a 1227730 ecm? Ive seen ones on ebay for $65, but who knows if they work.

 

I definitely respect RBobs expertise, but does anyone else have an opinion? What else should I be looking for? Is there an easy way to TRULY tell if the ECM is bad, or could it be the chip?

 

Pull the ECM down and take the full cover off (10 screws, 1/4" hex head). Start the engine and leave it idle. Gently press on the main board at various locations. Note any changes in how the engine is running.

You may find a spot or two where if you hold pressure on the board the engine will run normally.

RBob.

 

Thank you. That absolutely did it.The ses light would turn off when I pressed down on the board and come back on when released. Months of chasing the problems are over

 

So will you repair the board? Cracked board or just cracked solder joints?

 

I looked at the board up and down, even under a magnifying glass. And I cannot see anything out of the norm. The board looks like it is in PERFECT shape. I am going to order a reman one to double check, and send mine out for repair.

 

New, well reman, ECM is installed and car is running great. Im thinking I will have to change the plugs and clean up the TB since it has been running rich and like crap over the past several months.