So I'm helpin' a friend out who isn't really able to fix things himself. he's got this 89 IROC w/5.7 TPI with automatic and 47K miles on it.

Had set O2 codes,MAF codes and a TPS code.He had an aftermarket chip in it,I swapped OEM back in, after I couldn't get my scan tool to retrieve any data.

The car runs pretty good under full throttle but under light loads it falls on it's face around 2000 RPMs and sometimes it dies when coming to a stop (not lock converter up stalling either).

My scan tool won't always read data on his car, but all of sudden will read ( no problem on other cars) and when it did I saw the TPS with a fixed .78 V and no change when the trottle is moved so I changed it and the O2 sensor for good measure. I cleared the codes and it ran fine for about 5 miles when the problem reappeared as a MAF code (33) only so I changed that and both relays associated with the MAF. I thought the problem was solved until he called and said it's still the same, stumbling at about 2000 RPM under light to moderate load. It didn't set a code till tonight but I didn't have my scan tool to retrieve them, but I'm perplexed why it ran poorly for about 15 miles without setting a code?

This car is clean, clean, clean and has low miles and I was wondering if I'm over looking something simple like a fuel filter or catalytic convertor problem because it sat for most of it's last 4 or 5 years.

Or is it more severe, like a bad ECM.

Any thoughts would appreciated.

Thanks,

 

I'd suggest checking connections to, at, and in the ECM. Start by confirming that the MEMCAL and PROM are fully seated. Verify that the ECM harness connectors are clean and fully seated. Make sure the ECM is properly grounded, and that the ECM harness grounds on the engien and chassis are intact. If none of that works, tap test the ECM to see if vibration causes a change in engine operation.

FWIW, 0.78V from the TPS is out of range, and will contribute to a MAF error code. The O² code could be related to a connection, or a weak sensor from age, contamination, and/or fuel mixture problems for various reasons (like poor injector spray patterns, low pressure due to a pump or filter, etc.)

 

Thank you, thank you, that sends me in a specific direction that I needed.

 

Okay so I had some time to chase down this problem some more, but to no avail.Here's what I did today. Pulled and checked the mem cal for proper seating and looked for obvious cracked solder pins on the board,nothing. Checked the wiring circuit from the ECM to the ALDL serial data line, good and no short to ground or open. ECM is grounded properly, no obvious harness problems under dash or under hood.
Checked fuel pressure; 42 at key on,non running. 37 running at idle, 32-ish running down the road and during problem too (new fuel filter BTW).
TPS at.55V and rolls up smoothly to 4.57V. Checked that cold and hot with no changes in voltage.

Open loop runs fine, closed loop sets MAF code,TPS code and under light to moderate load stumbles, almost like a bucking right at 2000 RPM.as you accelerate past 2000 it runs fine and if you were to drive the car pretty hard you'd never know something was wrong as you drive right through the 2000RPM mark on the way up and it doesn't fall on it's face.

I was able to get a data stream intermittently on my scanner and even going down the road, what I noticed is that it happens right at .78-80V on TPS and the integrator and block learn go wildly high to 155-160 during this event too.

Strangely, one time on a test drive while it was bucking or falling on it's face at 2K RPMs it was setting no codes on that drive. And occasionally on coast down it'll stall right out.

MAF gps at idle is 8 and at light cruise is 14-18 gps.(If I remember right).

I'm about ready to just get a new ECM for this guy but I'm tired of spending his money and getting no results. I'm just thinking that there is a problem with the ECM's 5V reference circuitry.

Any body have ideas??

 

I have the same problem right now, so if you know what you did to clear up the problem let me know.

 

Sure, because I couldn't get any data stream with any regularity I decided to throw some more money at the problem and get him an ECM. I couldn't diagnose what my scanner couldn't read.
The long and short of it is that the new ECM fixed the problem completely, even the intermittent fan cycling problem that I didn't mention earlier. He's happier than a pig in mud now, and so am I.

I still think it was a 5V.reference problem with the ECM, but I'm still very new to these TPI's.

Good luck!