90 Beretta GT 3.1L rough idle

[DukenukemX]

Decided to try some things out. BTW, bellow is the picture of the old gasket. You can see what happened.



Took the ECM out of the car today, it's a A1 Cardone rebuilt.


Going to try and replace the PROM chip, since I have a spare lying around, but I saw this when I opened it up. Looks like the round red capacitor looks chipped. Not sure if that's an issue.

[robertisaar]

actually, that isn't a capacitor.... it's a varistor(MOV). you'll also find them in surge suppressor power strips, they're designed to conduct a WHOLE lot of current in a very short amount of time to try and get the circuit back under a specified voltage. if the alternator voltage spikes up for some reason, this part MIGHT save your ECM. the one in my 16149396 was actually gone before i even got it, and i've been using it on my testbench for quite some time without issue. the most it ever sees is only 12.6ish volts though.

chipped could be a manufacturing quirk and nothing being wrong with it... or it could be an indication that it has had to come into use at some point and got hot enough for the casing to chip off. you really can't tell if it is still functional without removing it and feeding it enough voltage(current limited at that) for it to conduct.

[DukenukemX]

I put the ECM back in and it won't start. Just cranks and cranks, but nothing. Put back in the old PROM chip with similar results. I took it apart further and the thing looks like a rebuilt mess. I Don't understand how removing it did anything to the ECM. I made sure to unplug the negative battery every time I removed and install the ECM too.

Anyway to test the ECM if it's bad?

[robertisaar]

by removing it, you may have turned an intermittent fault into a permanant one. it was going to happen eventually, you just accelerated the process.

if the PROM was bad, backup fueling should have at least allowed the engine to start and run to some degree. sounds like even that has been knocked out, which would imply a power/ground issue.

[1988GTU]

Bent pin(s) upon reinstalling is highly likely.

[pocket-rocket]

I can second that. I've done it by mistake years ago. Luckly it was my own car.

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[DukenukemX]

Bent pin(s) upon reinstalling is highly likely.

I'm looking at the pins and see nothing bent. I didn't have a hard time getting the computer out. More likely I agitated the ECM when I removed it. Something was already failing, I just moved it along. At least I hope.

Pepboys had the best price for a new A1 Cardone rebuilt at $60 with core. They'll have it for me by tomorrow. I hate these rebuilts with a passion, but what choice do I have? I plan to take some extra precautions this time, as I notice these ECMs have MOSFET's with heat sinks. The heat sinks have no thermal paste. I may just use some extra thermal paste I have lying around to make sure they stay cool. I did the same thing to my C5 Vette with no issues, even though nothing was wrong with the ECM besides heavy oxidation.

These 80's designed GM ECM's were built like crap, and went out often. At least they did for my cousin and myself. Don't know if anyone else has had similar experience? Recently a lot of injectors went bad, and would short. Which essentially shorted out the ECM, and would shutdown to protect itself. Literally one injector went bad after another, and I only replaced one injector at a time. I'm willing to bet that the injectors may have damaged the ECM.

[pocket-rocket]

I haven't had one from the early 90s go bad on me, but when I see them cheap I grab one. I think I have 2 spare '9396 for my car. I also have an ICM and coils on the shelf just in case.

Edit: lol, typed go nad instead of go bad and my dumb-phone didn't correct it.

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[DukenukemX]

Got the new computer and car starts, but still runs bad. The problem still seems to be the TPS. I just got the new TPS in the mail, but I saw that something was wrong with it. For one, the spring wasn't springing. So when I turned it, it stayed in that position. I threw it in the car, and it reports nearly 0 voltage, but so does the other TPS sensor.

The oldest TPS sensor will actually produce voltage at closed idle. The first TPS test log is with the sensor off the car, and I have someone turning it with a screw driver. At some point the voltage is reported correctly, and the TPS is at 0% open. The person had a hard time playing with the TPS sensor and the screw driver. At 0.35 the TPS is at 0%, and the engine runs normal.

The second log is with the sensor on the car, and me playing with the gas peddle with the engine off for a while. Reports 0.45 and the TPS is at 2.75%. When I start the car and give it gas, the engine seems to run ok at higher rpm, but at idle it's crap. Accorindg to this article, the idle TPS voltage is .5 to .9 idle. Yet the computer likes .35 for 0% throttle.

Starting to wonder if it's the TPS or it's the throttle? The throttle is closed or damn near closed, but I wonder if something can be adjusted? I'm also afraid to put in the other PROM chip, cause it could have taken out the old computer. I'll take another look at it. I have an Ebay seller to deal with as well. BTW, the rebuilt computer from PepBoys is BWD rebuilt, and looks pretty good.

[DukenukemX]

Could it be that to install the TPS I have to turn the TPS and push against the spring? Like a good bit of turn before I match the wholes and screw in the bolts. Should I need to turn the TPS at all to install it?

[robertisaar]

could be a different design than what i am used to, but i don't remember a spring in any of mine? i can't remember the specifics on the gen2 60V6 though, the last couple i've dealt with were gen3.

if the ECM is showing 0% TPS at .35 volts, then that is where it is calculating the 0% value from. i want to say that is close to the lower limit allowed before the TPS Low DTC will get set.

did you reset ECM memory after installing and seeing ~.45 at idle?

[DukenukemX]

Good news, the engine is running fine. What's strange is I don't know how. I decided to do two things. Firstly, I put in more coolant and bleed the system a little. It was low on coolant, and leaks a little through the thermostat now since I pulled it apart. Yea I gotta get a new thermostat.

The next thing I did was check the rocker arms on the front valve cover. I was hearing a knock coming from there and decided to check it out. Pulled each one off and made sure each push rod was seated proper. They were all seated properly, so I just put them back on and tighten the bolts. Started the car and it runs better then ever.

I'm guessing I didn't put the rocker arms back on correctly when I was doing the intake gasket? Yet the TPS is reading 1.57% open still, but it runs great and idles at 800 rpm. BTW 5 year old TPS is on the engine. I haven't done anything to the stopper yet, but I have a log of the engine running. I plan to take pics of the new ECM with thermal paste, and a new log with the battery unplugged.

[robertisaar]

strange.... you may notice after driving for a while that the 1.57% TPS value might drop down to 0 eventually. there is code to update the minimum TPS position, it requires you to be coasting down from a significant speed with no throttle and pulling a decent amount of vacuum to indicate that there is no throttle being applied.

[robertisaar]

the only issues i see in the log now are running extremely rich at idle without O2 correction and the TPS low DTC being set.

[DukenukemX]

I've disconnected the battery and let it idle relearn for a while. A did some short distance driving and it runs really great. I plan to upload one more log once it's driven some distance. I wanna see if you're right robertisaar about the TPS resting to 0% after some driving. As for what must have happened, my guess is that one of the rocker arms wasn't positioned properly and it wasn't opening up a valve. Either that or the valve wasn't opening by much. Must have been an exhaust valve, as the pressure would have backed up through the intake, which cause the map to detect air, which caused this mess to begin with.

As for why the ECM stopped working, I guess there's a reason why I never used that old PROM chip. To the trash can it goes. Too bad, that was a custom performance chip tuned for 91 octane and removed the speed limiter. I have some pics of the new ECM, and it's 10x better then anything A1 Cardone rebuilds. I didn't even need to use thermal paste, cause they used a thermal pad instead. I'm not messing with that.