Here's the problem with my stock 1988 IROC. It has a a Hypertech Thermomaster chip in it and that's the only modification to it. Anyway, the SES light goes on and off all the time. And it's not flashing as if it were in "field service mode" to pull the trouble codes out of it.

I was driving in traffic when this happened. The engine idle was going as low as 500 rpm but did not die. I drove home, tried to pull the codes but it wouldn't let me.

With the key ON engine OFF, the fan was already on after shut down AND when the engine was cold. I tried to pull the codes with a jumper wire, but the codes would not flash on the SES light. Also, the SES light would stay on then turn off periodcally with the key ON egine OFF and when the engine was running WITHOUT the jumper wire in teh ALDL connector..

I disconnected the battery to dump the computer's memory. Even worse. The car started very nice and everything seemed fine until it went into closed loop. When the engine went into closed loop, the fan immediately went on (engine was barely warm) and the engine idle went down to 550 RPM and it was noticeable. No rough idle, just and idle where you know something's not right.

And, after I shut the car down and tried to restart it, it would crank but no starting. In the engine's present state, i have to give it throttle like starting a carbureted car in order for it to fire. I can hear the fuel pump working and the engine stays running when I get it started and stays running until i shut the engine off.

What's happening? I even tried disconnecting the MAF sensor since it was always the source of drivability problems in the past. My MAF sensor was just replaced 7 months ago with a Bosch made unit. Usually, when you disconnect a sensor, the engine runs fine. The engine runs poorly with the sensor(s) diconnected. Thus, telling me the senors may not be at fault.

A friend of mine feels that things are pointing to a faulty engine computer. Anyone had a similar experience? Thank you.

 

BB,

You'll probably need to access the ECM. Since you've already been there to replace the EPROM, you know how to do it. Once you have the ECM exposed, disconnect the battery as a precaution. Carefully unlatch and unplug each of the wiring harness connectors, noting their positions and orientations before removng them. Inspect the connection pins/clips, and the mating connections on the ECM mainboard. Clean anything that appears to be questionable. Reseat each of the connectors, unplug, then reseat them again. This unplugging and reconnection can clean any marginal contact areas by its wiping/scraping action.

Once the connectors are fully reseated and latched, check the ground connection to the ECM case with an ohmmeter to a good known chassis ground. There should be zero resistance. If you have an ohm or two of resistance that can't be attributed to the meter leads, start looking at all chassis grounds. Once you have verified a good ground, reconnect the battery and try it again.

Another thing to check is the MIL lamp flash when the ignition is first turned on. When powering up the ECM, the MIL (SES lamp) should briefly light, go off, then light again until the engine is started. This flash indicates that the ECM has passed some rudimentary on-board self diagnostics, an I/O scan, and EPORM checksum test. If you don't get this flash, there is a problem with the ECM, its connections, or the EPROM.

 

I have taken the Hypertech PROM out and replaced it with the original stock chip. That seemed to cure the problem.

Nope. Problem is still there. When the engine is cold and in open loop, it runs fine. As soon as it goes into closed loop, that's when the symptoms return.

I haven't actually cleaned out the pins on the ECM nor have used a test light or checked the grounds. I will give it a try.

For some strange reason, my CD changer doesn't work either. Coincidence? Or maybe it's the bad ground? I'll check it out.

Thank you Vader.

 

Well, after troubleshooting, it seems that I have bad ECM. I can't find no blown fuses, broken wires, or bad grounds. We'll see what happens.

 

I found the problem for those who may want to know, my IROC had a bad computer. I bought a NAPA rebuilt unit and plugged it in. Car runs better than when it was running normal before the problems arose.

 

Thanks for the update. It's always nice to know how these turn out and what the causes are. Hope that it doesn't return, or you'll really be digging into the spaghetti.

 

What a difference a new computer can make. I didn't drag race it on the track, but on the road. Wow. Of course, my car still has to "learn" my driving style but man, it just feels stronger when I floor it. I just hope I won't be disappointed on a drag strip.