Since about a week ago, my car has developed a misfire. It doesn't misfire immediately after starting, but it takes a while for the engine to warm up -- maybe 10-20 minutes. After that it starts misfiring and then worsens little by little. If you wait long enough, the Service Engine Soon (SES) light comes on. At that exact moment when the light comes on, both cooling fans kick on high, which I thought was strange. When this happens, the temperature gauge reads slightly over 160 -- cooling doesn't seem to be an issue.

I attempted to get my car scanned after it through a code. However, I ran into a problem -- my PCM is a Madz28 OBD2 to OBD1 conversion -- basically I'm running an OBD1 PCM with the OBD2 connection inside the car. When I tried to have it scanned, it said the link failed.

After this, I stuck the original, OBD2 un-tuned PCM in and let the problem repeat itself, except the fans didn't come on. I had it scanned and it told me the O2 sensors were bad. But then I remembered that I'm missing rear O2 sensors due to my headers, so the stock PCM would throw that code anyway. So when I had the stock PCM in, I don't believe it ever threw the code for the misfire since the cooling fans never kicked on. (Maybe the cooling fan part could be related to having an OBD1 PCM? Maybe not?)

Does anyone have any idea what my problem would/could be? I had the injectors cleaned/new plugs/new wires/new optispark about 30,000 miles ago, so it would suprise me if any of those were the culprit. I've already looked at the wires, all of them seem fine.

I'm stumped for the moment... looking for some good advice on what to do next...

Thanks for any help in advance.

 

Have the icm tested mabey?

 

Were you using a scan tool that is compatable with OBDI? My 95 is OBDI with an OBDII style connector (came from GM that way). You fans are coming on because the SES code also sets the computer into limp mode; limp mode sets everything to "conservate" settings to protect the engine (backs out timing, turns fans on) until it's repaired. Reading the SES code is you easiest way to get to the root cause. Sounds like it could be heat related; as posted above the IC Module is a possible root cause.

 

The first time I tried to scan it (with the OBD1 PCM, OBD2 connector), the link failed due to the scanner being OBD2. I'm guessing if I tell them I have a 95 Z28 they will use an OBD1 scanner with the OBD2 adapter and then I can tell what is wrong with it while I have the OBD1 PCM inside? That would eliminate my un-tuned OBD2 from falsely telling me the rear O2s are bad (since they are not there due to my headers/no cat).

I'm going to change out the O2 tonight and see if it happens again... then I will just tell them I have a 95 and get it scanned that way.

Haven't thought about it being the ICM -- last time I had a misfire (and it ended up being the optispark), we changed out the ICM and coil but neither were the cause. I'll post later tonight letting you guys know what's going on so you can continue helping me. Thanks so much for the advice thus far.

 

Update: Put in another passenger side 02. The problem only seemed worse so I put the old one back in. It didn't help it any -- it was backfiring terribly. So then I was going with the backup plan of putting in the OBD1 PCM and taking it to AutoZone to get it scanned. They had no way of scanning an OBD1 with an OBD2 connector inside, OBE1 95Z28. How have you had yours scanned before?

I'm leaning towards ICM or optispark.... optispark... ugh.

 

So I finally got a cable and was able to scan my car -- still saying the right O2 sensor is lean. I checked the voltage and it is a constant .004 volts. Replacing the O2 sensor did not fix the problem. Is it some wiring issue somewhere or could it still be the optispark?

 

Sounds like you have an O2 wire burned up on your headers.

 

Sounds like there is a short to ground in the wiring. If there was an open circuit, the reading would be a constant 450mV.

 

Yeah, that's what I was thinking. I was looking at a couple sets of wires running by the headers -- the ABS one looked a little burned but the O2 set didn't look so bad. How hard is this going to be to fix? I'm taking it to the shop on Monday anyway to get my cam kit installed -- went with the hotcam kit. I was thinking it was the optispark so I ordered a $600 DynaSpark... hope they'll take it back...