I usually dont have to ask questions about engine problems but this one has me puzzled. My 94 Z28 has developed a problem where I can drive it till it just starts to get to maximum operating tempurature and then it starts to misfire and then it will die. It wont restart after that. Fuel pressure was checked before and during the time that this occurs and it is fine. I got a chance to check for spark while it was not starting and there was no spark from a spark plug wire and nothing from the coil wire..... however when I got to that one it began to spark shortly there after and would run again. I figure one sensor is getting hot and then this happens. Things that have been replaced now and recently: Opti, plugs, wires, coil and ignition module. Another note is I unplugged the MAF when it wouldn't start and it made no difference. I'm hoping that one of you might have some answer to this, thanks.

 

How about some history to the problem. When it occured. What was changed as a result of the problem. Of the parts you changed, which ones where changed before the problem occured. What you checked thus far that you haven't told us about.

 

Might check the O2 sensors; the car runs in open loop when cold and switches to closed loop (fuel trim decided based on O2 readings) when at operating temp.

 

I know that you might not want to hear this, but it sounds like your OPTI-CRAP.

 

This problem started about three weeks ago and the only parts replaced since it started are the coil and the ignition module. I replaced those because of no spark out of the coil and have heard of the hot-soak problem with the ignition module. The opti and other parts were replaced about seven months ago. OBE1: I would doubt that it is the 02 sensors because it would still run, just not right..... it won't start due to no spark to the plugs and that would mean that it isn't in closed loop anyways. Hammersz28: I truly hope that you aren't right but that has crossed my mind as well. To All: No SES light till after it starts to misfire and I have no way to read them because of no scan tool.

 

okay, so it seems that this is again the opti. can't remember where I bought the last one, was somewhere back east. some guy that sells stuff through his house, genuine gm parts that I found through here. any help would be nice.

 

Have you checked for the presence of the IC signal(1-4 VAC) on the white wire going to the ICM when the engine is cranking?

How about the presence of 12 volts at the three wires on the coil with KOEO.

 

speedygonzales: I finally had time to check that since it is now my two days off from work. The IC signal does in fact disappear when it dies and isn't present while cranking after it dies and won't start. There is 12V at the three wires on the coil, so it seems that does in fact point to the opti, am I not correct in that thinking? Looks like I will be contacting DAL again for another opti :-(. Good for DAL, Bad for me.

 

Wo hold on there buddy. Just because the IC signal drops out, doesn't always mean the opti is the bad guy. For the opti to cause the IC signal to drop, it would have to drop the feedback signals. And if that were the case, it would throw codes for that. So are you consistantly getting a DTC 16 or 36?

Why don't you monitor the feedback signals from the opti when the problem occurs to see what they are doing. If they stay but the IC signal goes, the problem is the PCM

 

Okay, so there are four wires coming from the opti to the PCM. Don't know about the codes, don't have a code reader and if the problem happens while driving it it will turn on the SES light after the first misfire until it dies and won't restart. If the problem doesn't misfire on the engine but won't start after a hot soak the light does not illuminate, of course, I'm not sure it would since it never starts up. So that brings me to which signal should I be looking at? Distributor Reference Low Signal, High Resolution Signal or Low Resolution Signal.... I'm assuming that the Distributor Ignition Feed is a 5V reference. Which wire(s) should I be looking at and what kind of signal are they? No osilliscope here either so no square wave patterns can be looked at. Frequency I can do as well as AC or DC voltages. Hope your still willing to help me out there, I appreciate it a lot.

 

I would focus on looking at the low res signal. The signal will be a 5 volt square wave. So like you said, you wouldn't be able to see it. But you can use the frequency portion of the meter. If it is a hot problem. You can start cold and monitor the frequency between the red/black and the pink/black wires going to the opti. Once it's hot, if it is the problem, you should see the signal start to falter right before the engine starts running crappy and then it will shut off.

That isn't the cause of the SES light but I suspect the hi res is dropping out also. The hi res sets the SES light. The low res will not but the loss of the low res will cut out the fuel pump and the injectors.

Let us know what you find.

 

Okay, checked those wires and the Hz on them. Here goes as to what happened. I checked the Hz at cranking, 1000 RPM and at 2500 RPM. Cranking it was 7.00 - 10-70 Hz, 1000 RPM was 53.65 - 55.00 Hz, and 2500 was around 160.00 Hz. I kept it at 2500 so it would warm up faster and I wouldn't have to be out in the cold so long. When the engine misfired and died it the Hz started to be erratic and then dropped to zero. Of course the engine died. Cranked the engine and it remained at zero. So no signal on the low signal coming from the opti, still sounds like the opti to me.
So that is what I found Speedy, got any further testing you think should be done before I order and replace the opti again?
Thanks for the help BTW, appreciate this site and you a lot.

 

The only other thing to confirm is the reference voltage supplied to the opti from the PCM is staying or going. If the reference from the PCM is dropping out then obviously the feedback signals will as well. It is 12 volts on the red wire. Check it from the red wire to the pink/black wire. If it is still there when it craps out, the problem is definately the opti. If it drops out when the engine runs crappy, it's the PCM.

Do you have the receipt for the opti you put in last if it's not that old? If you do the last test to confirm it's bad, maybe you can get a replacement at no cost. But then it doesn't matter, PM me and I'll tell you how to get your money back regardless.

 

The reference voltage does not crap out from the PCM to the opti. speedygonzales you have a PM.

 

try the crank trigger sensor if it has one