Hello all. I've got a '97 z28/6 speed that developed a really nasty misfire the other day. It's down on power, eating more gas, and the SES light occasionally flashes at me.

When the engine is dead cold it seems to run ok, the warmer it gets, the grumpier it gets.

It has headers (already checked for exhaust leaks), no rear 02's, no cats, no egr, no AIR system. I bought the car in March and it has ran great up until the other day with all of that done to it.

Front o2 sensors are newer AC Delco units. Plug wires are MSD, no arcing, and none are burned through. Plugs are newer double plats and gapped correctly.

Replaced the coil because it was the original, no change.

Rented a fuel pressure test gauge. Got 43 psi key on/engine off, 40 psi with engine idling, 45 psi at WOT. Changed the fuel filter just to be safe.

Cleaned the MAF too just to be safe, also no change.

The opti was supposedly changed before I bought the car. I don't know what brand it is.

If anyone has any ideas on what it could be or what else to test please let me know. I don't have the money to start randomly throwing parts at it so that option is out.

 

If you are just getting the random misfire code it most likely caused by a component that can affect all cylinders. Check the ignition first: ICM, coil, opti. If the ignition is good, then the most likely suspect is a lean condition. From your post the fuel pressure, and O2 sensors are OK, so that leaves the MAF, or bad injectors. Unplug the MAF and run it with it disconnected, this will not cause any damage, but will force the PCM into open loop, and bring up the MIL. If the engine stops misfiring with the MAF unplugged, it is an indication that one or more sensors are sending bad data to the PCM. If that is the case, use your scanner to verify that what the PCM sees from the sensors makes sense.

 

Thanks for the info. I'll try unhooking the MAF and see if there's any change when I get home.

The coil has already been changed, and I tried a working ICM from my friends '96 Camaro. No change with either of those. (His also ran fine with mine installed)

Is there any diagnostic test for the opti? I've looked all over and can't find one. And given how much genuine ac delco/delphi units cost I can't afford to replace it if it doesn't fix the problem.

The scanner I'm using is a cheap ELM based usb unit I got from amazon. It reads and clears codes fine, but I've never been able to get the live data logging to work, the readings for All the sensors bounce all over the place. And I'm not sure if it's the scanner or the PCM that's doing it.

 

Had the same issue with my S10. Started using non-ethanol 91 octane, problem solved.

 

Just took it for a spin with the MAF unplugged. Soon as the engine got warm it started missing like crazy again. Shaking and shuddering and i swear i can smell raw gas coming out of the exhaust.

 

maybe you have a leaky injector?

 

I left the gauge on it for about 20 minutes with the car turned off and it was still holding pressure. If they were leaky it should have all bled off, right?

 

I am having 100% of the same symptoms. how did you fix the random misfires?

 

I never did. After almost a year of seemingly no one one the planet being able to figure out what was wrong with it I sold it. It was the 2nd LT1 powered car I've owned and they were both nothing but trouble.

 

Have you tried some of the routine maintenance stuff like was ruled out on the original post?

Mine disappeared with a simple plug, wire and cap and rotor change. I also changed the coil but I don't think that was the culprit but I also am not a big fan of OEM coils. During removal I inspected the parts I removed and I noticed that due to the condition of the cap and rotor I don't think a simple plug and wire change would have rectified the situation. The original poster did not do anything with the OPTI from the posts so they could have had a similar situation I had.

Obviously everyone's mileage will differ.