Ok fellas, I'm at my witts end. Car runs amazing for 15 minutes and then bam grows a miss fire. So far I've replaced the 02 sensors, icm, coil, plugs, and opti. Unplugged the maf when it started doing it to confirm that it's doing it in closed loop too. I can't figure it out. I was really hoping that the opti which I changed last would fix it. Any help would be awesome.

 

bad injector? is it a bad misfire? disconnect each inj..until you find which bastard is misfiring.

 

Yea it's a bad miss fire that only happens once the car gets hot. Like I said after 15-20 minutes of driving.

 

I had this happen and found out the water pump would only leak on the opti once pressure was built in the cooling system. It would run great until operating temperature and then start to stumble.

 

when u disconnect the injectors one at a time the misfire should get worst.(2 misfires) whichever cyl does not change is likely to be the problematic one.

 

Check the resistance values on the coolant temp sensor in the water pump yet? See if it's where it's supposed to be cold/ ambient and check it again when it starts misfiring. You should be able to do it in the driveway. Stuff gets hot of course- it's easier to get at it from below when it's warm (ramps). See if the value it has when it's warm is where it should be for the range of 180-215 degrees. Resistance value table here:

http://shbox.com/1/4th_gen_tech2.html#ect

Something to eliminate that it doesn't sound like you've checked yet...

 

I notice you didn't mention replacing plug wires...did you? It's pretty easy to burn them on the headers causing misfires if you don't have the over the valve cover wire conversion.

 

Coolant temp sensor is new and yes changed plug wires.

 

In that case I change my vote to the dreaded OPTI! I had a misfire that would not go away at 5200 RPM until I got a MSD opti.

 

What happens when you unplug the MAF is the car will die if it's running. But it will still go into closed loop while running speed density. Only way the car will go open loop after it's warmed up is if an O2 sensor is completely dead or unplugged. If you want to check if it could be an O2, either hook up a scanner and check the sensors switching or crawl under and unplug them, one at a time. If you unplug one O2 sensor and it smooths out, check the connections for any corrosion, breaks in the wires, and finally swap the O2 sensor.

Also, what brand O2s did you swap into it? Bosch are sheet.

 

OP

I would REALLY check for any arching at plug boots. It can be hard to see and you may need to look from below also while car is running. Even though you replaced plugs/wires one may not be on all the way or a plug boot is arching to exhaust. or arching between heat shields over boots (if you still have the OEM ones on).

You really need to use dialectic grease on both ends of plug wires

I chased a misfire and replaced everything you did only to discover a plug boot arching with a heat shield boot cover. Could not see it from up top even in dark while spraying wires with water mist.

It certainly could be another cause but REALLY check for arching from wires.

 

Fuel pressure is good. Rechecked for arcing plug wires and came up with nothing. It's really driving me crazy that it only does it when it's hot i.e 15-20 minutes of driving. Until then it runs awesome and then the miss has a slow onset until it's just down a cylinder. I've found from searching that numerous people have had this problem but nobody on any site ever posts the fix.

 

I did notice that when it starts missing my fuel trims go from left 128 right 128 to 120 126. I don't know. I'm about ready to just get rid of this thing. It's driving me batty.

 

Did u try to narrow down the mis to its possible cylinder by disconnecting inj. electrical?

Did u replace the distributor cap beyond just the opti unit alone?

Who's plug wires did you choose?

 

Have you checked or changed the ignition control module that will act up after it gets warm very common

 

So I want to be a guy that finishes up and posts fixes to elusive problems. My buddy called his guy at gm and they suggested the PCM. I figured what could it hurt and bought one for $100 and whamo we seem to have a fix. Going to drive it around on a good long cruise and make sure but drive for about 30 minutes tonight and nothing. Runs strong.

 

That's pretty weird....how many times was it re-tuned?

 

Oh, I see your PCM was "Tuned by GregRob".....lol, he must have done it!

 

[QUOTE=ahritchie;16510360]Oh, I see your PCM was "Tuned by GregRob".....lol, he must have done it!

Lol. Maybe he sabotaged me.

 

Seemed wierd to me too but it's been two days now and no miss fire. It had been tuned 4 times.