Ok guys. I need a little help.

Background.

Few months back, started having a slight stumble at 2k under light throttle and load. I'd hit the throttle, and it would clear up. Scanned it and had misfires on cyl 2 and 4. Not too much later, I got caught in a late afternoon T-storm and the car died. Thought it was the opti, and I needed to do the plugs/wires anyway. Completed those. Cylinder misfires were almost zero, but some occasionally on 4 (no stumble) Started blowing ignition fuses (10A). Took a while, but I found that the wiring harness down the passenger strut tower was burnt and melted from the long tubes. Replaced ICM, CKP, O2's, and coil in the process of elimination. Occasional stumble. Finished repairing the wiring harness with high temp wire (fiberglass insulation 875 degree). Drove the car and it stumbles badly (Short term fuel trims are in the -20 to -30% range). When it gets around 2600 or so, it takes off like a BOOH. Fuel trims drop to 0% (not WOT either..only maybe 1/2 throttle). As soon as the rpms drop back below 2600, it starts missing again, and fuel trims shoot back up trying to remove fuel. Scan shows it has a misfire on cyl #4. Pass. side O2 was working fine, but now is sluggish. Staying in the 400-500mV range. I'm not so sure the misfire is not causing that (does it on a warm or cold restart in OL).
I have checked every sensor (MAP, TPS), cleaned IAC, and replaced the fuel injectors (almost 200K on them anyway). I've tested the injector harnesses (ok). I have spark on plug #4.

To recap, I've changed everything in the ignition system, new fuel injectors, new opti (Delco), new ICM, new CKP, new coil, and new plugs/wires. I've checked everything I know to check as far as wires (no damage, resistance oK), spark (good spark, no arcing), fuel (injector harness ok). What else would cause one cylinder to misfire? All the other cylinders look ok. OL idle is somewhat rough, and IAC counts are close to 100 to compensate for the misfire. CL idle is rough (fuel trims same as above). Basically, nothing I've done has helped. The engine will run all day, just stumble badly until I hit 26/2700.


At this point, I'm wondering if it is something mechanical. I have about 120K on the lifters/rockers/springs. Could I have a bad lifter that will not pump up until the oil pressure gets high enough, but not bad enough to tick? I have S/A rockers (noisy), so it can be hard to detect a bad lifter. I don't want to tear into it and not find anything. I'm at a loss. What am I missing?

Fred? Shbox? Any ideas?

 

plug could of got messed up when you installed it or you could of hit it while putting it in and changed the gap...either way take plug back out and check it...

 

I wish it were that simple. I swapped plugs w/ cylinder 3 to see if the problem would follow. I didn't damage or hit it. Plugs have less than 500 miles on them.

I had thought today that my harness repair may have messed up the O2 voltages. But it still misfires in OL when O2's don't matter..

 

O2's do matter in open loop, because its using the long term fuel corrections it learned in closed loop. If the long terms are screwed up due to bad O2 sensor info in closed loop, its going to miss all the time.

 

The Long Term trims should be zero since I had to disconnect the battery when repairing the harness. I only drove it 40 miles after that (ran like crap). Since then, I pulled the PCM fuse to see if I saw anything different on the scan data (same readings and misfire).

Any other things I missed?

 

I too had a problem with the melted wires on the passenger side shock tower. If I remember correctly, in addition to the O2 wiring in that bundle, the starter wire and the knock sensor wires are in there as well.

When an O2 is unplugged on the wire has a break, the O2 signal will default if you will to bias voltage around 450. Sounds like your problem. One quick and easy test is to unplug the O2 and short signal to ground wire and see if the PCM sees it near 0. If not, you know the wiring is a problem.

Also disconnecting an O2 will force it into open loop. See how it runs then.

More ideas.

Don't discount a compression problem. Check that bad cylinder compared to another cylinder. A burned valve can give you a problem.

I didn't see whether you had your EGR hooked up. A sticking EGR could cause idle problems but appear to go away at higher speed.

 

That's correct, CKP sensor, both O2's, Starter wire, knock sensor, and ground wire.

You may think I'm crazy, but as a last ditch effort to avoid pulling the valve covers and intake, I seafoamed the engine last night. I can't say 100% that it is what improved the situation, but it did get rid of the constant misfire on cylinder 4. I still think it was a sticking lifter. Car runs relatively smooth, but fuel trims and IAC counts are still out of wack.
At least I am back to a situation that I can address. I'm getting misfires on all 4 passenger side cylinders, which I know is sensor/harness related. I'm getting much fewer misfires than I had before. One odd thing is that it is only misfiring at or near idle.

My pass. O2 voltage is in the 350-450 mV range and sluggish. The short term fuel trim is at 50% (before the seafoam is was -30-40%) I"m going to first swap the R and L O2's to verify the sensor is ok, then I'll start looking at the harness more.

What's an EGR valve? I don't seem to have one on my car

 

I kept fouling plugs on the passenger side. Bad O2 and I'm swapping the opti this weekend. I will post results.

 

I think I have everything sorted out now...FINALLY. I'll post the process of figuring it out just for anyone else who is in a similar situation.

Original problem was burnt wiring harness on pass. side shock tower. That shorted out 1 O2 sensor, causing slight stumble and misfires on bank 2 (more prominent on cyl 4 and 8. Replaced O2 after looking at O2 voltages from PCM. Burnt wires started grounding out ignition during heaving rain, and blew ignition fuses. Found bad plug wire (#8) and went ahead and replaced opti, plugs, and wires. Thought it was fixed (couldn't see harness damage). Started blowing more ignition fuses. Replaced ICM, coil and CKP sensor b/c they are only things connected to 10A ignition fuse). Finally found damaged harness after blowing 20 A fuse. Harness was burned by header, but did not contact header. Repaired harness using high temp 18ga fiberglass insulated wire. After repair, severe stumble misfire. Looking at Autotap, only cyl 4 would misfire. Bank 2 fuel trims were -30%. Thought injector #4 might be bad, so replaced them. Cyl 4 had a sticky lifter, and I corrected it using Seaform as a last ditch effort (thank goodness...I didn't want to pull the intake). After Seafoam, stumble much improved, but still had slight stumble. Autotap showed misfires on cyls 2,4,6 and 8. Short term fuel trims were +20-30%. Bad pass. O2. The last short circuit also toasted the O2 wires going into the sensor. I replaced with old sensor I had. IAC counts are still high (90-100). Engine bogs when A/C comes on. Bad IAC.

Now I need to order new O2 and IAC. Car is driveable, but bad IAC takes some of the low end away. Nightmare is almost over. If anyone has any questions as to why I did what...feel free to ask.