Ok, here's the deal...

My car runs luike crap once it warms up, I get a misfire code 300. So far I've replaced:

Spark plugs
Wires
Water Pump
Optispark
Ignition Coil
ICM
Front O2's

My mechanic can't figure it out, it all started after about the 5th nitrous run, I hit the juice on a nice deserted backroad and there is a "pop" from under the hood and the car shuts off. Great, what did I do? Turned out the fuses to "Gages" and "IP Dimmer" was burned, so I replaced it and away I went.

The next day the car was very hard to start, I had preignition and it coughed and sputtered, but fights pretty hard. Cranks fine though. I bypass the MSD, same problem. Replace the coil again, same problem. It started eventually, but it took 3-4 tries. It has plenty of fuel and 40PSI at idle.

I replaced the O2's and Opti and it got a little better, starts fine now, but still has a code 300, random misfire. What else could cause this???

It starts fine, runs ok when cold, but once it warms up it gets really rough. I've replaced every piece of ignition system, bypassed the MSD, and still no joy. My merchanic had no clue what else it could be.

Can anyone think of what this could be? It's at the dealers now, but I am on a tight budget and need to know what else it could be.

Thanks,
Chris

 

Injectors?

 

I dunno, driving to the mechanic for the Opti swap it felt like it was running on 3 cylinders...knocking BAD and even sounded like it was hitting the hood! I was surprised I made it there, it could barely go uphill.

I don't have any injector codes, but the dealer can test the injectors for me if they suspect them, right?

 

Could be that you blew an injector fuse too?

 

If you had a N2O backfire you could have killed the MAF. It could also cause some of the injectors to lock. You should check them.

i would also check for compression on all cylinders because you could have blown a hole in a piston. It's a cheap test and will save you from wasting time and money if you check it first.

 

i bent a pushrod and kinda did the same thing till the pushrod came out of the hole and the cylinder just stoped firing.

 

If I killed the MAF though, would it throw codes or not? I checked all the fuses in the dash and under the hood, all good so far. I was thinking it shouldn't be a holed slug or anything mechanical, as it runs pretty good when it's cold, but I could be wrong. I also don't hear any knocking or clunking when the hood is up, so that's encouraging at least.

Never thought about a bent pushrod though, would that happen from hitting a rev limiter at 6000 rpms?

Just scared that the easy stuff has all been addressed, and it's gonna be something scary this time.

Crap.

 

either way to partially knock it off ur list; see if there is any obvious physical damage to the MAF. Its easy enough.

 

Make sure there aren't any holes in the bellows behind the MAF allowing unmetered air into the system. Check your intake and head gaskets.

Pop=Bad sound.

 

If you are getting random misfire code only I think I would do a compression test sounds like you may have hurt a couple cylinders. Also like other posts recommend maybe some injector damage. Good Luck!

 

If the MAF were bad then just unplug it. I think the PCM will revert to running on internal data tables. As far as internal damage, perhaps a leak down test will tell you what happened. Unlike a compression test it will tell you WHERE your loosing compression from. But like a compression test, It isn't fun to do since you have to go through the plug holes.

 

Well, I carefully cleaned the MAF with alcohol before it went to the dealer, all the wires appeared to be intact. Was pretty dirty too.

I did run the car with the MAF disconnected, unlike my 5.0 it not only ran, but ran pretty well! I didn't drive it though, just let it idle in the driveway, so I don't know how the power output was affected.

Yeah, I don't really have room for my car in the garage to do the compression check, but I do have the gauge and tools to do it. If the dealer doesn't find anything else wrong I'll probably take it to a cheaper mechanic to have the leakdown/compression test done.

I had a bad intake air sensor on my Mustang that flooded the motor with fuel once, a different (read: fired) mechanic did a compression check and found no compression in any cylinder, and told me the motor was bad!! I swapped out the sensor, changed the oil, and was good to go with 145 psi all around. I wonder if something like that is happening now.

The dealer quoted me one hour diagnostics, so at least I won't get screwed from approving a thousand dollar estimate or anything. I just want to know what's wrong so I can fix it myself. I'm sure they'll check the injectors, but they're scheduled to work on it tomorrow, so we'll see.

 

A guy on one of our truck board ran nitrous in his vortec headed tahoe. He got a misfire on one cylinder after his nitrous and excessive timing caused a valve to chip/crack and not seal properly.

 

Yeah, I thought about that, but the Nitrous Mastermind was pulling something like 10-12* of timing, and was jetted pretty rich so I'm not too worried it burned something....

I dunno, anything is possible. I'd think a hurt piston or valve would result in an individual misfire code, not the generic code 300. Wouldn't it run poorly all the time (not just hot or cold) if it were something mechanical?

I am starting to wonder about the temp sensor begin bad, could it be faulty and not throw a code? It might help explain the better cold performance, and if it's running as rich as it seems to be, the poor performance and misfires when warm.

Thoughts?