Hey everybody. New to the forum and am looking for some help. I have an '01 Monte Carlo with the 3.4L and about 80k on the odometer. It has an intermittent random multiple misfire, P0300, only at idle and I can't for the life of me figure out what's wrong. The strange thing is that it seems to come and go and doesn't seem to happen at any particular time except randomly at idle. Here's what I've changed so far, plugs, wires, all 3 coils, ignition control module, catalytic converter, intake manifold gaskets (upper plenum and lower), cleaned the injectors and checked the resistance of all of them, cleaned the egr valve, did a compression check (ranges from 155 to 175), and probably some other stuff that's not coming to me now.

Any help would be greatly appreciated. Thanks fellas

 

All right... hate to ask the stupid questions, but here goes. You sound like you're a pretty good troubleshooter yourself though.

Your'e sure it is misfiring and not just the computer randomly reporting? Only thing left is that PCM that I can think of. There are some people here a little better informed on the sort of thing than myself. Somehow or another, the timing is off. That's about all I can come up with. Whether that is something the computer has screwed up or what, I don't know, but I really think the timing is off.

 

It is most definitely misfiring. The idle is very rough and the tach is all over the place. Usually, as soon as you give it some gas it smooths right out and stops misfiring but every once in a while it won't go over 1,500 rpm's and just stumbles, coughs and farts and usually stalls. I spent all day today replacing the intake manifold gaskets because I was told they were known to leak. I got it all back together, took it for a 30 minute test drive and all seemed good. A few hours later, I started it back up and sure enough, it was idling rough again and the check engine light came on. I can't seem to narrow it down to any particular operating condition or temperature. My butt is thoroughly kicked from this.

 

I'm usually pretty good with the 3100/3400 engine. You've done a wealth of stuff to it that is definately not bad (just not a good feeling when it doesn't solve your problem). Here's some questions and thoughts:
1. - Did you recently do any service to the car right before this problem began?
2. - What brand plugs did you put in? AC Delcos are the best. If you put BOSCH plugs in, change them NOW. I've had enough personal experience with BOSCH plugs, I'm never using those again. Heck, those could cause this problem (even brand new). BOCSH makes a lot of good stuff, plugs for GM v6's, not not so good. Sadly, coils are rarely the problem (I have coils on a '94 Grand Am with 210,000 miles and two of them are still original, GM used the same coil part number on the 3100/3400/3800 N/A).
3. - Have you checked for a vacuum leak?
4. - Have you changed the fuel filter? Air filter?
5. - I know on the '94-'95 3100 v6, the tube that is part of the upper intake that goes to where the EGR sits would plug up with crap. I know that tube still exists, I'm not sure it's as problematic as it was in '94-'95, as GM changed the EGR, but did you clean that while you had the top end dis-assembled?
6. - When you put the car back together, did you replace the O-rings on the injectors?

Since it's a random mis-fire, we need to look at things that effect EACH cylinder. Not getting another air in, not enough fuel to the rail, or the coils not firing right (which that should be addressed by the fact you changed the coils and ICM).

Hope some of what I suggested helps.

OH and welcome to the forum

 

BTW - I looked up the generic info to troubleshoot this code. Here's the link:
OBD-II Trouble Code: P0300 Random/Multiple Cylinder Misfire Detected
And it suggests the following causes:

A code P0300 may mean that one or more of the following has happened:
- Faulty spark plugs or wires
- Faulty coil (pack)
- Faulty oxygen sensor(s)
- Faulty fuel injector(s)
- Burned exhaust valve
- Faulty catalytic converter(s)
- Stuck/blocked EGR valve / passages
- Faulty camshaft position sensor
- Defective computer

You did plugs and wires (and so long as they are not BOSCH plugs, you're good there). You also did coils and ICM and CAT.
I forgot to suggest an upstream O2, the one on the rear exhaust manifold (I believe it's recommended to change that about 70,000 miles, as it wears, it's readings become less accurate and can effect fuel economy, if you change it, stay with Delphi or AC Delco).

They also suggest a faulty camshaft position sensor. My experience is if it's that sensor, it trips a different code. I'd save that for one of the last things to look at.

 

^^^^^^

That would be were I'd focus my attention if it hasn't already been addressed. Good luck - hope you get it figured out.

 

Thanks for all the responses, much appreciated. I'll go in order here.

This started happening seemingly out of nowhere. It didn't have anything serviced except the PCV valve was replaced but it started happening some time after that so I don't think they are related. Plus, I double checked to confirm it was the correct p/n.

I also own an '06 Trailblazer I and know how picky they are with spark plugs. Anything but AC Delcos and it's a gamble so I made sure this got Delcos as well.

I have checked extensively for vacuum leaks using a propane bottle and brake clean and can't find anything. I've even induced vacuum leaks to see if it would start misfiring and it doesn't . It truly has a mind of it's own. All the intake manifold gaskets appeared to be sealing good as well. They didn't show any signs of leaking.

The fuel filter is one of the first things I replaced (forgot to mention that) and I checked the pressure at the rail too. It was about 2-3psi below spec but I think that is a negligible difference. The air filter is squeaky clean as well.

I read somewhere about the EGR tube clogging up so I checked that the other day. I didn't completely remove it but from the top end, I can't see anything obstructing it.

It was all reassembled with new o-rings on the top and bottom of the injectors. I removed each one from the rail to inspect for blocking......found nothing.

 

It's broken. I found a guy in NAPA a month or so back talking about a '94 Grand Am, he didn't mention the engine, but eh said it would run about 45 seconds, then start misfiring, and wouldn't drive right at all. Every now and then though, you would get lucky. It ended up being his PCM if I recall correctly. I know they're expensive, but what else is left to check? Have you pulled the plugs to see if one of them has fouled? If it wasn't, it is now.

 

Nope, no fouled plugs. They all had normal wear.

 

In the 3100 world the ONLY way you might see the EGR blockage is by removing the throttle body, most of the time there is a clump of crud sitting in that end of the passage way. But it's also possible the blockage is a tiny clump stuck somewhere in the passage that you won't see.

The way I've used to clear it was remove the EGR, using carb clean, spray that in the cast aluminum passage going to the upper intake. Use a large length of something easily flexible (I use about a 2ft section of old speedometer cable). Work that cable in and around, twisting and pushing until enough of it has gone tin that you KNOW it made it to the other side of the passage (I like to then spray more carb clean and run the cable back and forth).

It can be a real pain the first time you've done it (the cable I use sometimes won't go right through).

I just looked at a GM shop book for a 3400, if your crankshaft sensor was changed, they recommend a re-learn process. But not the case here (and requires a scan tool).
Looking at the rest of what's in the list on that manual that you have not done is:
Loose/Bad ground connection
Clean EGR Passage
MAF Sensor (I would start with the MAF sensor cleaner)
Crankshaft Position Sensor
PCM