Any ideas what could cause this? It seems to only happen when starting from a stop, when the engine is under low rpm load, when it is using low end torque, it will sometimes stumble and almost die then get back up.

It seems to do it less or even stop doing it entirely once the engine has been running for 15 minutes or so. Its worst first thing in the morning.

Please help! There are no trouble codes either.

Update: it stumbled or misfired or whatever once or twice today while idling, so I guess it's not only at takeoff <img src="/forums/images/graemlins/zombie.gif" alt="" />

I've been mulling over your situation. But it's hard to diagnose without a code. If you have a code reader and haven't hooked it up, do so and check the pending codes to see if something is tripping the ECU, but not consistent enough trigger the CEL. If you don't have a reader, you can take it to ZombieZone or O'Really's and get it checked for free. <img src="/forums/images/graemlins/notooth.gif" alt="" />

I would suspect that something inconsistent is happening... could be a starting-to-fail sensor (O2 or something in the fuel/emissions circuit), a poor connection somewhere in that system (the wires are notorious for corroding at the base of the connectors), bad spark plugs or plug wires, the timing belt wearing out (stretch or slip, though this is usually a constant problem), or a bad catalytic converter (my primary suspicion - if the cat is worn and the honeycomb/biscuit inside busts loose, it can intermittently plug the exhaust system and cause the engine to run rough and/or backfire). I'm just vomiting some possibilities on the table... <img src="/forums/images/graemlins/barf.gif" alt="" />

I'd check for pending codes, then have a muffler shop check out the cat if there are no codes pending or there's something that points that direction (i.e. O2 sensor related or the fuel system too rich or too lean). You can also whack the cat with a mallet from below to see if something is rattling inside or sounds loose.

Last thought - if you did some work on it recently, go back through to check for loose connectors if you disconnected anything or worked close to a connector that could have been bumped, loosened or the damaged. I've done that a few times... <img src="/forums/images/graemlins/baby.gif" alt="" />

Hope that helps... <img src="/forums/images/graemlins/patriot.gif" alt="" />

Thanks, I'll look into those. The cat is hollow, so it's not a cat problem. I'll check for pending codes thanks!

Check vacuum hoses too, they get brittle and crack. Also check ground wires, especially near the battery, had trouble with things like that causing backfires, rough idle, even overheating once. Then things like fuel filter.

Update:
There don't seem to be vacuum leaks, vacuum looks good.
Catalytic converter is a non-issue, my cats are hollowed out so there is no way for blockage there.

I did some sensor graphs with my OBDII scan tool. Throttle position sensor looks good, MAF looks good, O2 sensor readings look good.

I checked short term and long term fuel trim, they get up to around 5% at times. I don't know if this is high or not.

The problem seems to be getting worse. Now it stumbles even while idling.

Disconnecting the idle air control valve doesn't seem to affect this, so I don't think the IAC is the problem.

More and more I am suspecting either fuel pressure supply problems OR... spark plug / coil / wire issues or arcing.

One thing I recall in the past was that the rearmost spark plug hole had a bunch of oil leaked into it because the valve cover seal had leaked oil into the spark plug well. Would oil in there with the spark plug wire cause problems? Does oil conduct electricity?

I will try to check the spark plugs and wires / coils tonight or tomorrow night.

I think I found the problem, here's the rearmost spark plug boot:

Oh yeah, that could definitely be the source of your troubles... <img src="/forums/images/graemlins/zombie.gif" alt="" />

Last time I change the valve cover gasket I didn't do a great job and then I have some overheating issues after that and here's the end result:




I Suppose overheating plus sitting in that oil plus being almost 150,000 miles old was just too much for that spark plug boot <img src="/forums/images/graemlins/lol.gif" alt="" />

Well I replaced the spark plug wires and boots on the coils but didn't replace the coils themselves those look fine. also replaced the valve cover gasket and was more careful this time and used some RTV because it might be a little bit warped I am not sure because I used to have overheating problems

with all that done it's running smooth as butter <img src="/forums/images/graemlins/smile.gif" alt="" />