Got a '96 5.9L that has developed a slight miss/stumble @ idle.

Here are the particulars...
1. Happens in any gear (including park and neutral)
2. Only happens @ idle. Once throttle is pressed it goes away
3. Tach will move ever so slightly
4. It is not consistent. Does not have a smooth miss and comes and goes at will (hot or cold doesn't matter). It may go days without doing it.
5. No CEL
6. It is a very slight stumble/miss
7. Did notice a slight film of clear oil in the inside of my intake

New Parts to rule out...
1. Distributor cap and rotor
2. Spark Plugs
3. Front O2 sensor
4. Cleaned IAC valve and throttle body (thoroughly)

I searched around and found a few possible causes (iac valve, TPS, CPS, plenum), but nothing definitive.

Anybody have the same issues?

 

have you tried running a can of seafoam through the gas tank? mine and my dads and my sisters trucks all have a "slight miss" but i don't think its anything to worry about. do you mean that the tach isn't frozen in one spot while running? what i mean about ours is that if you watch the tach you can see it move maybe 5 rpms while its at idle.

 

1. I ran a can of Seafoam thru the system back in April.
2. On the tach, you can barely see the needle move when the engine misses/stumbles. This tells me that the problem is engine related and not tranny related.

 

Cleaning the IAC is not an end all solution(it may still be bad), but you also need to check the plenum.

 

Not to hijack, but my tach will twitch every now and then at idle. And i dont feel a misfire or anything when it does this. It'l twitch up or down. It twitches Quick too.. IAC? Or what? Even when it does this i dont feel a bump in the idle. Is it doing somthing and im just not feeling it? Its weird. I dont hear anything when i listen to the exhaust either. I need to ad that i replaced the TPS with a BWD TPS from advance. No change at all. I will clean the IAC to see if that helps. If not. Ill replace it. What i really want to do is replace all the engine control sensors. Like IAC, MAP, and the o2 sensors. And also want to replace the cat before that gives me probs. lol, but money wont allow that..

 

Another thought might be the injectors.

IMO I don't think there's anything wrong with the tach. Its computer controlled so it could just be something with the PCM making it do that. Just my $0.02

 

Do you have access to a scanner that can check fuel sync?? Might be off a tad. I think mine is at +8 and it does that a bit. Only way to fix it is with a scanner and twisting the disty to get it dialed in.

 

I don't have a scanner to check the dist +/-, but I have noticed a few additional points to consider.

1. I relocated the ignition/coil wires per the TSB
2. I have concluded that the engine will not miss/stumble when cold, only after it has warmed up to operating temp.

Does this help jog anybody's mine

 

Are you sure the CEL bulb is not burned out, and when you looked in the intake did you have the TB off at the time and look with a small mirror and flashlight, or did you just look through the butterflies?

 

Intake/Plenum needs to be resealed. I had same problem.