OK....Now I've been trying to beat this idle stumble for the last 3 months nearly. I have done every single test I can think of, replaced numerous componets, and racked my poor little brain. I have a miss/stumble at idle up to about 1500 rpm or so. It doesn't miss under load or while at a sub 1500 rpm cruise (like in 4th gear going 40 or so). Every now and then the idle "bumps", like the car is gonna die, but then returns to its irregular miss feeling. I bought an updated Distributor (I wish I had taken pictures, this thing is built better than the MSD I looked at), I've replace IAC, Checked TPS, new wires, plugs, cap/rotor, O2Sensor. My fuel pressure reads 45 psi at idle, and 55 psi with key on engine off. I checked compression across the board (fig'd I had a lifter not pumpin up) compression was like 150-155 on all cylinders. I have my timing set at 10* advanced with the ETC disconnected (for the cam). All of my plugs are white, meaning the ceramic part, but not LEAN white, more like perfect AF ratio. A balance test reveals NOTHING, no change in idle quality despite which plugwire is disconnected. I have NO vaccumn leaks, I regasketed EVERYTHING in an attempt to locate a leak, there were none (there are none). I have no check engine light, snapon scanner reveals no codes. I tried it with 2 different ECMs, same idle quality. Anybody have ANY ideas what this could be??? I've talked to 2 of the guys I work with, and it's got us stumped.

 

Disconnect your EGR valve hun. 95% of the time its the problem.

 

I've tried that (it's an obvious troubleshoot). I disconnect vaccumn and the idle doesn't improve. Yesterday evening it idled correctly for the first time in 3 months. The weather was pretty cold outside, perhaps 40* with about 85% humidity. BUT, tonight the weather was the same, and it's idling like crap again . I thought I was on a lead, having to do with IAT. I know my motor is running slightly lean (even at 3 psi above normal on AFPR). The cam in this motor isn't THAT big, it has about 12* more duration than stock. The car ran fine for about 4 weks after the cam swap, I raced it at the track, then 4 weeks after THAT the probs started. None of the guys I work with have had any luck finding the problem (All mechanics by tradde, like myself...although I'm a young buck). I suppose the valve could be sticking (EGR), but I've never seen that ahppen (I'll block it off and see). ANyhow, any more suggestions would be appreciated (my motor has all of the same systems as the L98's).

 

It sounds like the EGR is sticking open, or flopping open when idling. I would almost put money on this is the problem. If not, then I would guess that something is moving on you, like ignition-wise or valvetrain wise. Perhaps the cold start valve is being gay?

 

What is the easiest way of doing this?

Scott

 

Disconnect the vaccum line that goes to it and put a piece of tape over the line. Sometimes the egr is stuck open, in which case sometimes you can tap it with a hammer at the base and losen it up.