misfire at idle?

[cboyfan2020]

hi guys....i have a 1993 chevy k2500 with the 5.7 tbi. new intake gaket,egr,plugs,wires, cap and rotor. Has a misfire/stumble at idle with thransmission in gear. Runs fine at other speeds and w/ AC on at idle. Timing is good and known good IAC valve installed with no improvement. Any advice beside a 5 gallon bucket of gas and a match?

[broughy84]

since you have recently done plugs and wires are you positive you don't have one grounding out on a manifold? also what type of plugs are you using, ACDelco are the best, and I would never use Bosch in a SBC.

[cboyfan2020]

Originally Posted by broughy84

since you have recently done plugs and wires are you positive you don't have one grounding out on a manifold? also what type of plugs are you using, ACDelco are the best, and I would never use Bosch in a SBC.

I am positive there is no plug wire grounding. they are ac delcos( only thing I will use in gm). I recently read a post on iatn about a truck (92 2500 5.7) with exact same problem and it was the ecm not controlling the IAC which was my last option so i will try that and let you know what comes of it. Thanx for the reply.

[2000CAYukon]

If you have access to a scanner, you can see what the ECM is commanding the IAC to do.

I had a hunting idle on my 90 GMC K1500 5.7 700R4 and it turned out to be a leaking PCV gromet (was hard as a rock) and leaking vacuum hose going to the MAP. Looking at a scanner, I could see BLM go lower and lower usually around 108, the idle would get real bad. The ECM was leaning out the fuel and my theory is that the bad connection at the PCV valve was causing the ECM to think the engine was running rich.

The idle is much better after fixing the vacuum leaks. Bottom line, the scanner will tell you what the ECM is seeing and doing.

Good Luck,
//2000CAYukon

[silicon212]

I am surprised that nobody has brought up a restricted or dirty injector. The truck should be TBI (L05) and there will be two injectors in the top of the throttle body. Gunk builds up inside the throttle body bores over time and the injectors are right in the midst of it all.

[cboyfan2020]

I have looked at all the usual and unusual suspects. I have a Modis and the only weird thingi going on is that every time the engine stumbles the IAC coil A will cycle on and off. Injectors flowing fine, ran smoke machine with no leaks.

[silicon212]

Originally Posted by cboyfan2020

I have looked at all the usual and unusual suspects. I have a Modis and the only weird thingi going on is that every time the engine stumbles the IAC coil A will cycle on and off. Injectors flowing fine, ran smoke machine with no leaks.

My next suggestion is to look for vacuum leaks and don't forget the brake booster ties into this as well. Does your A/C system work well?

[cammer]

Exact same problem on a 94 5.7L TBI - IAC does seem to react to the 100rpm stumble but not fast enough to keep it from stalling. A vacuum guage shows the vac drops 2in at every stumble, but is a steady 16in at all other times. No pushrod clacks or other noises that I'd associate w/a sticking valve.
I can't seem to correlate the stumble to any other telemetry I see on the scanner, except that the TBI pulse width opens up a bit from (.9 to 1.1ms) with the stumble, seems to be a reaction *to* the stumble rather than the cause, though. MAP voltage also climbs about .15v at the stumble.

Any other ideas?

[maxwedge]

I'd start a new thread here to get more replies this one has died down.

[gwjturner]

cboyfan2020,
Take a look at your ignition coil. Specifically look at the area between the actual coil and the metal frame on both sides. Usually the coil is wrapped in black insulation. If there is a whitish powdery spot or area between the coil and the metal frame, the insulation has broken down and the coil is arcing (grounding) to the metal frame. It DOES NOT happen with EVERY firing of the coil but the whitish color indicates that it IS happening. This is a very common problem and will drive you nuts trying to find the misfire. Simply replace the coil. I'm still getting my sanity back from this issue almost 10 years later!!!!!

[cammer]

All,
Thanks for the help - turns out it was the distributor pickup coil - long shot, but it cleared the problem. Oh, to have a tracing scope/scanner.......