Miss at idle

[SoCalSlaughter]

Ive been hunting a miss I have on my 1990 Chevy Suburban 5.7, its more noticeable during a cold start. When I listen to the exhaust I can hear a "puff" every few seconds. and the motor shakes alittle bit. Every sensor on the motor is new, cap, rotor, wires, plugs. Even swapped everything back and same result. New O2 sensor, tried old one to make sure. Sprayed carb clean around tbi base and both sides of intake manifold with no change. Im hoping its not piston rings or a valve problem. Any ideas?

[SoCalSlaughter]

Also Ive tested the map sensor, drops 1v for every 5 pounds of vac I applied. Coolant temp sensor read 322ohms at normal operating temp. Intake is pulling 19 pounds of vacuum at idle. Spark plugs gapped to 35., all spark plugs look normal, none stuck out more then another.

[Rufton]

Vacuum 19 inches steady at idle?

[SoCalSlaughter]

yes, steady at idle. I have a gauge hooked up to the manifold

[Rufton]

I was able to catch faulty ignition system red handed by watching spark. During miss, spark became visually erratic. In my case (different engine type) it was a faulty ICM.

[SoCalSlaughter]

Ive tested and replaced the ICM, same result. Im thinking I have compression loss. Im going to do a check tonight to confirm

[Rufton]

Do your plugs show an antifreeze tint? Did all your old plug's color look normal w/o any scale? Did you confirm vacuum normal during revs, etc.?

I like to do a couple compression readings on warmed engine. I reverse order of cylinders on test 2.

Radiator and cyl leak down tests are good also.

[SoCalSlaughter]

The old plugs I pulled out had alittle bit of oil on them, I know some of the valve stem seals are bad, other then that the plugs looked alittle tan. And the vacuum will drop when engine is reved, or while driving. But in gear at a stop the vacuum is the same, 19.

[Rufton]

I replaced ICM from parts store; after some hair pulling I confirmed new ICM was also bad. The second bad ICM had a different personality though. No way to test my ICM but yours may be testable.

[SoCalSlaughter]

I had both of my ICM tested, multiple times. I was getting a code 42 (Lost communication with distributor) But fixed all that, code is gone. But the miss is more noticeable when the motor is cold, runs good once warm.

[Rufton]

Did you google vacuum tests like:
http://www.autospeed.com/cms/A_2393/article.html

[Rufton]

Originally Posted by SoCalSlaughter

The old plugs I pulled out had alittle bit of oil on them, I know some of the valve stem seals are bad, other then that the plugs looked alittle tan. And the vacuum will drop when engine is reved, or while driving. But in gear at a stop the vacuum is the same, 19.

But your new plugs that miss have no oil fouling?

[SoCalSlaughter]

When I snap my throttle the vac will drop low but come right back up, when Im driving and go to WOT the vac will drop to just about zero, then off throttle will go to about 22.

[SoCalSlaughter]

The new plugs have less then 40 miles on them, but still has the miss with the new plugs

[Rufton]

When you do your compression test it'd be good to see if plugs are already oily.

What is your oil consumption rate?

You can check valve guide wear in car w/ wiggle test; take precautions to avoid dropping valve.

O2 gauge very handy but may be approaching that overkill zone.

[SoCalSlaughter]

It does have a puff of smoke on cold starts, doesnt eat up to much oil. Maybe half a quart every 2 weeks? I have new stem seals but havent gotten around to doing it yet. I had the valve covers off a few days about, none of the springs looked broken, nothing was loose and nothing makes noise

[Rufton]

If your engine is worn it may be time to step up to a warmer plug. Sometimes a warmer plug can allow a worn engine to run w/o fouling plug for several more years.

If you have time, closeup plug pics may be helpful.

[SoCalSlaughter]

Just finished the compression test. Took all the plugs out and they all seem orange. Compression test results

#1 - 150
#2 - 150
#3 - 145
#4 - 145
#5 - 140
#6 - 140
#7 - 150
#8 - 145

[SoCalSlaughter]

Took the ICM to Aurozone and I tested it myself (I know how to use the tester haha) Tested it 11 times and was good every time