1990 c1500 5.7tbi Rough idle when warmed up Help Wanted!

[hbryant123]

Hello all, first post here, I am needing some help with a rough idle issue I am battling!
When warming up the engine runs smoother but has a noticeable roughness in it, once at operating temp it idles rough, even worse in gear with the ac running, it almost seems like a miss at times only at idle! driving down the road it runs great, climbs hills great, pulls my lawncare trailer with ease! The engine does not smoke, use oil or make any noises! it has 185,000 miles on it.

Here is what all i have went over and replaced over time!

The engine currently has no SES codes and starts up easily!

Spark plugs are ac delco about 25,000 miles on them, i took each one out they all look to burn the same.
Wires are delphi-packard all of them ohmed out good, when removed one at a time they all caused the same change in idle.
cap and rotor are new
icm and pickup coil have been replaced
coil has been replaced
Coolant temperature sensor test out good
i swapped the map sensor with a known good one
replaced the 02 sensor with an ac delco 02 sensor
i pulled the iacv and cleaned it
fuel pump was replaced with a delphi unit 3 years ago, currently has 11psi of pressure before the filter.
i replaced the pressure regulator 3 years ago also
i pulled the egr and blocked it off with a plate as a test to see if the idle changed
replaced the pcv valve
i sprayed carb cleaner around the intake and throttle body to check for changes
i checked the vacuum of the engine, it set on 18 but would drop as it would rough idle/miss

The injectors dont drip but the spray pattern i am not sure about, ive never seen a what the pattern should look like, it seemed cone like ! but not really a fine mist! seemed like it didnt much to change the idle, i stuck a screwdriver in the spray and just that little change made it idle rough, I am wondering if the 32 year old injectors are to blame!?

Any Advice is welcomed!

 

[Erik the Awful]

When warming up the engine runs smoother but has a noticeable roughness in it, once at operating temp it idles rough

Check for vacuum leaks. Your air fuel mixture is richer when the engine's cold, which covers up vacuum leaks. Once the engine warms up the mixture leans out and you start running rough.

 

[hbryant123]

Ok so I sprayed carb cleaner around the intake and around the throttle body, also sprayed around vacuum lines, anything I thought is related to vacuum, I never noticed a change in idle, could the intake be leaking on the bottom side? Can I test that somehow? Thanks

 

[LA2SD]

Did you replace the grommet when you replaced the PCV? Those things get hard after a while, and don't seal.

Also check for cracks or hardness around the rubber boots for the vent tube that runs from the PCV to the TBI.

 

[hbryant123]

It seemed to fit in the grommet pretty tight, I will double check it after work!

 

[DerekTheGreat]

I think the ignition control module is to blame for that. Does it idle rough all the time or just sometimes when warm? Sometimes my truck idles real choppy and other times like this morning, it's smooth as polished glass. When I observe the timing (EST wire plugged in) during those rough episodes it is rather wobbly, like the ECM isn't consistent with the advance, always changing by four degrees retard or further into advance. Perhaps that's the cause, but I'm no engine guru and never investigated further. However, I have swapped ICM's and noticed a difference for a while. The Standard Motors one that's in there now has been the best of all I've tried. Changing my coil out helped too.

 

[hbryant123]

I think the ignition control module is to blame for that. Does it idle rough all the time or just sometimes when warm? Sometimes my truck idles real choppy and other times like this morning, it's smooth as polished glass. When I observe the timing (EST wire plugged in) during those rough episodes it is rather wobbly, like the ECM isn't consistent with the advance, always changing by four degrees retard or further into advance. Perhaps that's the cause, but I'm no engine guru and never investigated further. However, I have swapped ICM's and noticed a difference for a while. The Standard Motors one that's in there now has been the best of all I've tried. Changing my coil out helped too.

I have replaced the Icm, it made no change, the rough idle happens consistently when it's warmed up!

 

[hbryant123]

I think the ignition control module is to blame for that. Does it idle rough all the time or just sometimes when warm? Sometimes my truck idles real choppy and other times like this morning, it's smooth as polished glass. When I observe the timing (EST wire plugged in) during those rough episodes it is rather wobbly, like the ECM isn't consistent with the advance, always changing by four degrees retard or further into advance. Perhaps that's the cause, but I'm no engine guru and never investigated further. However, I have swapped ICM's and noticed a difference for a while. The Standard Motors one that's in there now has been the best of all I've tried. Changing my coil out helped too.

I have replaced the Icm, it made no change, the rough idle happens consistently when it's warmed up

 

[BeXtreme]

This sounds exactly like intake manifold gasket leaks. 18 in. hg. is low. I'd bet you have an intake manifold leak or a leak around the throttle body gasket. With that kind of mileage, if you have never changed them, it's probably time to do it whether they need it or not.

 

[hbryant123]

Other than removing the intake is their a way to check it and be sure, I'm trying to keep from throwing money at it with no results ! Could I run a camera down the throttle body ?