Diagnosing Rough Idle Hell

[DylChrFla]

I’ve been battling a rough idle with my 1991 305tbi C1500 pickup for months now and I’ve gotten no closer to solving the issue. When cold it idles smooth and warms up normally. At idle when fully warmed up it has an intermittent miss/shake. When it does miss it puffs some smoke out of the tailpipe. However, the motor seems to run fine while driving. The motor is 100% bone stock with 150,000 miles and has flowtech block hugger headers installed. I have tried everything from basic tune-ups to throwing parts at it that are known to have issues. Timing with the EST unplugged is dead on 0° BTC. There are no CELs. Recently I bought an ALDL connector to datalog but I don’t have baseline numbers to work from. If anyone has experienced this same issue or has good TunerPro RT data to share please help a guy out.

List of new parts/work done:
Plugs & Wires
Cap & Rotor
Ignition Coil
Coolant Temp Sensor
Rebuilt TBI
Fuel pump
Fuel filter
TPS
O2 Sensor
PCV Valve
Replaced vacuum lines
Air Filter
Cleaned battery grounds

 

[RichLo]

I dont see an Idle Air Control valve/solenoid on your list yet.

 

[red98]

I’ve been battling a rough idle with my 1991 305tbi C1500 pickup for months now and I’ve gotten no closer to solving the issue. When cold it idles smooth and warms up normally. At idle when fully warmed up it has an intermittent miss/shake. When it does miss it puffs some smoke out of the tailpipe. However, the motor seems to run fine while driving. The motor is 100% bone stock with 150,000 miles and has flowtech block hugger headers installed. I have tried everything from basic tune-ups to throwing parts at it that are known to have issues. Timing with the EST unplugged is dead on 0° BTC. There are no CELs. Recently I bought an ALDL connector to datalog but I don’t baseline numbers to work from. If anyone has experienced this same issue or has good TunerPro RT data to share please help a guy out.

List of new parts/work done:
Plugs & Wires
Cap & Rotor
Ignition Coil
Coolant Temp Sensor
Rebuilt TBI
Fuel pump
Fuel filter
TPS
O2 Sensor
PCV Valve
Replaced vacuum lines
Air Filter
Cleaned battery grounds

Does your ICM pass at the parts store? I don't see one on your parts-shotgun list.

 

[DylChrFla]

I dont see an Idle Air Control valve/solenoid on your list yet.

As far as I can tell from resetting the IAC and data logging, it is functional but it could be possible that the IAC is not working 100% right.

 

[DylChrFla]

Does your ICM pass at the parts store? I don't see one on your parts-shotgun list.

That could be the issue too. TunerPro shows no errors with it but I’m not sure how sensitive it is.

 

[90 Silverado]

Take it out and clean it, they are usually gummed up. Clean up the port the pintle sits in on the tb housing also.Use throttle body cleaner only.

You have go through the idle relearn process also when its reinstalled.

Doug in P.R.

 

[DylChrFla]

Take it out and clean it, they are usually gummed up. Clean up the port the pintle sits in on the tb housing also.Use throttle body cleaner only.

You have go through the idle relearn process also when its reinstalled.

Doug in P.R.

I’ll give that shot tonight and see if it helps. Thanks

 

[PlayingWithTBI]

OOPS - double post!

 

[DylChrFla]

Update: I removed the IAC valve and inspected it. Wasn’t gummed up really but I cleaned it with throttle body cleaner and put it in to test. I reset it and the problem remained. I ended up putting a new one in and still no change.

 

[PlayingWithTBI]

IDK what happened with my post, I saw 2 posts and changed one to double post, now the other one's gone.

Anyway, you're describing a fairly common issue with the ICM and/or coil, it runs fine until it warms up and starts stumbling. TunerPro won't tell you the ICM is bad but, you may trap a sudden spike in RPM during a data log.

The ICM will send a random pulse to the ECM which, in turn, thinks the engine is running faster so it'll change the SA and injector DC to coincide with your SA and VE tables for that speed.