1994 B3000 v6: Slight Eratic Idle

[Research Rat]

My 1994 B3000 with 97k miles has a lingering slight erratic idle.

Parked with transmission in neutral, the truck will idle at 700 rpm for a few seconds, drop down to 650 or 600 rpm for a few seconds, then pick back up to 700 rpm. This repeats over, when both engine is warmed up or when engine is cold. This occurs with all accessories off-ie air, stereo, lights, etc. I'm assuming that the motor should be idling at a constant rpm level and not oscillating, even though it's slight.

According to the manual, an erratic idle can be symptomatic of a few malfunctions: loose fuel/manifold, vacuum lines, iac sensor, maf sensor, cps sensor, tps sensor, air temp sensor.

I ran the truck with the hood open and sprayed soap mixture along the manifold/fuel rails and didn't notice any bubbles. I pulled the IAC sensor while the truck was running, the idle immediately died down-so it appears that is working. All other sensors are connected as well. The PCV valve rattles, so appears to be working. All vacuum hoses appear to be in good working order.

I did not notice that the top of the plastic housing on the MAF sensor was cracked-ie the part of the sensor where the plug connects. But I pushed on that while the engine was idling and didn't notice a difference.

Any thoughts on where to check from here?

Thanks!
-Scott

 

[Research Rat]

 

[pillen140]

use carb cleaner to check for vacuum leaks, not soapy water. that would be the first thing i try. followed by cleaning the iac valve. depending how bad "slightly" is

 

[Research Rat]

Hey Pillen,

What's the difference between using carb cleaner vs. soapy water for checking for leaks?

Do you have any recommendations for cleaning the IAC valve? The manual states a few times not to clean it with solvent.

Thanks!

use carb cleaner to check for vacuum leaks, not soapy water. that would be the first thing i try. followed by cleaning the iac valve. depending how bad "slightly" is

 

[Rogue_Wulff]

When carb cleaner meets a vacuum leak, the engine idle speed will suddenly increase. A vacuum leak won't bubble with soapy water, as it's actually sucking air in, not letting it escape.