Vortec 350 idles way too high

[battyice]

I am working on a 1999 GMC 3500 for a friend with the Vortec 350, the problem is that the truck idles too high. As soon as you put the truck in gear, it takes off and will go about 35 mph without touching the accelerator. If you shift into neutral at 35 mph, the engine will run about 1600 rpms and stay there until you stop, at that point it idles down to 1000 rpms.

I have checked for vacuum leaks, found one at the PCV valve elbow but fixing it didn't cure the idle problem. Throttle body seems to be working fine, I cleaned it with throttle body cleaner and that seemed to make the problem worse.

I tried starting the engine with the idle air control valve unplugged to see what it would do, the engine raced to 2500 rpms and stayed there until I shut it off.

The truck has been well maintained and has roughly 120k on it, the idle problem has come on gradually. The check engine light is not on.

Any suggestions?

[cheyenne264]

Maybe a tps. that would explain the high idle. and why it takes off like that.

[4x4k20]

replace the coolant temp sensor they will cause this problem also replace the wiring connector that hooks to it napa sells them with the wiring end harness ..oh and check for codes agian

[LiftedVictory]

yes coolant sensor, it is probably internally open, which would make the computor see an extremely low temp, therefor always running in cold start enrichment. replace then scan for hidden codes.

[battyice]

Does anyone know the voltage/resistance spec for the ECT? If possible I would like to test the sensor rather than just replace it and find out that wasn't the problem.

[JeremyB]

At 5 degrees F 21450
32 degrees 9420
50 degrees 5670
68 degrees 3520
77 degrees 2796

[justinp95]

Tps

[battyice]

Update: I hooked up a Scanguage II to the truck and discovered that the TPS reads "0" at all throttle positions, if you move the throttle enough once and a while I would get a brief reading of "20". The wires leading to the sensor check out fine so I will replace the TPS this weekend and hope that is the end of it.

[montessman1]

looking at that info it would indicate your tps is bad, but a real voltage would be nice, if you have that info it would really help, get back to me if you need help with it

[battyice]

The plot thickens. I replaced the TPS, and nothing changed. The truck still idles too high, the TPS output on the scanguage still reads "0". I checked the wireharness for broken wires and didn't find any. With the truck at operating temperature, I tried unplugging the TPS to see if anything changed...nothing, I unplugged the MAF sensor...nothing changed, and no check engine light. Does it make any sense that the truck would run the same with the MAF, IAT, and TPS sensor unplugged and not throw a check engine light????

I am wondering if the PCM has a problem.

[Koots]

Truck should have run like A$$ with all those thing unhooked. sounds like something is definitely not right. Do you have a spare ECM you can try to test out?

[4x4k20]

at least swap the coolant temp sender it not but about 30 bucs

[battyice]

The coolant temp. sensor output was accurate on my scanner. Started out at ambient temperature and leveled out at ~195 when the engine was warmed up.

[montessman1]

did you remove and clean the i a c valve. sounds like it may be stuck, what kind of scanner are you using , is your e.g.r valve stuck open even just a little bit? clean the throttle plate?

[montessman1]

might still be a vacuum leak, i have had a few with intake gaskets causing a running problem before they leaked antifreeze, the gaskets warp and the center ports suck external air, still doesnt explain the no check eng light, i will look in to it a little tomorrow when i go to work