My truck revs up by itself and stays revved when stopped. Bad IAC valve?

[HopHead357]

I have a '99 K1500 5.7L with a NV4500 5 speed manual transmission that I swapped in about 2 years ago. This issue started with the old 4L60E that was in it before. With the 4L60E at about 10mph and 1500rpm, the truck would stay at 1500rpm and continue to drive at 10mph with my foot off the gas. Now with the NV4500 when I go to stop at a light and engage the clutch and take my foot off the gas the truck stays at what ever rpm it is at any where from 10-30 seconds. this doesn't happen every time I drive it, maybe only about 10% of the time. When I go to shift and engage the clutch and take my food off the gas, it will rev up by itself about 500rpm, this happens about 50% of the time. Any idea what the problem could be, maybe a bad IAC valve? Any help would be much appreciated.

 

[iEnding]

My IAC valve went bad a while back but I have an automatic and it would idle at 1400 rpm and sometimes almost die out if I let go of the gas to fast. Replaced it and BOOM everything went back to normal

 

[SAATR]

Could be the IAC, a bad TPS, or possibly a sticking throttle plate or cable. With the truck in neutral and the parking brake set, try to reproduce the issue by moving the throttle by hand, under the hood. It could possibly be the throttle cable itself binding, but you would feel that in your foot as a hard spot or the pedal not returning to its normal position. You can disconnect the cable from the throttle body and slide it in and out by hand, to see if there is any excess resistance in it. If that's all good, and the throttle plate is clean, then I would pull the IAC and clean it and its passage thoroughly. While you're there, ohm out the TPS and make sure that the resistance change is smooth with no jumps or hanging in one spot.

Is your CEL on? Do you have any codes? That would help narrow it down considerably. Something like a large vacuum leak could cause this kind of issue, but you would get a lean condition code from it.

 

[stroker52]

my old 96 with the V6 and 5speed had this problem.. i changed everything even took the TB off and went to the chevy dealer and got some top engine tub (X66) cleaned the hell out of the TB. so now i had a new IAC, TPS, cleaned TB.. still did the same thing so i hooked up the scanner to watch the IAC in real time. it was acting slow i drove the truck around with the scanner hooked up with a buddy watching it and it would do it sometimes.. so i pulled the PCM and had it flashed problem gone!! from what ive been told the vortec trucks have this problem alot..

 

[HopHead357]

Could be the IAC, a bad TPS, or possibly a sticking throttle plate or cable. With the truck in neutral and the parking brake set, try to reproduce the issue by moving the throttle by hand, under the hood. It could possibly be the throttle cable itself binding, but you would feel that in your foot as a hard spot or the pedal not returning to its normal position. You can disconnect the cable from the throttle body and slide it in and out by hand, to see if there is any excess resistance in it. If that's all good, and the throttle plate is clean, then I would pull the IAC and clean it and its passage thoroughly. While you're there, ohm out the TPS and make sure that the resistance change is smooth with no jumps or hanging in one spot.

Is your CEL on? Do you have any codes? That would help narrow it down considerably. Something like a large vacuum leak could cause this kind of issue, but you would get a lean condition code from it.

It's not throwing any codes or anything. I'll have to try cleaning the IAC and testing the TPS and throttle cable tomorrow. Hopefully this does the trick or I'll have to try a new IAC then TPS.

my old 96 with the V6 and 5speed had this problem.. i changed everything even took the TB off and went to the chevy dealer and got some top engine tub (X66) cleaned the hell out of the TB. so now i had a new IAC, TPS, cleaned TB.. still did the same thing so i hooked up the scanner to watch the IAC in real time. it was acting slow i drove the truck around with the scanner hooked up with a buddy watching it and it would do it sometimes.. so i pulled the PCM and had it flashed problem gone!! from what ive been told the vortec trucks have this problem alot..

I had the PCM reprogrammed when I did the transmission swap. It was doing it before so I'm hoping it isn't part of the problem.

 

[HopHead357]

It's been a while, but I finally got around to trying to solve this problem. I cleaned the IAC valve and te throttle body last night an it seems to be doing better. I know if the problem once I get some more miles on it.