idles high when first started
 

I have a 97 GMC with a 5.7 I believe. Everytime I start it it idles way up then comes down. Revs up to about 1700-1800 RPM for a few seconds then comes down and everything is fine. This happens everytime it is started warm or cold. Any thoughts? Thanks, Brian

Re: idles high when first started
 

I do believe this is normal on a cold start up and it may do it slightly longer when really clod. My 98 does the same thing, but it does not do it when its warm. I wonder if the coolant temp sensor (near the thermostat) is having issues and starting to fail where it may be giving low readings (high resistance) back to the pcm? The pcm should set the service engine light if it goes too long.

Re: idles high when first started
 

Yea it does do a little longer when it's cold but that's normal. Never did it until recently. Now every start it revs up hot or cold. No service light though. I'll try the coolant temp sensor and see what happens.

Re: idles high when first started
 

I will bet it is the coolant temp sensor on the front of your intake manifold. These are notorious for going bad and they tell your ECU that the motor is still cold and dumps fuel into the throttle body. I have two motors with them and changes both out and made a world of difference. Computer will throw a code too if you can get it scanned somewhere.

Re: idles high when first started
 

So I swapped out the temp sensor and it still does it. Any other thoughts???

Re: idles high when first started
 

Perhaps a faulty throttle body position control sensor? Are you having any troubles with the shifting? Like it seems to take higher rpm's for it to shift than normal?

Re: idles high when first started
 

Damn that's weird I was going to say temp sensor as well. I wonder if the TPS is maybe starting to cause issues?

Re: idles high when first started
 

I don't notice any higher rpm's before shifting but it does rev up a little after I put it into park. Example: Drive around for while. Pull into driveway or parking spot. When I put it into park it will rev up a little then come back down. Not as high as when I start it but some.

Re: idles high when first started
 

I had a Throttle Position Sensor (TPS) start flaking out on my 95 5.7 4L60-E 2WD Silverado. It started with the high idle speed and then started delaying the shifting. Shifting pressures were high giving me harsh shifts. My Powertrain Control Module (PCM) was going to default programing because it did not have a defined closed throttle signal. I replaced the TPS located on the passenger side of the throttle body. Runs like a new truck.

Mark.