High Idle

[Tazman]

I always have the ECM set the idle on my truck and it has always been near 600 rpm. Yet lately after it has been running for fifteen minutes or so the idle is around 850 rpms. Any ideas why this is happening?

Thanks

[rich weyand]

Sounds like the temp sensor isn't reporting properly.

[Irish_Alley]

how cold is it?

[cali6shooter]

I'm having the same issue in Cali and just replaced water temp sensor the one directly below thermostat housing and sometimes the idle is so high can't get into revers just grinds the tranny

[Tazman]

80 degrees at 2:30 in the afternoon

[Irish_Alley]

Lol well that eliminates that
Try sensor testing

[Tazman]

When it was cooler my truck would never get warm enough to turn on the heater. Would that be just the high temp thermostat or could that be the temp sensor also?

[LTZ C20]

All the temp sensor does is act as a thermometer for the gauge (for you to see) and the ecm if equipped (for the computer to see).

The heater not getting hot can be a few things, but if you have a thermostat with too high of a operating temperature then it could cause no or little flow thru the core, creating a no heat situation. Reason being, it will never open so the coolant won't flow out the top end and into the core, it will just stay flowing thru the lower end of the engine.

[Irish_Alley]

All the temp switch (on the side of the drivers head) does is act as a thermometer for the gauge (for you to see) the temp sensor (on the intake beside the thermostat) is for the ecm if equipped (for the computer to see).

[LTZ C20]

Whoaaa lol more moderator voodoo magic goin on here.

[Irish_Alley]

Whoaaa lol more moderator voodoo magic goin on here.

na its just a quote

[LTZ C20]

Ok lol.

[Tazman]

Plugged in my code tester and up came code 33 which says the map sensor is failing. I had replaced this just over two years ago but this has a warranty, so with no cost to me my truck is running smoothly again.

[Oldchevguy]

Vacuum leaks can cause high idle , check all vacuum lines and carb or throttlebody gaskets . For example check vacuum line to brake booster for cracks or loose connections.

An easy way to check for vacuum leaks is with a hand held propane torch. Turn it on without lighting it and direct the nozzle all around the intake manifold and anything that connects with a vacuum line. If there is a vacuum leak the idle speed will change, you can see how much by directing the nozzle into the carb or throttlebody. I also use carb cleaner to do this but the torch is prob safer.

Just be careful, the small amount of propane from the torch is usually not a problem, but if you are like me anything can become a problem ha ha . Good Luck

[Dirka]

Adding a question to this thread concerning idle.
I have a 77 K10 and my idle is higher in park than in gear, by 600rpm, would that be due to some vaccum leaks I've read here, or is it normal, or something else?