High Idle

[SafetyGuy]

I've come across an issue and I don't know where to start.

My Truck a 2012 Sierra 4.8L runs and drives great, Usually, after a long highway drives my idle will slowly begin to creep up. (Normal Idle is around 530 RPM at Operating Temp). Sometimes it makes it as high as 800 RPM at idle. If I shut the truck off and let it sit 5-10 mins the idle will come back to normal. 

I've cleaned the throttle body. Air conditioning on or off does not make a difference, and operating temps are 190-225. 225 in heavy traffic situations 

Any suggestions on where or what to look for. 

[Supreme Pizza]

I would start by spraying starting fluid around the intake manifold gaskets when it is idling higher than normal. If it idles even higher with the starting fluid, you have a bad intake manifold gasket. Sometimes the engine heat can warp the metal just enough to open up a tiny leak, at certain temperatures. If the gasket is actually bad, you could probably seal it externally without pulling the manifold. Keep a fire extinguisher handy during this test, and don't spray the exhaust manifolds. A straw in the can will help direct the fluid to the correct spot. 

[SafetyGuy]

I don't believe the intake gasket is the culprit. The truck runs too good and doesn't have any other symptoms. I'm not sure I like the idea of spraying highly flammable starting fluid over a very hot engine.

 

I should add that it doesn't always exhibit this symptom 

[Supreme Pizza]

I doubt that it is the intake manifold gasket as well. However, it takes only 20 seconds to find out, and it is very safe to do so as long as you are careful, and don't SOAK the engine in it. It just takes a little bit of fluid for the engine to rev up. You are using a small enough amount that even if it caught on fire, it would go poof - and the fire would be gone. Intake gasket leaks as well as exhaust leaks can sometimes come and go according to temperatures, as the materials expand and contract. You could also smoke the intake manifold to check for leaks as well. I'm not saying that the manifold is your problem, but it is just about the quickest and easiest thing to eliminate. It takes more time to check the engine oil. Vacuum leaks can cause a high or wandering idle. 

 

    Just a suggestion. 

[DONWT15]

Check your vacuum lines first. PCV hoses, brake booster, things like that. Could be cracked or swells up when warmed up.

[Supreme Pizza]

Ya, 15 might be right. The vacuum leak could be a hose or something rather than the intake manifold. You can find those leaks with starting fluid as well.