high idle problem

I have a 88 bonneville with 54K on it that I just got. It has been sitting about 2 yrs. it ran well when I picked it up and idled down OK.

Now it has been sitting in my garage for 1 1/2 weeks and I wanted to change the oil. I started it and it went into fast idle, no problem. let it run a few minutes and it would not idle down. shut it off and changed oil.

Restarted after oil change, upper radiator hose is getting warm, but engine will not idle down at all.

Where do I start looking

If the coolant is getting warm and there's no air in the system then it sounds like an issue with either the idle control valve or coolant sensor.

If you have access to a scanner you can verify the engine is going into closed loop (ECM control of fuel) and see what the coolant temperature sensor (CTS) is reading (cold = open loop, richer choke type operation, higher idle). A high resistance at the 3 wire sensor CTS tells the ECM the engine is cold. The ECM then positions the idle control valve to attain the desired idle.

The idle control not seating fully because of carbon-dirt can cause the idle to stay high. It can be removed from the throttle body for cleaning along with the seat inside the throttle body.

If you want, with no scanner I can post the resistance values to manually test the CTS but they're not an expensive part if you want to take a stab at it. It's located below the thermostat housing threaded into the front (left bank) side of the intake.

I will change the CTS and go from there.

Appreciate the info.

high idle most common causes, a large vacum leak, stuck tps (throttle position sensor) faulty or disconected map sensor (manifold absalute pressure) faulty or an air leak after maf (mass air flow), or carbon build up around throttle plate.

Upon further inspection, I found tape on the wires for the temp sensor. I wiggled them a bit and started the car and the car idled down. So it looks like I need to clean these connections.

Also When I pulled the oil cap off while the engine was running, the idle dropped. Never seen such a thing before. Seems to me there is some sort of restriction for the engine to breathe. The air filter is clean, PCV is working. Any ideas?

Its common to have the engine drop idle when you open the oil cap with it running on the Vin C 3800...

Its common to have the engine drop idle when you open the oil cap with it running on the Vin C 3800...


Thanks. Appreciate the info.