I have a minor issue with the idle that has me stumped. The truck starts fine. When cold it idles around 1400 rpm. The fan eases back as expected after 30 seconds or so. The idle used to drop down to 850 rpm when the engine warmed up, but now it stays around 1500, and this is for a cold start.

If I shut the truck off for a few minutes after the engine is warm and restart it, the idle drops down to 850 where it belongs. I can also use the clutch to slow the engine and that will bring the idle down from the 1500 range to the 850 range.

Occasionally the idle will creep back up to 1500 rpm after it was idling fine for a while at 850.

It’s a rebuilt 3VZE engine with 3,000 miles on it. I’ve replaced the TPS and it tests fine. I’ve cleaned the throttle body. I did an EGR delete to simplify things. The AC isn’t working so I removed the AC belt and capped off the AC idle-up vacuum lines. I capped off the power steering idle-up lines to narrow down the list of potential causes. I replaced the timing belt and water pump recently, so I had to flush and fill the coolant. I’m positive it’s burped correctly. I also replaced the fuel injectors recently, but that didn’t affect the idle. I thoroughly cleaned the intake plenum and I’ve replaced all the remaining vacuum hoses and tested for leaks with propane. Found none. I replaced the o-ring on the idle adjust screw on top of the throttle body, so that’s not leaking. The screw does adjust the idle but that adjustment can’t help with occasionally high idle speeds. I’ve triple checked the timing and it’s set at 10 deg before TDC. I know that the timing belt is installed correctly and I rechecked that nothing slipped.

A lot of the related parts have been replaced as well. The spark plug wires (NGK), plugs (Denso), ignition coil (OEM), distributor cap and rotor (OEM), coolant temp sensor (OEM), oxygen sensor (Denso).

The only thing I can think of that’s left is the idle air control valve on the throttle body. I’ve heard the valve can degrade over time so it’s not able to fully close any more, causing a high idle. But if that were the case, the idle would always be high, wouldn’t it? Could a degraded valve close some of the time but not always?

One last thing to note. It PS idle up wasn’t working so I replaced the idle control valve when I replaced the PS pump, but I didn’t fix the idle-up. For right now it’s disconnected. I think this is a different issue, but throwing it out there in case it might be relevant.

Any suggestions for what to check next to run this problem down?