97 GMC Suburban k2500 idiling at 2500 RPMs

Suspect a vacuum leak, in particular the sides of the intake manifold. A smoke machine will be helpful in locating it. It will let you inject white smoke at 2 psi into a vacuum hose, then look around the intake manifold gaskets. If it's leaking underneath, you might see the smoke coming out from the oil fill cap.

A scanner can also be used to read live data. Look at the idle air control motor to see how far it is opened. If it is closed, the Engine Computer is trying to bring the idle speed down. If it is commanded open, the computer is requesting the higher speed.

Caradiodoc

"Darn the bad luck". Then you're going to need a scanner to see what the Engine Computer is trying to do.

One thing you can try is to unplug the IAC motor while the engine is running, then you can stop the engine and pull the motor out to see where the pintle valve is positioned. You have to do that before you stop the engine because as soon as you turn the ignition switch off, the valve will retract in preparation for the idle flare-up at the next start.

You can also pull the valve out to extend it. It will pull real hard but it can be done. Reinstall it that way but don't plug it in. Start the engine, then see how fast it idles. If there is no vacuum leak, you should have to hold the gas pedal down just a little to keep the engine running. If the idle speed is still too high, there has to be unmetered air sneaking in. Try pinching off vacuum hoses, especially the hoses to the brake booster and the cruise control servo.

Don't overlook the coolant temperature sensor and the intake air temperature sensor. That one sits in the fresh air tube near the right hood hinge.

Sorry I overlooked your original comment about no vacuum leaks.

Caradiodoc

No. When a belt or chain jumps, typically the valve timing is late. That results in very hard starting and low power. Retarding valve timing just a little, say one degree, will increase low end torque. "L" for low and "L" for late. That is designed in for motorhomes. Advancing timing up to one degree increases top end torque. That would be typical for police cars. A jumped belt changes valve timing much more than one degree, ... More like 15 - 20 degrees.

Caradiodoc