Engine revving up and down when warm.

I bought a 90 Tracker that someone put a 91 engine in. When started, it idles real high, then after a couple of minutes (when it warms up), the engine starts revving up and down. I looked down into the throttle body and it looks like it is getting an extra squirt of gas, that’s what is causing the engine to rev.
Does anyone have any idea what is causing this to happen? Please help….. this is driving me crazy. Thanks !!!

Generally the ECM controls when the injector squirts fuel into the throttle body and there are numerous sensors that tell the ECM when a cylinder is ready to accept fuel. I would try tracing the various sensors and find the one that is miscuing the ECM. It may be a mismatch between a 1990 ECM and 1991 sensors.

good luck.

The same idle up and down problem happened to me and it was the idle air control valve. I heard that you could clean them but I bought a new one and the problem stopped.