1998 geo tracker gear power issue

If the fuel and air filters, along with the cat con are new, you might try a fuel pump pressure and volume test.

Another thought is the high voltage system. If you have a rotor and cap, there could be carbon tracking that would only show when under load, or a wet damp day. Also in this area is the ignition coil. If you open the hood at night, with no lights on, you may see a series of sparks from the coil, cap, or wires. Especially if you were to load the engine by putting it is gear, and giving it a SMALL amount of throttle. ONLY try this with an automatic transmission, and a competent assistant to hold the brakes hard.

GA from me...

It will idle up and down, say from 15000 to 20000 rpm.

Wow! thats amazing! my jeep would blow up if I hit 6000 RPM. Even a rotary Wankel engine would have some troubles at that rpm.

jokes aside, what size engine does it have? It is a geo after all, not built for power. I've seen a few Geo's with 3 cylinder engines, which aren't anything to take to the race track.

Have you inspected the rest of the fuel system? does it have a carburator? If its carburated, make sure the Carb is clean and leak free. Any leaks in the intake side of the engine will cause problems. Is it fuel injected? If its fuel injected, are the injectors clean and are the injector seats leak free?

Do you have any engine trouble codes?

The erratic idle sounds like something linked with the emissions system, and i would think there would be a good chance that something will show up when you scan the computer for codes.

Good Luck!

Suggest you look for any disconnected or loose, cracked vacuum hoses.

If it is fuel injected it could be the Throttle position sensor.