98 Cherokee

First easy step: pull the TPS plug at the throttle body and clean the contacts with contact cleaner. In a pinch, you can use WD40 for a cleaner. Spray the cleaner into both halves, plug an unplug them a few times, then blow both side good and dry. Use some dielectric grease (as in spark plug boot grease) on the rubber seal. Give it a day or so just in case the computer has been confused and you may have to repeat the cleaning but this has become an annual rite for me.

Is the Check Engine light on? If not on, does it work? (It is REQUIRED to come on before you start the engine -- key to ON but not START -- and is supposed to go out at all other times.)

If the light does not come on before the engine is started, then it is burned out, and you need to pull codes. If the light is on, then you need to pull codes.

If the light works properly, then you need to look around for a small-hose vacuum leak. Look at the hose ends on hoses that connect to the intake manifold -- don't forget to check both ends. Also check the same hoses for cuts or breaks in the middle. The extra air from a vac. leak can be countered by the computer adding gas. It's possible for a small leak to be countered with more gas and keep the air/fuel mixture within specification, and not turn the Check light on. It's not likely this is the problem because such a leak ought to turn the Check light on, but it can happen if the conditions are just right.

I'd _expect_ the Check light to be on if the problem was the TPS or other sensor, so I'd be looking for a vac leak because there is no specific sensor to monitor this, and the resulting Lean condition can be countered with more gas. The Lean condition would be detected in the exhaust stream, and the computer would turn the injectors on longer to counter the lean condition. More air and more gas is the formula for more speed.

While looking for small-hose vac leaks, be sure to inspect the throttle linkage to be sure it is returning properly. If it is sticking open, then you will get the symptoms you report and no Check Engine light. You take your foot off the pedal, the throttle closes -- but not all of the way. The throttle does not know you are not holding it open, so by definition there is no error to report.

That's funny, I have the same problem in my JGC. When I start it up, it surges. It comes back to normal, but surges again. When I finish driving and place it in park, the tach is around 1200. So I give the accelerator a quick kick to the floor and the RPM comes back down to normal idle. What gives?

If that doesn't fix the problem, take a can of TB cleaner and with the engine on, spray little puffs around every rubber hose. If you find an air leak the engine will stammer.

Another thing to do is disconnect the brake booster hose and plug the hole, or clamp the hose off. (just don't cut it or loose anything into the intake rail) If that lowers the idle then it's either the hose or you might have a leak in the booster diaphram.

A vac leak will run your idle up.

So as others have said, trace every hose that runs to the intake rail. The hard lines tend to fail at the bend.

Also spray around under the TB. It is possible the gasket has sprung a leak. Cheap and easy to replace, but don't just lean on the screws to remove the leak as that can bend the TB ears and cause even more problems.

This is a very good tip. It will simulate the small-hose leak that I was talking about earlier.

Learned to check that back in 94. My old Chevy C-10 pulled that stunt several times. Thing is on a carb it causes you to die at idle.

OK....thanks for all the advice I will check it all out-very helpful....Larry

I had a variant where the hole in the diaphram was so large that the engine stalled when the brake pedal was pushed.