Rough Idle question (1998 Cherokee)

I have recently cleaned my throttle body and still have a rough idle. The rough idle is intermittent and especially occurs when I am in park and then hold on the brake and switch to reverse or forward without pressing the accelerator down and let it sit for a second. When it happens, it is usually so rough it wants to just stall out if I don’t give it some gas.

Any ideas?



Sounds like you might have a vacuum leak. Maybe around the brake booster?

Is your check engine light on?

How many miles are on it? How old are the plugs & wires? Distributor cap? Fuel filter? If you have a means to do it you should check the fuel pressure. I would also clean the idle air control (IAC) valve which which is supposed to adjust when your at idle and load is put on the engine - such as when you put it in gear.

Describe better how it runs in park at idle. Is it only a problem when the engine is cold? If you just sit at idle in park and just press the brake does that do anything? Or is it only once the transmission engages?

You can also get this from an EGR system malfunction (even without check engine lights/error codes). Temporarily disconnect the vacuum line from the EGR valve & plug the line & port on the valve. See what happens.

Would an exhaust manifold leak that has been growing and growing over the past year or two cause this?

Also, it is a 1998 Cherokee Sport with about 223,000 miles so the fuel filter and pump are not accessible (they are in the tank I believe). How do I check the fuel pressure and what should it be?

When I cleaned my throttle body, didn’t I clean the IAC as well or is that something different?

Also, the problem I believe occurs whether cold or hot. If I just sit at idle and press the break, that doesn’t affect it. It will either be a rough idle in park, or the rough idle happens when I put it in Reverse/Drive.

Finally, please explain your EGR test/fix. You are telling me to intentionally plug my line??? What will this do and won’t this harm the car?

Thanks guys!

It sounds like you might want to drop $20 on a basic repair manual - available at most auto parts stores. (It will probably be a Haynes. You can get better ones online, but Haynes is ok for the basics).

To test fuel pressure you need a fuel pressure gauge. It screws onto a test port on the fuel rail. For the pressure specs, you get those from the manual, or google is your friend. Make sure you don’t have an external, in-line fuel filter before you ignore something simple.

The IAC generally bolts onto the throttle body. When your foot is off the “gas” the throttle plate is closed. The IAC spans ports before and after the throttle plate and puts the right amount of air through for idling. Those ports can get gunky and the valve itself gets gunky. When you go from park to gear (D or R), load in the engine is increased and the IAC has to adjust a little. If it isn’t then the engine is getting starved for air. You would have to remove it from the throttle body to clean it. You will probably need a new gasket for it. If you can’t find one you can cut one out of something like Karropak gasket paper.

Temporarily taking the vacuum line off of your EGR valve to find out if the symptoms go away will not harm anything. If you permanently disable it you can get pinging which can do engine harm over time. But simply pulling the vacuum line from the valve to find out if the symptoms go away is a 5 minute operation. Plugging the vacuum line - temporarily - is just to make sure that the vacuum the solenoid lets though through can’t act like a vacuum leak and mess up the “test.” Plugging the port of the valve - temporarily - is to make sure that if the valve is sticking open some, it can’t mess up the “test” in the same way. You could actually drive around that way for a while without hurting anything. But - yes - it is a terrible idea as a long term thing.

I cleaned up the IAC just now; when I cleaned the Throttle body last winter, I had neglected this because the screws were in an awkward position so I just had sprayed a bunch of Throttle Body cleaner in there in hopes it would clean up the IAC. Now that I properly cleaned it, seems to have fixed the rough idle. Thanks!

By the way, I didn’t notice any gasket on the IAC…should I go get one or just leave it really tight as it was since it took 15 years for it to cause a problem the first time?

The gaskets are normally some kind of paper-like material. Sometimes they end up stuck on both parts and are wrecked when you take the IAC off. Sometimes they adhere well to one and not at all to the other and are just left intact. In this case you might not have noticed and you’re probably good to go.

If you end up with a leak from that you’ll probably get symptoms like a vacuum leak - surging/falling idle. If you put it back on and it idles smooth, I’d just leave it alone.

Thanks for the follow-up. Glad you got it fixed.