I have a 98 ZJ with a 5.2. For some reason the rpm fluctuates while it idles. This doesn't happen all the time but enough to spark my curiosity. The plugs, wires, rotor, cap, and coil are all new. I run cleaner through the injectors regularly. Any ideas on t
what might cause this?

 

Is the defrost or A/C on while this happens?

 

Welcome to the forum!

When was the timing chain last changed?

 

The heater was on last time it happened and I doubt if the timing chain was ever changed. It had 230,000 on it when i bought it and now has 256,000. I have replaced a lot of part that were obviously original. It also has a whistle under the hood from time to time that I suspect is a vacuum leak but it is right behind the A/C pump and I dont see any vacuum lines there.

 

Wadaya think, Busted? Plenum gasket?

 

That is what I'm thinking. It whistles more when it is cold and then goes away as it warms up but doesn't always make a sound. some days yes, some days no.

 

Most likely.

 

Great! Thanks guys..