jayo310

Hey guys. My 99 XJ Sport has been acting funny as of late. With the occasional putt-putt of the exhaust along with the even more occasional cylinder 1 misfire code (sometimes on, most of the time off), my RPMs have just started to act funny.

The putt-putt of the engine usually happens when in park or neutral, but disappears once I put it in drive. It kind of sounds like a steam locomotive train. But this might be a separate issue. Now, my RPMs has been occasionally running rough from 400-700 at a stoplight, and when I throw it in neutral, the RPMs surge to about 1500. When I resume driving, everything feels pretty normal until the next stop light - putt-putts, rough idle, and surging RPMs. It goes away, then comes back.

Another funny thing... When I'm going around 40-50mph and let off the gas to cruise, I notice the RPM dropping like normal, then surges to 1200, then drops again, then surges, so on and so forth. This pattern repeats every two seconds.

I'm totally confused of whats going on here. I've done my due diligence with changing out to champ plugs, cables, distributor, fuel injectors, exhaust manifold, o2 sensors, cleaned IAC, no vacuum leaks, fuel pressure is good, cylinder compression is within range, and TDC is dead on.

Any ideas? I was wondering if its my TPS. I have a mopar CPS waiting for me to install. Thanks!

BruceB

Have you done a compression test?

jayo310

Yes. Compression is good, within range of acceptable.

Turbo X_J

Firing order correct?
Plugs gaped properly?
Check the intake manifold/gasket.

jayo310

Firing order is good .Plugs have been gapped correctly. And the exhaust manifold is new. Intake manifold is in great condition. Vacuum lines are good .

dave1123

Check the torque on the manifold bolts. They may have loosened after settling in. Sounds like the IAC compensating for a loss of vacuum.