98 XJ Idles rough sometimes

[Todd799]

My 98 XJ 4.0 idles rough, but only sometimes. Runs great 90% of the time. Sometimes when it starts up it will miss real bad and stall, after I start it again its fine. Sometimes when its just idling it will drop to about 500 rpm and start missing. It seems like the computer is trying to fix it cause it will rev up but then drop back down. I haven't had any problems driving usually on start up or just sitting and idling.

[logdog]

CPS is going bad.

[Todd799]

Thanks I'll try that and let you know how it works out

[joey99jb]

Sorry I am a noob to all these anacronyms, but what does CPS mean? I too have a 98 XJ and sometimes at idle it gets rough but, but only sometimes like Todd779. Mine drops in rpms and then come back up. Mine has never stalled before but if I fix the CPS will this "idling" problem go away? I guess its the same question. Thanks.

Joey B.

[jneary]

the crank sensor usually fails when hot. if you dont have a crank sensor fault stored, dont waste your money. i would suspect the throttle body is gummed up and possibly the ais motor may be failing. remove the ais motor and thoroughly clean the air passage as well as the throttle body itself.

[johnnyc]

i would suspect the throttle body is gummed up and possibly the ais motor may be failing. remove the ais motor and thoroughly clean the air passage as well as the throttle body itself.
That's what I was gonna say. I just took off and cleaned my TB last night. I took the IAS motor off and cleaned it up (it can get gunked up).

While I was doing this, I discovered a vacuum leak at the manifold. But that's not relevant. Or is it? Anyway, it's running very smoothly now.

[johnnyc]

Sorry I am a noob to all these anacronyms, but what does CPS mean? I too have a 98 XJ and sometimes at idle it gets rough but, but only sometimes like Todd779. Mine drops in rpms and then come back up. Mine has never stalled before but if I fix the CPS will this "idling" problem go away? I guess its the same question. Thanks.

Joey B.

CPS is Crankshaft Position Sensor. It's mounted on the bellhousing on the driver's side. It can be a real bear to reach. The connector ties into the harness right where the TPS connector is. Follow the wires down.

Not sure of the symptoms of a bad CPS, though. I was just gonna change mine out because I have 132K miles and I'm going on a long road trip next month.

[XJCasper]

Sorry for the stupid question. I plan to clean my throttle body this afternoon, among other replacement parts. How do I identify the AIS?

Thanks
Jason

[CharlesS]

The IAS is on the passenger side of the throttlebody.... about 3" long 1" diameter, held to the t-body with (2) T20 (I believe) Torks screws.

Charles

[Osprey413]

I have sort of the same problem with my '97. Sometimes it runs perfectly, sometimes the RPM's drop and then shoot up. This happens, atleast as far as i can tell, at completely random times. I can be driving down the highway and have it miss real bad and the Jeep will jerk or i can be idling at a stop light and it will suddenly start to jump around. sometimes it only does it once, sometimes it is so bad that the Jeep dies. My computer is throwing a code for "temporary loss of CPS or CKS" but i have already tried replacing the CPS (97's dont have a CKS, or i would have tried that too). Also, it seems like i can make it stop missing if i move it from drive to neutral. What exactly does the IAS do?

[XJ88NO]

I have this problem in 6 years with my 88XJ.I have rebuild all of sensors.
But the last was cps,after that the car have been ok,but now is the same again.I have buy many parts to my Cherokee in USA,so I can fix it near.
Very best
from
Per
88xj.no