• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

rattle under load & slight rough idle

JeffU

NAXJA Forum User
Location
Atlanta
97 XJ with 120K miles. Complete new tuneup and sea foam through manifold. Also have run several different injector cleaners through gas. Still have a very slight rough idle at 750 rpm. Also engine knock/ping under load. Is the factory TSB flashing the computer for knock sensor setting the only fix?
Thanks,
Jeff
 
Do the newer XJ's have a knock sensor?

I have an '01 that had a rough cold idle. I don't know if the same PCM reflash that I had done applies to the '97. It didn't make much of a difference in the idle in my case. Fuel economy seemed to go down a little. I seafoamed it and the cold idle improved for about a week. I have been running Fuel Power every tank and the problem seems gone for good. I had to run several tanks HEAVY on the FP to get it cleaned up though. Techron concentrate wouldn't touch it, even after several bottles. New plugs did nothing. My engine combined with my driving pattern and the fuel I use seems to carbon up very quickly without the FP. YMMV
 
I’m having the same issue with my ’91. I can tell you what didn’t fix it: plugs, wires, cap & rotor, coil, TPS, CPS, MAP, IAC, PCV and new vacuum lines. I’m pretty much out of options at this point. If you find the magical cure please share the info. What is the factory TSB flashing the computer for knock sensor setting and how do I get it?
 
JeffU said:
97 XJ with 120K miles. Complete new tuneup and sea foam through manifold. Also have run several different injector cleaners through gas. Still have a very slight rough idle at 750 rpm. Also engine knock/ping under load. Is the factory TSB flashing the computer for knock sensor setting the only fix?
Thanks,
Jeff

There's no knock sensor on '91+ 4.0's. The factory do have a computer reflash for '96+ OBD II 4.0's that retards the timing to allow 87 octane fuel to be used without pinging. Unfortunately there's also a slight performance loss.
If you don't want that, you could try running premium fuel to see if the pinging goes. Then step down to 91 octane and 89 octane until you find the lowest octane gas that still prevents pinging.
 
ZmOz said:

Thanks, I didn't think so.

A little OT...

It sucks that the reflash may have retarded my timing. My mileage went down a little so that makes sense. Since it didn't fix my idle I want the old software back. My guess is the only way to do that is to buy a used PCM. I suppose I could call the dealer and ask them if they could put the old version on. Luckily I didn't pay the $53 to have it done. It would pay to buy one if I could even gain 0.5 MPG considering how much I drive.

This got me thinking. Will PCM's from other years work in an '01? Is there a best year? PCM's seem pretty cheap on ebay.
 
Back
Top