Anyone know what year they stopped using the knock sensor on the XJ 4.0? A buddy of mine thinks he has lifter noise, but it only happens at power, and it is nice and quiet at idle. I think the lifters are fine; I haven't gotten in to it the diagnostic far enough yet, to say what is making the noise we are hearing, but I did notice that Napa didnÆt have a knock sensor listed for my '99 XJ, but they do have one listed for his '96 XJ, which brings me to my original question of when it was removed from the 4.0, why? I would assume something is programmed into the computer to compensate for it?

Thanks,

Sean

I'd betcha it is high compression from soot built up on the piston top. Run some valve cleaner for a few of tanks and it goes away most times.

A good test is to rum premium and see if the clatter goes away.. If yes it is the soot.
Big JIm

They always have a knock sensor... Napa my not carry one or if you got the pimpled faced teenager, he may not be the best source...

Modern EFI runs the spark the closest they can to detonation as possible in order to get the most performance/efficiency out of the motor as possible. Without the knock sensor this is not possible.

Have You checked for an exhaust manifold leak?
How miles are on the jeep?
Does the jeep do it under high RPM load or low RPM load? Does it have an oil pressure gauge? If so what is the reading?
Is he running the correct oil?
Does the jeep have it's normal amount of power?

LOL you asked a bunch of the same questions I was asking him last night, I just looked at it in the parking lot for about 15 minutes after pool league and then went for a 15 minute ride, so like I said I am just beginning the diagnosis process.




Since you asked, itÆs a 96 4.0 with the 5 speed, with 240K miles, running 10W30, it has great oil pressure according to the gauge, it stayed solid around 40 ish psi at idle, and went up to 55 ish psi at cruise. It only does it under high rpm load, say entering the highway, and then quiets up at cruise, then it does it again if you stomp on it at highway speed and when you pull your foot out, it goes away again. It does sound like detonation to my ear, and I had a few things I was going to have him try, but we havenÆt messed with it in day light yet. It still has plenty of power too. One of the first things I was going to check was if I could hook up the scan tool (not sure if jeep waited till 97 to go OBDII or if this one might be OBDII) and see if it was setting any codes, and then see how long its been since he gave it tune up. I really only went to the Napa web site to see what the sensor looked like so I could verify it was actually hooked up, I was just surprised to see it for the Æ96 and not see it for the æ99. I was thinking the running high test for a tank too, and see if there was any change. I did bring up the exhaust leak last night too before I went for the ride, and it doesnÆt sound like any exhaust leak I have ever had, but IÆll check that too.




This is what I was thinking as well, but I canÆt seem to find a knock sensor available for a Æ99 4.0L anywhere.

Actually here is an interesting little blurb I just picked up off of Wikipedia, might have to do a little more researchà..


In 1991 Chrysler Corporation, then the owners of the Jeep brand, redesigned the RENIX engine control computer and raised the intake ports approximately 1/8″ for a better entry radius. Chrysler also enlarged the throttle body and redesigned the intake and exhaust manifolds for more efficiency, and the fuel injectors were once again replaced with higher flowing units. The camshaft profile was also changed. The net result of all these changes was an engine that made 190 hp (142 kW) and 235 lb-ft (312 Nòm) of torque. Badging on Jeeps equipped with this engine read "4.0 Litre HIGH OUTPUT". The new cam profile combined with altered computer programming eliminated the need for an EGR valve and knock sensor, but make the engine more sensitive to alterations, especially where emissions are concerned.

Clatter seems to be a problem with 3 and 4 litre Ford engines.. The fix for them is cleaning off the tops of the pistons with valve cleaning agents. Everyone that has complained on the Ford site I attend and has used 3 or 4 tanks with the cleaning agents in them has lost the clatter noises.
Big JIm

Jim, I hear ya, and I agree on the carbon build up on the pistons as a likely possibility. I really didn't think I knew enough info wrt his vehicle to come here and ask questions about the noise yet, that was just a byproduct of my actual question of when and why the knock sensor went away between Æ96 and Æ99 4.0 XJ. Because I had never heard of an EFI vehicle opperating with out one either.

IÆll hit you all back with what I find out, and with what resolves his issue.

<img src="/forums/images/graemlins/cheers.gif" alt="" />

Sean

The NAPA listing is wrong, there is no knock sensor on any '96 4.0l. They were only used on '87-'90 XJ Renix 4.0l as Bigbird79's post said earlier.

On '96 4.0l engines, TSB 18-24-98 applies to reprogram (Reflash) the PCM to eliminate light spark knock under load. What this reprogramming does is allow the timing to be retarded up to 4 degrees from base timing using the DRB3 scan tool. High end aftermarket scan tools may be able to retard the timing, too after this flash update. Note: If your knock is heavy, the TSB recommends further diagnosis.

Like BigJim said, cleaning the carbon from the pistons can be very helpful. Mopar sells a very good combustion chamber cleaner, you can get it at your local dealer.

I've had cracked flex plates sound like a knock or heavy ping, too. You can usually isolate one by loading the engine against the brake with the trans in gear and listening for the noise at the bellhousing area.

Exhaust leaks from a cracked manifold are usually more pronounced when the engine is cold and under a load at low rpm.

Hope this helps!

Thanks Marty. I was coming to the same conclusion that the Knock sensor went away in Æ91, but I wasnÆt quite to that point yet. IÆll have to take a look at the TSB too, and see if his computer has been reflashed yet. I would hope it had been done by now. I only see the buddy once a week at pool league on Tuesdays, so itÆs a slow diagnostic process right now.

So far my current plan for when I see him Tuesday is to start with a tank of high test, and if that makes the noise go away, then a few tanks of regular with some cleaner should do him fine, provided the computer has been updated. If the high test doesnÆt take care of the noise, I will be back here with more info after I have poked around under the hood for more than 15 minutes.

I have sprayed water mist down a carb of a small block Chevy and Ford while running before, and that seemed to clean everything out of the cylinder as well too. Anyone ever used this method on an EFI engine?

I was leaning away from a cracked header, because like you said it wasnÆt there when cold at idle where I would expect a cracked header to show itself, but it was dark and I didnÆt look closely then, so it hasnÆt been completely ruled out yet.

As for the flex plate crack, this is a 5 speed, not an Automagic, so that wont be a problem.

<img src="/forums/images/graemlins/cheers.gif" alt="" />

Sean

I don't know about you guys...but I'd be coveting them 4*.. I'd run the chamber cleaner a while before setting the timing back. I've seen 4* make a few more horses in chevy engines. So I'd be reluctant to retard the timing from the original setting.
Big Jim

Which is why I want to read to the service bulletin TSB 18-24-98, to find out the specific details. And also why it really surprised me Chrysler stopped using the knock sensor, it has always been my understanding, the ideal situation is to have as much timing as possible.

Sean