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.