btz0

Sorry to add another knocking thread to the forum, I have searched extensively the last few weeks as to avoid having to make a thread about this but I haven't had much luck finding an XJ with similar symptoms. I have found many videos that sounded the same to me, piston skirts, piston slap, cracked/loose flexplate, rod knock not so much, lifters not so much, basically it's rather difficult to diagnose. I will pull the valve cover off when I have time but it is currently my DD so I can't take on anything that will take too much time.

Anyways, I digress. My XJ makes a knocking sound, sounds like a tapping coming from the front of the engine, not so much the bottom end. Haven't got the stethoscope on it, but it is marginally quieter from below the Jeep. Sound is loudest on passenger side of engine. When my XJ is cold, it knocks constantly at idle. It's not a super loud knock like a broken skirt or a bad rod, but it is noticeable. Once my XJ sits at operating temp for a bit, the noise fades away to the point that it is barely noticeable. From what I've read that's just pretty well normal piston slap on these motors.

The issue is though, if I rev the engine manually I can clearly hear the knock in the low RPM range regardless of temperature. It increases in frequency with engine speed, and decreases when the motor decelerates, but this only happens in the low RPM range. Beyond about 1500 rpm or maybe even less, hard to tell as I can't see the gauge, it completely disappears. Whether it's being drowned out by drivetrain noise or if it's actually gone is hard to determine.

Other issues are also present, but not sure if they are related. Here they are:

Relatively erratic idle - Bounces up and down 25-50rpm constantly, likely due to the multiple exhaust leaks. Working on getting this fixed soon.

Cylinder 2 misfire at idle: Jeep misses at idle. Never has missed under load. Code only happens every few days, clears itself then comes back. Has only ever happened while idling when cold. Wondering if it's related to the knocking sound. (new spark plugs, new wires, new cap and rotor, next item on list will be injector upgrade)

Occasionally stalls out when started - Only happens in cold temps. By cold I mean below -20C. I live in Canada so I assume that this could just be due to temperatures. -10C and above it has never stalled, turns over very fast and finds idle right away. Can prevent it from stalling by getting on the gas.

Other than that, this jeep is great. Oil pressure is good, but occasionally drops at idle. Has never bottomed out. It's strange, sometimes it's fine at idle and other times it gets a little low. Wondering if it's just a sender or something, not really a big concern. Perfect pressure when cruising. Doesn't burn or leak oil. Doesn't burn or leak coolant.

No discernible power loss. Hard to tell as I bought the Jeep with this knock, but for a box on wheels it seems plenty powerful. Shifts at redline when to the floor and has no issue putting the power down. Much more power than my old XJ on 33s with stock gears (terrible). Fuel mileage seems alright, haven't had it long enough to know.

I have confirmed there are multiple exhaust leaks. Jeep sat for 2 years before I bought it, recently replaced thermostat. Original owner said that it was overheated before. Haven't had any issues with coolant loss yet. Recently changed oil, was dirty but no metal shavings or anything. Jeep is a 1998 XJ with only 170000km on it, or 105000 miles. Any insight would be appreciated, I am sure I am leaving information out. Sorry for the long post, trying to be thorough as best I can.

CCKen

Remove the distributor cap and check for damage to the rotor and cap.


Check to see if there is excessive play in the distributor shaft by moving the rotor from side to side, rotationally, and up and down. The distributor shaft may be the knocking you hear, and the cause of rough running.

btz0

I checked the distributor today and everything checks out. The shaft only moves slightly. I put a stethoscope on it and heard nothing like the sound I am hearing from the engine. The knocking noise was most clearly heard with the stethoscope through the exhaust manifold, and it was heard evenly throughout the valve cover. Also could hear it loudly on the injectors but not sure if that's relevant. All injectors made the same sound.

Any other ideas?

CCKen

When the engine is cold, you may want to remove the valve cover and examine the valve springs, rockers, and pushrods. Turning the crankshaft at the damper until each cylinder comes to top dead center compression to relieve tension on that cylinders valve train.


Look for broken valve spring(s) and or excessive play between the push rod(s) and the rocker(s). You may have a defective lazy lifter that is not pumping up until the engine heats up - especially on #2 cylinder.


Go back on with a Felpro (Blue) gasket.