1997-2001 XJ minor low end knock at idle question.

Hey guys. Was a member here a while ago but just created a new account since i forgot my old log in info. Anyway, I have a question. I buy and sell cherokees for a living. I have done around 200 or so in the past few years. I do all my own repairs. One common thing I run into often that i have never gotten an answer on is this. Half of the jeeps I check out have this issue where at start up they have this minor knocking noise coming from the oil pan. After the engine warms up it goes away for the most part. It seems to go away when you rev it as well but its hard to tell because its not very loud but definitely not a noise that should be there. On occasion this will be accompanied with ZERO oil pressure at hot idle. Its not lifter tick as i know the difference. But its also not a genuine rod knock either. In some cases ive pulled the plate off the bell housing to check for a lose screw on the TC but unfortunately i have yet to have that be my solution. Anyone know what this is and has figured out the issue? I know I said i do my own work but im not an ASE tech. Just a garage mechanic with a ton of experience with this engine. Thanks guys!

maybe piston slap?

2000&2001 4.0l cherokees have 0331 cylinder heads that have flawed castings and crack in the valve cover area letting coolant mix with the oil then destroying the bearings, thus the noise and low oil pressure.

What a weird problem. Only thing really different is that engine long brace at bottom of the crank. Not sure what year that started.

Could it be in the timing gear area?





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Green Mesa XJ said:

What a weird problem. Only thing really different is that engine long brace at bottom of the crank. Not sure what year that started.

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Stud girdle. They started using those in '96.

Green Mesa XJ said:

What a weird problem. Only thing really different is that engine long brace at bottom of the crank. Not sure what year that started.

Could it be in the timing gear area?

Click to expand...

No kidding. I cant make much sense of it. The only thing i can think of is wear in the crank bearings. When the engine is cold the oil isnt as thin when its hot so not as much oil is getting into the gaps between the crank and bearings thus causing a minor knock. When the oil heats up the flow is better so the noise is quieter or gone all together but due to the gap in the bearings the hot thinner oil is making it through much faster thus dropping in oil pressure. Thats my theory at least haha.

Its the craziest thing. Its on so many of them. Some people dont even notice it but because ive had so many its very distinct to me.

Oil filter choices? Many have no anti-drainback valve......

Are you sure it isn't piston slap? That seems pretty common with these engines and many of us have been able to run engines with it with no problems. I have read about people replacing pistons and curing it without too much trouble. Zero oil pressure at hot idle isn't abnormal for a high mileage 4.0, from what I've read, though it might mean it is time to move to heavier oil.

Paul

Mine has made the noise, kinda like a weird tapping/rattling noise? itll do it on start up, and sometimes I can hear a weird rattling noise when accelerating from a stop. and my oil pressure runs at 40 while driving but sometimes drops to 0 while idling. no idea what it is, I just turn the radio up, planning on building a 4.6L this spring