Lower end knock, Help!

[MJ Madness 87]

Hey y'all, long time Jeep owner, first time 4.0 owner, question, I've got an 87 Chief with 127xxx miles on it, granted not all of those miles were gentle, it's got a 10" lift and all that, well it's had royal purple ran through her since day one, recently i've had some financial difficulties and can no longer run synthetics, had to downgrade to a conventional oil, after changing the oil from royal purple to Kendall HD40 i'm getting a slight engine knock from the lower end, it's only at idle and low RPMs and only when it's warmed up, when it's cold in the morning it purrs like a kitten, I can't figure out for the life of me what it is. Should I be alarmed or is it something that y'all think will work itself out over time? I've still got normal oil pressure, haven't lost any power that i can tell, and doesn't seem to be causing any issues, so i don't think it's a rod knock but idk.

[sinkrun]

Oil pressure?

[big66440]

Could be a lifter tick? my Comanche's lifters are VERY picky with the motor oil I use and how clean it is. Do you have a mechanics stethoscope you can use to pin point where the noise is coming from? One trick I've seen done to check for "rod knocks" is to disable the cylinders one by one while the engine is running and making the noise ( simply disconnect and then reconnect the injectors one by one) and the one rod that has a spun bearing will quiet down while that cylinder is disabled.

[Megadan]

Synthetic oils, even of the same viscosity, flow more easily than conventional oils.    My engine does the same thing, cold it runs without a tick or knock, but once it starts to warm up it gives a mild knocking sound from the bottom end.  My lifters are fine, my pushrods are straight, and I've checked and rechecked my rocker arm torque specs multiple times.  I've come to the conclusion that I have an odd piston slap issue that only happens when warm, which is kind of backwards.  I've just learned to deal with it. I still have decent compression and it's pretty even across the board.  Lots of guys on here and other jeep forums will say they have the same thing.

FWIW, synthetic on a budget, run Rotella T6 10w40.  My engine absolutely loves this stuff, and you get get it at wally world for about $20/ 5qt jug (or cheaper even).  It's a great oil, and lots of guys on here use it too.

[yxmj]

Just for info...is it an auto trans....maybe it is just a cracked flex plate

[Zebvance]

I had a what I thought was a knock, then I found out the start was loosed and knocking around at idle. tightened the bolts up and it was fixed. I would start checking everything if I was you first. 

[FrankTheDog]

Friend picked up a 2000 XJ for $75 delivered.

Mechanic he bought it from said it was knocking real bad. He went to swap the motor and when he went to loosen the torque convertor bolts he found them all loose.

He asked me what he should do because the engine was already to pull.

I told him to put it all back together and start it and see. No more knock.

My 92 MJ has a slight knock that was much worse when I bought it 5 years and 100k ago.

I just kept driving it and figured I'd worry about it when it failed. Hasn't yet.

[gogmorgo]

Another word on the above. My MJ knocked sporadically when I first got it. Took me a while to figure out where it came from, as the knocking was also accompanied by symptoms similar to a fuel pump failure. Turned out the bellhousing bolts had come loose, and that fixed the knock.

Six months later I noticed another knock. Turned out it was a cracked flexplate. It ran and drove fine, and took me about six months before I got around to replacing it, and the new flexplate cured the knock.

Three months after that, it started knocking again. My oil pressure had been low for a while, but I pushed it a little too hard and the the knock started. It was also accompanied by the magical appearance of glitter in the oil. Unlike the other knocks, this one got worse very quickly.

 

As long as your oil is free of glitter, you'll likely be fine, although the knocking does indicate something's not right somewhere, and you'll probably still want to look into it.

[MJ Madness 87]

Thanks for the help everyone. And to answer mj junkie it's a five speed.