4.2l engine knocking

mbalbritton

mbalbritton

#@$%!
Joined
Mar 22, 2005
Location
Lakeland, FL
Great…..

New to us Jeep fires up and runs fine. When cold, she purrs away quietly and sits at about 50psi on oil pressure.

After a few minutes of running and starts to come up to temp, a knock becomes audible. Louder as rpm’s increase. But quiet at idle. Oil pressure drops off and with a hot engine will read 0 at idle and about 20-30psi up around 2-3k.

It doesn’t knock at idle, so I’m wondering if the advance isn’t working and I’m detonating? I would think rod knock would happen all the time. I haven’t pulled the valve cover yet to check valve lash and rods. Other ideas??
 
I would drop the pan and look for a cracked piston skirt. Oil psi dropping is going to be bearing wear. Probably cam bearings.

Time for a rebuild or a 4.0 stroker swap.
 
Jeff B said:
I would drop the pan and look for a cracked piston skirt. Oil psi dropping is going to be bearing wear. Probably cam bearings.

Time for a rebuild or a 4.0 stroker swap.
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Mine had the same symptoms. Ran it for several more years, then the piston skirt finally failed in a big way and she was done. If it's anything like mine you either have some time to come up with a repower plan or you can do a quick in frame rebuild.
 
no head work?

www.cleggengine.com

AMC L-6 258-4.2L Master Engine Rebuild Kit, 1971-1990

This Master Engine Rebuild Kit Includes: Complete Gasket set (Including seals), Pistons, Rings, Rod Bearings, Main Bearings, Cam Bearings, Oil Pump, Freeze Plugs, Cam Kit (Camshaft, Lifters), Timing Kit
www.cleggengine.com www.cleggengine.com

when I built my stroker. it was 30$ a hole to bore it and they cleaned the block which was iirc 50$ I guess with disassembly and re assembly and checking the connecting rods for round along with cam bearings it's not to far off.
 
Jeff B said:
no head work?

www.cleggengine.com

AMC L-6 258-4.2L Master Engine Rebuild Kit, 1971-1990

This Master Engine Rebuild Kit Includes: Complete Gasket set (Including seals), Pistons, Rings, Rod Bearings, Main Bearings, Cam Bearings, Oil Pump, Freeze Plugs, Cam Kit (Camshaft, Lifters), Timing Kit
www.cleggengine.com www.cleggengine.com

when I built my stroker. it was 30$ a hole to bore it and they cleaned the block which was iirc 50$ I guess with disassembly and re assembly and checking the connecting rods for round along with cam bearings it's not to far off.
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Includes head too. I thought that was the difference in short vs. long nomenclatures.
 
mbalbritton said:
2-3 weeks and $2600 for along block rebuild of my 4.2l sound about right? I pull the motor, and drop it off with them and I reinstall in the Jeep.
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Yeah, sounds about right, maybe a hair cheap.
 
For that money I would think you could junkyard v8 swap it and have more performance...
 
mbalbritton said:
So, go from one unknown to another. Cool. 😂
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The 4.2 isn't an amazing engine by modern standards. I tried to convince myself otherwise with mine and wish I would have saved the cost and done a swap sooner.

I had the Mopar multipoint injection and several other mods to mine. It was torquey at low rpm, but still gutless as a daily driver, and in the end I only got 118k miles before it blew.

I'm only encouraging you to explore other options for the same money or less before you spend $2,500 and still have 1970's technology. I would think you could at least by a running donor vehicle with a fuel injected 4.0 and have everything you need for a swap then part out or scrap the donor and come out much further ahead in performance with less investment.

Now if you have a period correct creampuff that you want to keep original then by all means carryon with the 4.2...
 
Clubbs said:
The 4.2 isn't an amazing engine by modern standards. I tried to convince myself otherwise with mine and wish I would have saved the cost and done a swap sooner.

I had the Mopar multipoint injection and several other mods to mine. It was torquey at low rpm, but still gutless as a daily driver, and in the end I only got 118k miles before it blew.

I'm only encouraging you to explore other options for the same money or less before you spend $2,500 and still have 1970's technology. I would think you could at least by a running donor vehicle with a fuel injected 4.0 and have everything you need for a swap then part out or scrap the donor and come out much further ahead in performance with less investment.

Now if you have a period correct creampuff that you want to keep original then by all means carryon with the 4.2...
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Don’t disagree with your logic and appreciate the thought. It has crossed my mind.

This is the wife’s Jeep (for now) and she’s just going to be running around town (5 mile radius) and honestly, she does need to be slowed down :D

But it is the “for now” comment about that makes me think a newer 4.0 with a AW4 sure would be nice.
 
Clubbs said:
The 4.2 isn't an amazing engine by modern standards. I tried to convince myself otherwise with mine and wish I would have saved the cost and done a swap sooner.

I had the Mopar multipoint injection and several other mods to mine. It was torquey at low rpm, but still gutless as a daily driver, and in the end I only got 118k miles before it blew.

I'm only encouraging you to explore other options for the same money or less before you spend $2,500 and still have 1970's technology. I would think you could at least by a running donor vehicle with a fuel injected 4.0 and have everything you need for a swap then part out or scrap the donor and come out much further ahead in performance with less investment.

Now if you have a period correct creampuff that you want to keep original then by all means carryon with the 4.2...
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I 100% agree with this. No way I would spend that kind of money rebuilding a carbureted 4.2. I would buy a cheap XJ, use the 4.0 from the XJ and the crank from your 4.2 and build a stroker. Sell what you can off the XJ, then throw the rest of the 4.2 inside the carcass of the XJ and scrap it.
 
I get the point. But there's also another consideration. I do not have the resources nor do I have the space or the location to do something like that. I'm also not looking for a performance machine, but rather something to drive. and this jeep made it 37 years on this motor.

These are all valid points and being considered.
 
mbalbritton said:
I get the point. But there's also another consideration. I do not have the resources nor do I have the space or the location to do something like that. I'm also not looking for a performance machine, but rather something to drive. and this jeep made it 37 years on this motor.

These are all valid points and being considered.
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In that case, my machinist has a 4.2 he took out of a running CJ to swap in a v8. It's been in his shop for a couple years. I think he wants $300 for it.

Are you in Lakeland fl? If so I'm driving to Tampa next week
 
Noel said:
Soooo…reseal pan and a new chain kit?
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I don't know. Could the loose timing chain sound like a knock or be the culprit of the knock, or just another worn out part out of many things in there. I'm halfway tempted to replace the oil pump and timing chain and see what she does. But my gut says the knock would still be there. First I'll rotate the crank and check everything as good as I can. If I don't find anything I may go that route first. those are parts that would need to be replaced anyway.
 
That chain is obviously shot. There should (have) be a rubber isolator in the cover that keeps it from slapping the cover. It's likely long gone.
At this point, I'd pull one rod cap at a time and inspect. If the bearings look good, it would be hard for me not to swap the timing set and a new oil pump and see what happens.
However, AMC sixes are prone to cam bearing wear with age and lots of oil pressure can be lost there.
 
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