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Unread 02-25-2009, 05:34 PM   #1
bilwil
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4.2 engine has strange knock.

OK forum brain trust!!
I am new to Jeeps,but have built a few motors in my day, but this has me stumped.....
Motor starts quiet then in 10 seconds or so starts knocking like a rod or piston. Pans off now and no metal in pan or strange looking piston's(viewed from down under). bearings look good, oil pressure @ 40 psi hot. Cam and lifters new, all rockers working( ran with valvecover off) checked with feeler gague while running. SO..... what did the old guy not think of, or what in a heep could make this tick tick in the lower end. Yea I should of pulled plug wires when it was running, just got into it thinking it was a piston...Has anyone heard of problems with wristpins? or the piston cracking on the top side of the wrist pin? or?
The motor is a remanufactured Mopar Claimed to have 80,000miles. Cly. walls straight ,1/2 thousand out, like new .040 over, .010 mains and rods. hope I covered everything.
OK guys let it fly>>>>>

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Unread 02-25-2009, 05:38 PM   #2
Burnerman
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Sounds dumb but, could it be a crack or leak in the exhaust manifold? Other then the timing chain I would've checked what you already have. Did you do a compression test?
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Unread 02-25-2009, 06:18 PM   #3
bilwil
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Exaust no leaks.New chain and gears, no compression ck, did clean up valves when I changed lifters, Lots of carbon, cleaned the valves and lapped the valves, guides were ok and reinstalled the old seals as they were in good shape. The tick/knock sounds come from right side like its around cly. 4 or 5 mid-block height. please allow for the old ears.
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Unread 02-25-2009, 06:44 PM   #4
jay-h
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load sensitive? Less or more loud with speed? You could try plastigage in the bearings to check

I had a (non Jeep) once that had a knock a few secons after startup, turned out to be a slightly loose damper pulley.
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Unread 02-25-2009, 06:56 PM   #5
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I'm pretty sure it's the piston wrist pins causing the knock. it's common in the 4.2's. My CJ had it real bad, so bad i pulled the motor and put in a 350.
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Unread 02-25-2009, 06:56 PM   #6
Burnerman
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A friend of mine at work had a CJ that broke the top rings. You said there is no ridge in the cylinders so I don't think that would be it.Did you try and push up on the pistons with a dowel while they were on the upstroke when you had the pan off? Any play would show a bad wrist pin. Short of pulling the head a dunno.....
Back when I was in HS I worked pumping gas at a local repair garage. The mechanic rebuilt a Chevy straight 6 for a guy and the engine had the same type noise. Everything was new inside that engine but you could hear a kinda "diesel" engine noise coming from the bottom end. I don't think he ever found out what the noise was. I do know the car ran fine otherwise.
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Unread 02-25-2009, 07:00 PM   #7
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sounds like a typical 4.2, just get a 4.0 ready for a swap, ur motors goin
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Unread 02-25-2009, 07:04 PM   #8
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Quote:
Originally Posted by scottsjeeprolet View Post
I'm pretty sure it's the piston wrist pins causing the knock. it's common in the 4.2's. My CJ had it real bad, so bad i pulled the motor and put in a 350.
could be wrist pins. Listen for load sensitivity. loud at acceleration/decelearation, quieter at idle. very loud when lugging.


If not load sensitive, look somewhere else.
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Unread 02-25-2009, 08:41 PM   #9
ctm
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Try this, old tech bulletin, do you have the 'new' style timing cover and cam pin/spring:

Camshaft Thrust Control On Chrysler 2.5, 4.0 & 4.2L (Jeep) Engines

AERA membership has reported a mysterious knocking noise on Chrysler 2.5, 4.0 and 4.2L engines used in various Jeep utility vehicles. The noise is loudest at the lower front area of the engine and has, on occasion, been reported in all of the above listed engines. The noise will disappear if the engine is operated at higher than idle speed.

Disassembly of the engine revealed that the camshaft gear was making contact with the timing cover. Recalling the engine's assembly, several machinists remembered encountering a bolt that had an 1/8 hole in it. Most were unsure where to install the bolt.

As of the 1991 production year, Chrysler has updated the engine to a new method of retaining the camshaft in the block. Prior model year engines were equipped with a camshaft thrust plate. This has been superseded by a special bolt, spring, plunger and front cover (see illustration). The plunger assembly maintains constant pressure on the camshaft, preventing it from moving
forward. Camshaft chucking will result if this thrust control assembly is not reinstalled. The pin rests against a small protruding pad in the front cover that is reported to be 1.000 from the gasket mounting surface.

A replacement Pin Package is available from Chrysler under the Part #83502890.

http://www.rebuiltautoengines.com/jeep-articles.html
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Unread 02-25-2009, 08:46 PM   #10
bilwil
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Jay-h
not load sensitive, seem to get louder the longer it runs, that said you can just hear it when driving inside the cab. stop at a stop sign or open the hood and it is very clear. but it always knocks when running. just quiet at a motor cold start.
I went out and checked the skirt clearance and they are tight. again the bearings are still in babbit no copper and not close, again great oil pressure.
I have to agree with SCOTTSJEPROLET that it must be a wrist pin, *#*^
CTM I have the pin and spring in ,KEEP Thinking The knock is around cly 4&5
Just hoping to leave the pistons in.

Thanks guys
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Unread 02-25-2009, 08:55 PM   #11
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Wristpin Wristpin Wristpin
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Unread 02-26-2009, 06:42 PM   #12
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Quote:
Originally Posted by bilwil View Post
OK forum brain trust!!
I am new to Jeeps,but have built a few motors in my day, but this has me stumped.....
Motor starts quiet then in 10 seconds or so starts knocking like a rod or piston. Pans off now and no metal in pan or strange looking piston's(viewed from down under). bearings look good, oil pressure @ 40 psi hot. Cam and lifters new, all rockers working( ran with valvecover off) checked with feeler gague while running. SO..... what did the old guy not think of, or what in a heep could make this tick tick in the lower end. Yea I should of pulled plug wires when it was running, just got into it thinking it was a piston...Has anyone heard of problems with wristpins? or the piston cracking on the top side of the wrist pin? or?
The motor is a remanufactured Mopar Claimed to have 80,000miles. Cly. walls straight ,1/2 thousand out, like new .040 over, .010 mains and rods. hope I covered everything.
OK guys let it fly>>>>>
Since you have the base off brig each piston to the bottom of it's stroke. Now move the crank and watch the piston move in relation to the connecting rod. Any wrist pin issues should show up when the piston doesn't move with the connecting rod.
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Unread 03-03-2009, 12:36 PM   #13
bilwil
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Good job "forum brain trust", # 6 Wrist pin piston galled on one side/end only. The piston / rod assy felt just like the other 5 assy's. only way to tell was to remove the pin from the piston / rod. again THANK'S to the jeep forum!!!
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Unread 03-03-2009, 03:24 PM   #14
cyco
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i had a chevy like that, was a crack in flywheel, after it warmed up in would flex an sound like a rod knock....was automatic
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