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Unread 07-09-2013, 02:37 PM   #1
C5B
Registered User
1992 YJ Wrangler 
 
Join Date: Jul 2013
Location: Fredericksburg, VA
Posts: 4
School of Hard Knocks

For the smart guys!

'92 Wrangler with 2.5L and AX-5. Its got 170,000 miles, but the motor ran like a watch. A year ago, a family member whom shall go unnamed, unwittingly drove away with it while the oil pressure sensor was uninstalled, waiting for replacement. The results were predictable, it made it about 3 miles, 5-10 minutes, before it stopped running. Once towed back to the house, the current knocking sound introduced itself upon restart. BTW, the video may feature the knocking slightly more strongly than it sounds in true life.

Assuming the bottom end to be wiped, I dropped the pan, and replaced the main and rod bearings, with a standard size set. Surprisingly, though some of the bearing were worse than other, and the main thrust bearing worst of all, the crank journals checked out pretty good, both on the micrometer, and using plastigage. Engine compression checks out good, with a 2.5% variation centered on 115 psi (one compression stroke, throttle open, all plugs out). Oil pressure holds 20 psi when hot. I have run it with the valve cover off, all actuation looks good, and was unable to damp or affect the sound by applying pressure on any of the rockers. Engine idles steady at ~700 rpm, at a steady 17.5 in. vacuum reading. Spark plugs look good, and the engine still runs smoothly, with about the same power as before. When I had the pan off for the bearings, I could not see anything obviously amiss with the rods/pistons/wrist pins (admittedly tough to see, but nothing clearly loose). Like wise, couldn't detect obvious damage to what could be seen of the cam lobes. I have had my stethoscope all over the motor, and cannot positively find the origin, though it may be coming from #3, or between #2 & #3.

I have not yet had the head off, or the front cover (would rather not). Is it a cracked piston, damaged valve train, or wasted wrist pin? Something else? How to check? At this point, I am continuing to run the motor on local trips, and it doesn't seem to be getting substantially worse (what's to lose?).

Thanks,
-C

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Unread 07-09-2013, 05:47 PM   #2
DaemonForce
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1983  
 
Join Date: Jun 2013
Location: Centralia, WA
Posts: 114
Wow, sounds better than my 258 after someone in my family ran it completely out of oil right before taking a 75 mile trip. Blew the head gasket and likely caused more damage to an already cracked block. It's not good.
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Unread 07-09-2013, 09:57 PM   #3
jnicewan
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1995 ZJ 
 
Join Date: Dec 2010
Location: Norman, OK
Posts: 2,079
Unplug injectors one at a time and see if noise goes away, that might point in the direction of a suspect cylinder. Its hard to tell what noises are through little laptop speakers but kind of sounds like a lower end knock.
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Unread 07-10-2013, 11:40 AM   #4
C5B
Registered User
1992 YJ Wrangler 
 
Join Date: Jul 2013
Location: Fredericksburg, VA
Posts: 4
Great idea on the injectors. Still not used to thinking in terms of MPI rather than carbs. Result: Sound maybe a little attenuated when #4 disconnected? Tough to tell, but definitely still present.
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Unread 07-10-2013, 03:47 PM   #5
bgredjeep
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1987 YJ Wrangler 
 
Join Date: May 2005
Location: Augusta, GA
Posts: 1,806
Sounds like my truck. (CTD)

The last time I had a gas engine sound like that I ended up draining 3 gallons of water/oil mix from the oil pan. Blown head gasket on an old s-10 blazer dumped all my coolant into the oil.
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Unread 07-11-2013, 08:52 AM   #6
C5B
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1992 YJ Wrangler 
 
Join Date: Jul 2013
Location: Fredericksburg, VA
Posts: 4
Currently no symptoms of blown head gasket. Oil clear, no overheating, compression strong, no steam out of the exhaust, no signs of gasses in the coolant. On a Model A site talking about engine noises, it talks about cam timing gears describing a very similiar scenario. But the 2.5 uses chain and sprockets, no? Anyone heard of cam/cam sprockets/timing chain causing something like this? Remember, I've already been through the bottom end.
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