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C5B 07-09-2013 01:37 PM

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

DaemonForce 07-09-2013 04:47 PM

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.

jnicewan 07-09-2013 08:57 PM

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.

C5B 07-10-2013 10:40 AM

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.

bgredjeep 07-10-2013 02:47 PM

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.

C5B 07-11-2013 07:52 AM

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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