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Old 11-14-2007, 11:21 AM   #1
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timing chain or main bearing?

Well I have an interesting one that is for sure. As many of you know I have a 1990 yj with the 4.2 and an ax15. I have a moderate knock towards the front of the engine near the bottom. I is quite a unique knock. It will only knock when excelerating and only from 2,000 rpm, to 2,800 rpm,s and it will also only knock in nutreal, 1st gear, and second gear. I am pretty sure it is not a rod bearing b/c they typically only knock when you take your foot of the gas. Any past experience or suggestions will be greatly appreciated.

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Old 11-14-2007, 03:11 PM   #2
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A good way to see if the timing chain is stretched out is to put a ratchet on the crank and with the distributer cap removed rotate the crand back in forth really slowly. If the crank moves and the distributer has a delay before it moves your timing chain is worn.You could also pull the cover and take a look at it. Most the time when main bearings or rod bearings are worn the oil pressure will drop a little or a lot. If you really want you could pull the pan and plastic gauge the main bearings. How many miles on it?

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Old 11-14-2007, 03:17 PM   #3
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Does it get any better or worse with the engine warmed up? Whats your oil pressure? How many miles on the motor? Any brass colored filings in the oil?
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Old 11-14-2007, 03:23 PM   #4
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There are 140,000. That is a good idea to check the chain I have not thought of that. Thanks for the advise. My oil pan is leaking so I think I will pull the pan and put rod a mains in. Its cheap insurance. I think I may go on and replace the timing chain as well. I have been holding out b/c I want to replace the factory 4.2 with a cummins 4bt. However I have my Camaro in the paint shop so this jeep motor needs to make it through the winter.
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Old 11-14-2007, 03:25 PM   #5
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The oil is clean. It does not do it when the engine is cold. and there are 140,000 miles on it thanks -Aaron-
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