another engine knock

i went to to look at a 91 XJ(laredo with 4.0HO) with 160k on the motor. when i got there and the owner started it there was a immediate loud knock that seemed to come from under the intake. the sound stopped after about 3-4 seconds and it knocked once more when he revved the motor a little. he said it knocks like that when it is really cold and when it starts but after that it quiets down and no knock. i took it for a test drive and no more knocks from the motor?? would it be worth buying and what do you think it could be?(i have a jeep but its a wrangler so i know nothing about XJs) i did search and i could not find anything that seemed like the knock that was happening with this XJ. any help would be appreciated

Its either the intake or exhaust gasket. Sounds like an exhaust gasket to me. After the engine heats up, the metal parts expand and self seal the gasket leak.

check the oil in it, if its really low or empty dont buy it, also look for small metal shavings on the dipstick, good chance its just the lifter valvues getting the lack of oil which is a common problem

Possible it could be piston slap due to out of round cylinder walls.my 03 4.0 actually did this at 20,000 miles or so when it was real cold out it ended up breaking my piston from the oil control ring down to the skirt.I installed this motor from a salvge yard purchase with actual mileage vin # documentation.I replaced the piston and it still made a slight noise , so now i just let the motor run for a bout 5 - 10 minutes before i drive it.No more noise when it warms up.Now that it is warmer out i only need it to run fer 3- 5 minutes now.Just let it warm up before you drive it.Or it maybe a rod knock look at the oil pressure next time it starts up and see how long it takes for the oil pressure to build up.If it takes too long like over 5 seconds its more than likely rod bearings and/or main bearings.

the oil pressure was good and normal.

when it was started ? and how long did it take to build?

markaboo929 said:

when it was started ? and how long did it take to build?

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what?:dunno:

the oil pressure.When the ingine is first started,cold,look at how long the OIL PRESSURE takes to build up and what it builds up to like 40 - 60 lbs? IF it has an oil pressure gauge that is.You did mention it is good and normal but did not specify WHEN IT IS FIRST STARTED.