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Old 09-07-2013, 09:52 AM
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Low compression and knock on #8 cyl 4.7liter

ok so i bought this 2001 jeep grand cherokee with the 4.7 v8, upon driving it home it overheated on me so i shut it off got a tow and let it cool off overnight.
Next day i figured out cooling issue and did a comp test to see if anything was terminally damaged, 130 psi-135 across the board except #8 was only 75, plug was wet with fuel but obviously not firing. coolant is clean and oil is clean. has a noticeable tick at idle and when driving it around 2500 rpm it starts to knock fairly loudly, but does not knock when throttling up when parked only when under load. other than the knock and the slightly lumpy idle you wouldnt know anything was wrong, id really appreciate any help, i will pull valve cover off if i have to but it looks like a huge mission, as its buried under ac lines, wiring harnesses etc.
Tanks again,
Will

P.S apparently this is a crate motor with only 10 000 k on it, at least this is what the guy told me. Needless to say he probably lied since it didnt even get me home the night i bought it.
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Old 09-07-2013, 12:27 PM
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Re: Low compression and knock on #8 cyl 4.7liter

sounds like a blown head gasket started all this
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Old 09-07-2013, 01:26 PM
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Re: Low compression and knock on #8 cyl 4.7liter

im hoping thats not the case as there is no oil in coolant or vice versa, it no longer overheats, the dude had forgotten to hook up the catch can tube from the rad and i never noticed it when i looked under the hood, thats why it overheated on the drive home, now that thats fixed its not losing coolant so im hoping head gasket might still be ok, cylinder beside #8 still has good compresson also.
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Old 09-12-2013, 04:04 PM
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Re: Low compression and knock on #8 cyl 4.7liter

Can you hear if the knock is coming from the upper end or the lower end? These engines can/will warp heads when overheated, but they can also drop a valve seat when overheated. If that happens, the rocker arm will fall off and will be laying on the head. A dropped valve seat can cause severe damage to the cylinder and piston. I would remove the valve cover first and see if both #8 cylinder rocker arms are in place.
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