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~75psi compression all cylinders
well my jeep is stuck here at college and its our class project now, and everyone is beatin on the ol 4.0 now! the symptoms are:
-rough idle -long cranks -check engine p0301 (cylinder 1 misfire) -no power until warm, but not near what it should be! anyways we did a dry compression test and got 74-80psi on all cylinders. already put new head gasket in before coming to college, and didnt change anything. could all the valve springs be shot? or cam timing barley off enough? help! |
Haha.. 2000/01? Sounds like a bad head..
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oops forgot that part..... no its a 98 which is what the 0630?
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Originally Posted by truckntractorhick
(Post 2021526)
anyways we did a dry compression test and got 74-80psi on all cylinders. already put new head gasket in before coming to college, and didnt change anything. could all the valve springs be shot? or cam timing barley off enough? help!
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Originally Posted by truckntractorhick
oops forgot that part..... no its a 98 which is what the 0630?
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The compression spec for the Jeep 4.0 engine is 120-150 psi, with no more than a 30 psi variation between cylinders.
You absolutely must try a different compression gauge to verify your numbers. It is not that unusual to have a fault compression gauge. Here's more on the procedure, and yes....throttle OPEN. http://lateblt.tripod.com/carcomp.htm |
Thanks for the link tjwalker, one thing I don't understand though: why is grounding the high side of the ignition coil important?
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With a different gauge and following that procedure and it's still that low --
Suspect very loose or jumped timing chain. |
Not sure about how the test was done.... luckily labor at my college is free so only parts= $. I know they use only snap-on tools and hopefully they did open throttle all the way idk..... they're gonna hopefully do a running test on Monday.
Edit: could it be all the valve springs? |
how many miles on the engine? also do a wet test. and check it when warm also. can be piston rings. use a boreascope to see inside the cyl and look at valves also. what kinda head gasket you useing? check timing with timing gun (snap-on) or good brand.
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one thing I don't understand though: why is grounding the high side of the ignition coil important? 1. with no load (sparkplugs) on the coil, the coil output voltage will go very high, most likely causing insulation breakdown in the coil - meaning you'll need to buy a new coil. 2. if you get too close to the un-grounded high voltage wire while cranking, you will come up with some cuss words you never knew you knew. You don't even need to touch it - it will reach out and touch you. check timing with timing gun (snap-on) or good brand |
Originally Posted by salad
(Post 2021801)
Thanks for the link tjwalker, one thing I don't understand though: why is grounding the high side of the ignition coil important?
eta: I see big h beat me to it, lol |
Originally Posted by the_big_h
(Post 2021957)
Two reasons:
1. with no load (sparkplugs) on the coil, the coil output voltage will go very high, most likely causing insulation breakdown in the coil - meaning you'll need to buy a new coil. 2. if you get too close to the un-grounded high voltage wire while cranking, you will come up with some cuss words you never knew you knew. You don't even need to touch it - it will reach out and touch you. The spark timing is not related to the low compression. The spark timing is set by the notches on the flywheel, not by the distributor position (like it was on older non-EFI engines. You used to be able to tell if the timing chain skipped a tooth if the timing was way off - but not with the 4.0) |
I put a felpro composite gasket in... I'm hoping its the mechanical timing of the engine because it did back fire once when I downshifted on the highway.
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Originally Posted by Radi
(Post 2021965)
You probably have the spark plug wires disconnected, with no load on the coil it can generate voltage much higher than normal when you crank and damage itself.
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