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Old 02-17-2008, 08:59 AM
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Compression problems

Hey guys I was wondering what the actual psi for a cylinder on a 1968 390 2v?
because my engine wont run so i checked spark, timing, valves and the fuel system and there all fine so i went to my friends to barrow a compression Gage. Well i hooked it up and my engines psi was around 60 psi doubtfully enough to even run it. So my question is will it still run? And is it the rings or the valve seals on the heads or would it be good to replace both?

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Old 02-17-2008, 11:14 AM
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Factory new compression pressure is 180#. The 60# you have is not enough for it to start with the starter. If you have a manual transmission, you may get it going with a steady push down the road, in gear with the key on. Has the engine been sitting for a while? If so the rings are probably stuck. The only way to tell for sure if its the rings or the valves is to do a dry, then wet compression test on all cyls. one at a time. The wet test consists of squirting a couple shots of engine oil in the spark plug hole then test compression on that cyl. If you have a substantial increase in compression, it will be the rings, if no major change, it will be leaking valves. DO NOT try to road start by towing--You have a 99% chance of rear-ending the tow rig if it starts.
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Old 02-17-2008, 11:19 AM
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Most engines have 140 to 160# of compression,
w/no more then a 10% difference between cylinders,
A low reading on one cylinder is usually an E-valve
low compression in two cylinders adjacent to each other is usually a head gasket, low compression in all cylinders usually means a rebuild is necessary.
When you do the test, you should have all the spark
plugs out & the throttle wide open, & the coil grounded or discoed.
Good luck,
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Old 02-17-2008, 11:25 AM
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If a little oil in the cylinders doesn't bring up the compression and it is still about #60 across the board , I would give the timing chain a good look .
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