360 Compression Test

Not yet, mid week it is supposed to be a bit warmer I will try to do it then, the way I have done compression tests in the past was warm engine, all plugs out, throttle wide open, one cylinder at a time. I have checked engine running fan belt off and oil filler cap off there was little blow by from the oil filler tube seemed to indicate the rings were sealing. I wonder if E10 gas plays into valve life when an engine sits with little use? When this engine runs it doesn't seem to burn the gas very well smells like an old truck running on old fuel.

This is an OLD thread I know, but I stumbled across it while seeing what everyone else online was looking at, and coincidentally thinking I needed to do a compression check on my Rambler to verify it's condition.  Here's a reproduction of my notes I took from an expert tech ages ago.

There's 3 different compression tests he ran and said each one could show you something a little different.  I don't have any actual compression tables written down because that varies by build.  What was (is) more important is the variations in readings.

First Check
The one everybody is probably most familiar with: Disable the coil primary circuit so you don't damage the coil , then remove all the plugs and test one cylinder at a time.

Crank it over till you get 5 "puffs" into the guage.  First puff should be 50% of the final reading or (a bit) higher.  If not, you've possibly got bad rings or a bad piston.  As a previous post mentioned, a few drops of oil can be added to the cylinder, and if pressure goes up that further indicates bad rings

NOW, if all initial "puffs" are low, you might want to check the crankcase for gas contamination; which will wash oil off the cylinder walls and give low readings.  If not that, then it may be something like a cam or valves issue

Also, the readings should all be within 30 pounds of each other.


Second Test:
This is an engine running compression test.  Hook the coil back up, replace all the plugs but one, hook the spark plug wire to a spark tester  (Again, to prevent coil damage - Note that this is more important on 80s & up cars with electronic ignitions, but still a very good idea).

Running compression should be 50 to 60 PSI lower.  If not, you probably have an issue somewhere in your valve train.  It could literally be any part of the valve train too.

As a side note here, you can further check the valve train with compressed air into the spark plug hole (and engine off).  It's a little tricky though, since you have to get the cylinder into a both valves closed position.  Anyway, at that point, push some air into the cylinder with an air nozzle on a compressor.  If air comes out the tail pipe, you have an exhaust valve issue.  If it comes out the throttle plate, you have an intake issue.  If it comes out another cylinder, you have a bad head gasket.


Last Test
This is a throttle snap test while the engine is running.  Compression should be 80% of the original test's high result.  If less, you probably have an issue with intake lobe on the cam, or an issue with intake lifter or valve.  IF it goes higher, either the exhaust valve isn't opening properly due to the parts just mentioned, OR you have a restricted exhaust.