From reading a prior post I learned a compression test will tell you if piston rings are bad and that the compression on a stock 440 should be between 130- 155 psi. What I wanted to investigate using a compression test is the state of the valve train. Will a compression test tell you if a valve is sticking or if something else is wrong with the valve train? Paul
What does a compression test really tell you
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commando1
Old Man with a Hat
First, tell us the problem, then we can start from there.
Triple Pickle
Senior Member
A leak-down test will tell you where you’re loosing compression. Compressed air is applied to the cylinder through the spark plug hole. If it’s a stuck valve you can usually hear it escaping out through the carb or exhaust. If it’s ring/piston related you’ll hear it in the crankcase. A gauge is used to see how much is being lost and how quickly.
rkrochen
Senior Member
Yes it can. Basically it identifies the cylinders ability to create compression. This includes rings and bad valves. You want even compression on all the cylinders. Do a dry test and a wet test where you squirt some oil into the cylinder and retest to see if the compression increases. Once you have the numbers let us know. As stated tell us more about the problem so we can steer you in the right direction.
As the old-line Chrysler service manager told me back in the 1960s, "All a compression test will tell you is the condition of the top (compression) piston ring". Not the lower rings, just the top one as that is where the basic compression is "sealed". Back then, "leak-down tests" were unheard of except in very high-end shops, plus requiring a good air compressor for an air source.
Another result is that it can indicate if an exhaust valve is beginning to burn, but unless you pull the cyl heads, you won't know WHICH one until it finally gets bad enough to cause a miss on that cyl.
I'm really not sure if "sticking" valves can be an issue on a running engine. If a valve sticks, even with hydraulic lifters, pushrod bending issues might be more prevalent, I suspect. Could be wrong, too.
Consider that if a valve is sticking enough that it takes the valve spring pressure + cyl combustion pressure, together, to close the valve, it's got to be pretty stiff to operate. Plus hard to open, too. At least that's the way it seems to me. NOT to forget about the heat such sticking would result in on that particular valve guide. In other words, something's going to break sooner or later.
A compression check is good to check "cylinder balance". BUT rather than worry about "FSM numbers", consider how close they are to each other. Compression pressure will vary with altitude, too, which can further frustrate getting to "FSM numbers", if that is the goal. You can check the "wear" by putting few squirts of engine oil into the cyl and re-checking. The more difference, the more wear. But ONLY on the compression ring.
You can also do a cylinder balance test by unplugging (Carefully!!!) one spark plug at a time with the engine at idle. Look for the rpm drop each time. Less drops indicate the less-powerful cylinders.
After conversing with the Chrysler service manager, I became less inclined to be worried about compression numbers per se, but more concerned about how they all related to each other. Not a determiner of if the oil rings had any wear on them, either.
If you do purchase a compression tester, get one which screws into the spark plug hole. The other "hold it in place" type probably worked better on inline engines, I suspect, rather than V-8s.
What is your basic concern in desiring to do the compression test?
Enjoy!
CBODY67
Another result is that it can indicate if an exhaust valve is beginning to burn, but unless you pull the cyl heads, you won't know WHICH one until it finally gets bad enough to cause a miss on that cyl.
I'm really not sure if "sticking" valves can be an issue on a running engine. If a valve sticks, even with hydraulic lifters, pushrod bending issues might be more prevalent, I suspect. Could be wrong, too.
Consider that if a valve is sticking enough that it takes the valve spring pressure + cyl combustion pressure, together, to close the valve, it's got to be pretty stiff to operate. Plus hard to open, too. At least that's the way it seems to me. NOT to forget about the heat such sticking would result in on that particular valve guide. In other words, something's going to break sooner or later.
A compression check is good to check "cylinder balance". BUT rather than worry about "FSM numbers", consider how close they are to each other. Compression pressure will vary with altitude, too, which can further frustrate getting to "FSM numbers", if that is the goal. You can check the "wear" by putting few squirts of engine oil into the cyl and re-checking. The more difference, the more wear. But ONLY on the compression ring.
You can also do a cylinder balance test by unplugging (Carefully!!!) one spark plug at a time with the engine at idle. Look for the rpm drop each time. Less drops indicate the less-powerful cylinders.
After conversing with the Chrysler service manager, I became less inclined to be worried about compression numbers per se, but more concerned about how they all related to each other. Not a determiner of if the oil rings had any wear on them, either.
If you do purchase a compression tester, get one which screws into the spark plug hole. The other "hold it in place" type probably worked better on inline engines, I suspect, rather than V-8s.
What is your basic concern in desiring to do the compression test?
Enjoy!
CBODY67
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1; its' an ongoing problem. Car was belching smoke (and raw gas) out of throat of carb and bowl vents on initial startup. After initial startup the car ran smooth and additional startups on the same day were uneventful. OK so one day the carb flooded and started leaking gas out of the EVAP cannister. Barely got the car home.
So I lowered the floats and I am now in the process of installing a fuel pressure regulator and gauge (which my mechanic claims is unnecessary because of mechanical pump). I will determine the pressure but in the meantime fuel is still dripping from the overflow tube that leads to the EVAP cannister. My understanding is that this means fuel is too high in the front bowl and under too much pressure (fuel is thus overcoming needle and seat). Floats and needle and seat are good - this I checked several times
So I'm thinking I have two problems. 1) Problem with fuel delivery - too much pressure causing fuel to run into EVAP cannister and out of bowl vents and 2) timing or valve issue - causing smoke and raw gas to belch from carb throat.
Any help appreciated - running out of time and faith. Paul
Have you changed he fuel pump yet? I don't seem to recall mention of that. Have I missed something?
The other thing is "What happened BEFORE" this all started? THAT might have some clues there, too. As you have been chasing this problem for a while now.
CBODY67
The other thing is "What happened BEFORE" this all started? THAT might have some clues there, too. As you have been chasing this problem for a while now.
CBODY67
rags
Senior Member
bad rings create blow-by, pressure in the crankcase. it'll breath heavy out the crankcase vent, push oil into the air cleaner housing. bad valves, springs, camshaft will show up on a vacuum gauge. a leak down test with compressed air is a great determinant of pressure loss but needs to be done with the rocker shaft removed. put air down the cylinders and it will roll the engine over so be careful. many years ago i had a similar problem. can't remember exactly what carb i had but thinking thermoquad. it was supposed to have a metal screen filter behind the fuel inlet fitting. i had the carb apart and just left it out. it then had flooding issues i couldn't resolve. stuck the screen back in, problem solved.
C; changed the fuel pump about a year ago. Maybe I'll just park the car for a while and simmer down. Paul
volksworld
Active Member
if you're dumping raw fuel into the engine from a carb issue it can wash the oil off the rings and....if you give it a compression test...will show low enough numbers on the washed out cylinders to make you panic and tear the engine apart thinking it needs rings...so dont do anything hasty....fix the carb issue first
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