compression and leakdown tests,what do they prove?

[magaoidh]

I am buying a car which has a low reading (when cold ) in #3 cylinder.I am yet to have a leakdown test.
Does this indicate the piston to cylinder performance only or is this in combination with the valves.
What is the significance of any leakdown results?
Please advise,thanks in advance.
Geoff

[nine_one_4]

[Dave at Pelican Parts]

Alfred, that's just plain annoying.


...Anyway.

One of the main things your engine has to do is suck air in, squeeze it, and then let it out again. The valves let the air in and out, and when they close they help to seal up the combustion chamber so that air doesn't leak out. The piston rings and cylinder walls help form the "bottom" of that container, so to speak.

The compression test measures how much pressure is made by the engine squeezing the air. The rings, the cylinder walls, and the valves all contribute.

The leak-down test will test how well the cylinder is holding air when the piston is at TDC (Top Dead Center). In some ways it is less useful than the compression test, in that it doesn't check the cylinder walls except at that one point. However, you can listen to where the air you pump in through the spark plug hole is escaping. If it hisses out the intake, the intake valve is leaking. If it hisses out the tailpipe, it's the exhaust valve that is leaking. If it hisses out the oil filler, you've got leaking rings. If it hisses out all three, well....

I hope this helps.

--DD

[nine_one_4]

Originally posted by Dave at Pelican Parts
Alfred, that's just plain annoying.


--DD

Dave, what's annoying about it? He's going to check out a car and these tests are completely relevant. I wasn't going to type in all that information. Hehe, but I did read in the Sandbox that you thought scanning from old books and magazines is part of my modus operandi but that's just my way of trying to provide credible information.

[magaoidh]

Dave , from what you are saying,if air escapes out the intake or exhaust its the valve seatings i.e. heads If its out the oil filler, its piston rings and cylinders i.e. the block.Could a lack of valve adjustment have an influence on the tests? Why would #3 improve when hot,natural expansion of moving parts?

[Dave at Pelican Parts]

The cylinders on our cars are separate pieces from the block (or crankcase). The compression and leakdown tests tell us nothing about the health of the "bottom end" of the motor.

A lack of valve adjustment can definitely affect the results of a compression test; they effectively change how long (and possibly how far) open the valves are. And if one valve clearance is tight enough, it might even keep that valve from closing all the way--which obviously will not be good for compression or leak-down numbers.

I'm not sure why the results would get better when warm, but the natural thing to look to would be thermal expansion.

--DD

[John Rogers]

Rules of thumb for compression tests: Make sure valves are adjusted to spec, run engine to normal temp, pull all the spark plugs, pull the fuel pump fuse, block the throttle wide open, test cylinders in order, noting how many pumps to get to the slowing down point and then the final pressure. Move quickly to the next cylinder and repeat. If the engine is relativly stock and in good shape, compression should jump right up quickly and be within 5 to 10# across all cylinders. When finished, be sure to unblock the throttle valve from wide open. If you have a radical cam such as in my 2L six, 906 cams, then the overlap of the open valves give a false low compression reading. In this case, a leakdown test with both valve closed is helpful to determine if the rings have broken or the valves are bent, etc.

[pbanders]

Alfred, thanks for posting the reference, there is some good info in there that I hadn't seen before. Too often, people with engine problems go after ignition and fuel systems, without ever having done a proper compression and vacuum analysis of the motor.

[nine_one_4]

Brad, I think it's amazing the information one can glean (as shown on pages 502 and 503) by using only a vacuum gauge.

[bob174]

Harkening back to my days in airframe and powerplant mechanic's school, the best method to use is differential pressure testing. This allows you to check the relative condition of the pistons, rings, cylinders and valves. IIRC you should see no more that a 10-15% difference in your input and output pressures. Someone correct me on this....

[Dave at Pelican Parts]

That sounds just like the "leak-down test". The problem is it only checks the cylinders at TDC, while the compression test goes through the complete cycle. They're both useful tests, and I personally don't feel that one is specifically "better" than the other.

--DD