Compression Test vs Leakdown Test

What gives a better indication of the actual quality of the sealing of the compression chamber, compression test or leakdown test? And why? Thanks in advance for your opinions.

 

actually, both are used in determining engine condition
compression test checks just that, basic indicator of valves and rings
if you don't get the published compression results for that particular engine, you can then introduce compressed air into each individual cylinder (at top dead center)
these results are useful for suspected valve/ring conditions.
air out the exhaust, exhaust valve/spring/seat/guide problem.
air up and out/through the intake manifold, intake valve/spring/seat/guide problem.
air out through the crankcase vent/oil fill cap, piston/rings/or cyl bore problem.
I might add that introducing a very small amount of oil into a cylinder can boost compression readings as well, indicating a possible ring/bore problem.
I would also add that no engine will have 0 % leakage, you always have some, just the nature of the beast.
I would say no more than 5-10 percent leakage would be considered normal in my book on any given cylinder.
The important thing is engine balance; you would like all the readings to be within say 10% of each other, compression and leakage.

 

Jim, you explained that well.

It seems with a compression test the absolute values are not as important as the relationship between them. I guess there are dozens of factors that could affect the absolute values (including the calibration of your gauge), so if my readings are all a little higher or lower than spec, I shouldn’t worry about it.

It seems by introducing oil to a cylinder it will tell you of a possible ring/bore issue, but the leakdown test could indicate if the leakage is coming from the rings, the exhaust valve, or the intake valve. Also, couldn’t you tell if you had a head gasket leak by removing the radiator cap and looking for bubbles or movement?

 

that is just one way, depends on the size of the suspected leak.
some don't show up till engine is warm/hot
it also could be cracked head or even the block, no proof it's the gasket.
oil in water or water in oil ?
one possible sign would be the constant need to add coolant with no apparent leaks in the cooling system; the coolant has to be going somewhere: external leak, crankcase, or being burned in combustion chamber, that latter of which might create white smoke depending on the size of the leak.
You can pressure test the cooling system (using pump-up system and cap tester) and let set overnight and check to see if pressure holds; not always a sure thing as the tester itself could bleed off overnight.
I think there are "sniffer" testers and different chemicals nowadays that help in determining head gasket problems. I have no experience using those methods.

 

While doing a leakdown test it’s a good idea to remove the radiator cap to prevent pressure buildup in the cooling system, should the oil cap be removed for the same reason?

 

normally you wouldn't remove either
unless your looking for the leakage
have you taken compression readings on an engine and you are suspecting engine troubles?

 

My thoughts were of possibly buying a gauge leakdown tester vs a compression tester. From what you wrote, and if I understand you correctly, it seems the leakdown test does have the advantage, and if I use the same leakdown tester at the same pressure all the time, I should get a real good idea of just what condition my Jeeps in.

While compression readings will always vary from engine to engine, the percentage readings from a leakdown tester should give very consistent indications on vehicles in similar condition regardless of compression ratio or other variables.

The leakdown tester with compressor may be useful for a future purchase of a non-running Jeep apposed to having the Jeep running to do a compression test. Although I have heard that you can do a compression test without the engine running, but I have never done this.

Jim, you’re a generous man, every time you post I learn.

 

Chuck, hope you don't mind my 2 cents; a $25 compression checker from Pep Boys is sufficient. You only need to be able to crank the engine to use it. It'll report engine condition well, and be able to distinguish between worn rings and other conditions. It's good to know the compression readings of any engine.

You can build a leak-down tester yourself - try Google with "leak down tester."

 

Tim, your opinion is always welcomed. You know, I was looking at the leak tester in a magazine and thought I should be able to make one. I have seen leakdown testers as high as $99 for a double gauge. I will buy the compression tester at Pep Boys and make my own leakdown tester, best of both worlds.

Thanks for the lead on Google, will check out.