Compression Test and/or Leak Down Test?

[Spetz]

Hi guys,

Would leak down test only show a problem if a compression test also shows a problem?

ie, if a compression test shows all cylinders are even and of acceptable compression, would a leak down test actually be able to show any other fault or is it a case that if a compression reading is good then doing a leak down test is a waste of time?

 

[Colt45ws]

My experiance is that I had a GM 3400 Van that had compression numbers well in spec.
I put a leak down on it and instantly knew which cylinder was the problem, even before I saw the bubbles coming out of the radiator.

 

[DoubleWasp]

Originally Posted By: Colt45ws
My experiance is that I had a GM 3400 Van that had compression numbers well in spec.
I put a leak down on it and instantly knew which cylinder was the problem, even before I saw the bubbles coming out of the radiator.


Opposite is true too. I have had phantom coolant leaks that became real obvious with a cooling system pressure test.

 

[exranger06]

A leakdown test is more thorough than a compression test. A compression test will tell you how much compression each cylinder has, but if a cylinder has low compression, it won't really tell you why. A leakdown test will tell you if the piston rings aren't sealing, if the intake valves aren't sealing, if the exhaust valves aren't sealing, and if the cooling system is leaking into the combustion chambers.

 

[Spetz]

Thanks guys,

My engine is blowing smoke and I plan to do a compression test. I just want to know if the compression results come up ok will there be a use to doing a leak down test, or will it show no issues like the compression test?

 

[exranger06]

Is it white smoke or blue smoke? White smoke means most likely burning coolant, blue smoke means burning oil. A leakdown test will tell you for sure. Leakage past the piston rings = oil getting into the combustion chamber and burning. Cooling system leaking into combustion chambers = burning coolant.

 

[Spetz]

I couldn't quite identify what kind of smoke it is.
It seems grey basically, and it has a smell to it but not really like the oil burning smoke?

If it is a head gasket, can it be a combination of oil and coolant causing an odd mix of smoke?

EDIT:
Also to my original question, since I have neither on of the tools (nor an air compressor for a leak down test), will a compression tester suffice for my situation?

 

[kc8adu]

mitsubishi 6g engines were well known for dropped valve guides.they would work well for mosquito fogging when that happened.
is it losing oil or coolant?

 

[Spetz]

It is actually for another car of mine that has the Mitsubishi 6A12 MIVEC engine in it.
It has had a full rebuild (heads and bottom end) but after the rebuild it has been put into storage and has not been started since 2009.

I am hoping the issue is something "simple" like valve stem seals etc. So long as it is in the top end I am not too fussed as I will be doing a timing belt service, so replacing valve stem seals with the heads still on is doable.

As for the 6G74 I have, I replaced the valve stem seals and since then the oil remains on Max throughout the OCI (6,200 miles)

 

[punisher]

In a case of excessive smoking, either a compression test, or a leak down test can be little or no help. Bad valve seals really won't effect compression or effect a leak down test. Sometimes a spark plug won't indicate a problem. Modern plugs are really effective at burning off deposits in mild cases of oil ingestion. I have seen plugged up oil ring drainback holes/slots that never effected either test, ditto for a broken 2nd ring. Once you put a borescope down the plug hole often you get that "a-ha" moment. Speaking of, as cheap as USB borescopes are nowadays, no excuse not to have one handy in the toolbox for having a look around. Sometimes a visual is the best way to catch a blown head gasket or oil burning cylinder.

 

[Spetz]

With valve stem seals, once I remove the intake manifold and I can see the intake valves, I assume that I should be able to see oil deposits there?

The car has been started twice in the past few days, prior to that not since 2009.
The first time it smoked a lot, but I had dropped 20ml of oil into each cylinder via the spark plug hole.
The 2nd time it smoked but nowhere near as much, however still quite a lot.

I'll look into a bore-scope as well.