Compression test "hissing" noise.

[Etzler]

Doing a compression test on a 2008 350 Mag inboard. Had to pull off the lower water hose off the bottom of the exhaust mainifold to reach the spark plug. When I would crank, with all the plugs out, I would notice a hissing sound coming from that hose fitting. It would last up to 30 seconds after I stopped cranking. What causes that hissing noise? All readings were about 175 psi.

 

[Bondo]

Ayuh,..... A leakin' exhaust valve, or valves,...

Put air pressure into each cylinder at Tdc to see which ones are leakin',...

 

[Etzler]

Ayuh,..... A leakin' exhaust valve, or valves,...

Put air pressure into each cylinder at Tdc to see which ones are leakin',...

How would there be any pressure in any cylinder if all the plugs are out?

 

[Maclin]

Bondo is describing a leakdown test. There is an air fitting that goes into the spark plug hole. When the cylinder is at top dead center on a compression stroke, and you introduce air pressure through that fitting the cylinder should hold pressure for quite some time before it leaks down. If it can't hold the pressure very long then you can listen for the hissing and that tells you where the leak is. Rings (crankcase), Exhaust or Intake (valves).

 

[Etzler]

Bondo is describing a leakdown test. There is an air fitting that goes into the spark plug hole. When the cylinder is at top dead center on a compression stroke, and you introduce air pressure through that fitting the cylinder should hold pressure for quite some time before it leaks down. If it can't hold the pressure very long then you can listen for the hissing and that tells you where the leak is. Rings (crankcase), Exhaust or Intake (valves). [/QUOTE

I understand the concept of a leakdown test.

With all the plugs out, there should be no pressure in any cylinder, correct? If no pressure in the cylinder what would make the hissing/ leaking air sound? I just don't understand how or where pressure could build in the first place with no spark plugs installed.

 

[Bondo]

How would there be any pressure in any cylinder if all the plugs are out?

Ayuh,.... The cylinder yer testin', has the gauge stuck in it,..... that cylinder builds pressure,....

 

[CamaroMan]

when the cylinder is at compression tdc both valves are shut and the air "shouldnt" escape anywhere. Applying pressure will tell u if a valve is stuck. A small amount of air will escape past the rings - a stuck valve will let air escape quickly (depending where its stuck). If the cylinder is good the air will leak out slowly - look for videos to see how.

 

[Etzler]

Ayuh,.... The cylinder yer testin', has the gauge stuck in it,..... that cylinder builds pressure,....

Actually all the pressure had been removed from the cylinder via the compression testers Schrader valve, when hearing the noise. That's why it's perplexing.

 

[Etzler]

when the cylinder is at compression tdc both valves are shut and the air "shouldnt" escape anywhere. Applying pressure will tell u if a valve is stuck. A small amount of air will escape past the rings - a stuck valve will let air escape quickly (depending where its stuck). If the cylinder is good the air will leak out slowly - look for videos to see how.

I will build a leak down tester and check.

 

[QBhoy]

Jeez....lot of confusion here !......
Pretty sure all who have contributed mean that you do the following;....
Leave all plugs in, apart from the clylinder you are testing. Crank her over ( after making sure she is warm enough and up to P&T's).....repeat for all 4,6 or 8 cylinders.....
Hopefully you are all within 10% of each other.
If not.....jeez, enjoy your boating and don't let it worry you too much, after all the sensible basics !
If you are defo doing the compression test to spec (many don't) and she fails, either enjoy what's left of her or sell on with honesty !
Atb
Allan

 

[QBhoy]

Jeez
Apologies....well, maybe.....
Just read the full post again... totally perplexed.
So the full compression test has good results ?
You defo need to do compression test each cylinder at a time and with the rest of the cylinders in full compression capacity.
The guys above sure know what they are on about....bottom line is....get a compression test done and go from there.
Failing this, a good mechanical ear will be able to tell you if she isn't firing on all cylinders.

 

[Etzler]

Compression and Leakdown test all had good results. The only think I can figure out on the noise is its related to the water pump and cooling system. It is generating some sort of pressure in the system with the residue water in the system.