Cylinder Leak Test Results . . .

[tpenfield]

I have been trying to diagnose an overheating issue with my 1991 Mercruiser 7.4L/Bravo 1

Having Checked all the water circulation issues to no avail, I did a leak-down test today.


Here are the results:

Cylinder------ Pct Leak
L1-----------------8 %
L3----------------35 %
L5----------------48 %
L7----------------17 %
R2----------------11 %
R4----------------52 %
R6----------------37 %
R8-----------------8 %

The engine generally runs well, but has seemed to lack power in the past few years ( about 10% less WOT speed). The overheating came on suddenly a few weeks ago. I have been trying to isolate it since then.

These Leak test results are looking like a top-end re-build at a minimum, but I would like to get any additional opinions. I plan on doing some more testing on the 4 'bad' cylinders to see where we are leaking.

Thoughts???

 

[dollarten]

Re: Cylinder Leak Test Results . . .

I have been trying to diagnose an overheating issue with my 1991 Mercruiser 7.4L/Bravo 1

Having Checked all the water circulation issues to no avail, I did a leak-down test today.


Here are the results:

Cylinder------ Pct Leak
L1-----------------8 %
L3----------------35 %
L5----------------48 %
L7----------------17 %
R2----------------11 %
R4----------------52 %
R6----------------37 %
R8-----------------8 %

The engine generally runs well, but has seemed to lack power in the past few years ( about 10% less WOT speed). The overheating came on suddenly a few weeks ago. I have been trying to isolate it since then.

These Leak test results are looking like a top-end re-build at a minimum, but I would like to get any additional opinions. I plan on doing some more testing on the 4 'bad' cylinders to see where we are leaking.

Thoughts???

You can find out where your leakage is while doing the leakdon test by just listening where the air is leaking to . Take off the oil filler cap and listen there, piston and/or are rings leaking and then of course carb and exhaust area to detect leakage in the valves If you have a head gasket out between two cylinders , that will be tough to locate but, by your readings , one would not think so If there is a head gasket failure at another area , this could induce an air stream in to the coolant flow and cause oveheating

 

[Pete104]

Re: Cylinder Leak Test Results . . .

Pull the heads. No doubt about it & just hope the block didn't burn cyl to cyl on either side.
What other tests?

 

[Volphin]

Re: Cylinder Leak Test Results . . .

Cracked heads. Should be very obvious when you pull them.
Time to upgrade to better heads/valves/springs and valve gear.

V

 

[tpenfield]

Re: Cylinder Leak Test Results . . .

Pull the heads. No doubt about it & just hope the block didn't burn cyl to cyl on either side.
What other tests?

Thanks for the replies . . .

Based on the results, the cylinders on the 'ends' (i.e. #1, #2, #7, & #8) look OK, the cylinders in the middle (#3, #5, #4, #6) are the bad ones. I am hoping that it is just a head gasket failure between the 'bad' ones. The standard compression test results are not too bad (134 PSI +/- 7 PSI on all cylinders) The leak test has been much more revealing.

I am going to re-test the 'middle' cylinders and verify where the air is leaking before taking things apart.

 

[dollarten]

Re: Cylinder Leak Test Results . . .

Cracked heads. Should be very obvious when you pull them.
Time to upgrade to better heads/valves/springs and valve gear.

V

Not to throw cold water on you thoughts But , the equipment that the engine cam with regarding heads and valve gear ehould be sufficient to operate the engine when there is enough cooling water availible . Even the best race quality engines can be ruined without enough cooling

 

[scoflaw]

Re: Cylinder Leak Test Results . . .

Cracked heads. Should be very obvious when you pull them.
Time to upgrade to better heads/valves/springs and valve gear.

V

That could be the answer if it were a small block chevy, they crack between the 2 center exhaust valves that are less than half an inch away from each other. Big block different story.

 

[tpenfield]

Re: Cylinder Leak Test Results . . .

Not to throw cold water on you thoughts But , the equipment that the engine cam with regarding heads and valve gear ehould be sufficient to operate the engine when there is enough cooling water availible . Even the best race quality engines can be ruined without enough cooling

I'm not sure what you said in the quote above ????

 

[dollarten]

Re: Cylinder Leak Test Results . . .

I'm not sure what you said in the quote above ????

Sorrey for the typo I meant to say that the cylinders heads and related item that " came" not cam with the original Bravo engine are suffiicient without upgrading to a better components and I will go on to say that if all is well in the cooling system there should be be no longevity problems . If they were overheated , that is another story .