blowback mercruiser 3.7

[mannieg702]

So I've had the boat 2 years and last summer it over heated due to a bad riser gasket and the coolant leaking. Took the heads off and had a valve job done while I was tackling the head gasket. I did a leak down test all cylinders where good in the green and held the air for quite sometime. I have nasty blow back from the breather wholebut only at 2000 or higher rpm. Everything pointed to a bad cylinder but now thats out the way what else can I check for? Engine has no power loss and its set a 4 dtc. I'm also running napa 40w oil. Any clues?

 

[Don S]

Re: blowback mercruiser 3.7

I think the term you are looking for is "Blow By" not blow back, and the problem is caused by compression gasses blowing past the rings and into the crankcase. The pressure in the crankcase then blows out of the valve cover fitting and into the carbs flame arrestor so the fumes go back into the engine.

Also not sure about your leak down test being in the green and the cylinder holding pressure for a long time ??????
That is not how a leakdown test is performed, and there is no green area, what you are looking for is a leak down percentage. None of them will hold pressure for more than a second, even on a perfect engine.

One other note, those engines do seem to have more blow by than any other engine I have come across, even with great compression.

 

[mannieg702]

Re: blowback mercruiser 3.7

The gauge read 20% to 25% comp loss for all the cylinders. I'm doing compression test tomorrow and report back.

 

[Don S]

Re: blowback mercruiser 3.7

20 to 25% is too much, you should be 15% or less.

 

[mannieg702]

Re: blowback mercruiser 3.7

Thanks Don I looked at my readings again and you are correct it was 15% loss. I was on the road when I replied to your last post. What else would affect blow by? I will do a compression test to see how that tests out and report back. I dont know if this makes a differnce i have a valve cover from a mercruiser 170 due to a broken tab on the original valve cover.