Blowby through vent tube? Mercruiser 3.7

[Dubed]

Good morning folks.
Have a 1986 3.7 190 hp Mercruiser.
I had entire engine rebuilt and have about 15 hours on it with break in oil and another 10 to 15 hours on fresh oil. I did prime the oil before first starting and again before starting this season. Engine was fogged before laying up through the winter.
It appears to have excessive pressure and oil coming from the vent tube to carburator. I will perform a compression test and leakdown test during the next few days. Is there anything else I should be looking for? I did notice when I started in the driveway on the muffs that there was a puff of smoke when started but I assumed this was from oil into carb.
Also I hear a clacking noise coming from the exhaust out the outdrive that I dont recall hearing before. Flapper is new
There is no water, just oil. Pressure tested the cooling system and all is fine.
Any help would be appreciated
Dave

 

[alldodge]

I had entire engine rebuilt

The rebuild was not done correctly.
Blow by is caused by the rings not seating. The cylinders should have been bored with new oversized pistons installed. If if was just honed and new rings with old pistons then this could cause it

Tell us more about what was done during rebuild?

 

[Dubed]

The rebuild was not done correctly.
Blow by is caused by the rings not seating. The cylinders should have been bored with new oversized pistons installed. If if was just honed and new rings with old pistons then this could cause it

Tell us more about what was done during rebuild?

I will have to find the paperwork on it and get back to you.
Best of my knowledge, it was bored, new pistons, rings, bearings, cam, head was re-done, etc. ect. It cost me close to $4K if I recall.
Will get back to you with compression test and leakdown results. A valve wouldnt cause this would it, just bad ring seal allowing the crankcase to pressurize??

 

[Dubed]

I will have to find the paperwork on it and get back to you.
Best of my knowledge, it was bored, new pistons, rings, bearings, cam, head was re-done, etc. ect. It cost me close to $4K if I recall.
Will get back to you with compression test and leakdown results. A valve wouldnt cause this would it, just bad ring seal allowing the crankcase to pressurize??

By the way, Thank you very much for your reply

 

[alldodge]

I will have to find the paperwork on it and get back to you.
Best of my knowledge, it was bored, new pistons, rings, bearings, cam, head was re-done, etc. ect. It cost me close to $4K if I recall.
Will get back to you with compression test and leakdown results. A valve wouldnt cause this would it, just bad ring seal allowing the crankcase to pressurize??

A valve not seating correctly will be a loss in compression but will not put more pressure into the block. If a valve seal is missing or damaged, this will cause burning of oil but still nothing inside the block

 

[Dubed]

A valve not seating correctly will be a loss in compression but will not put more pressure into the block. If a valve seal is missing or damaged, this will cause burning of oil but still nothing inside the block

Thank you. Thats not good. As long as theres nothing else for me to look for. I just wanted to get my ducks in a row before I call the motor shop and see what they are prepared to do about it.

 

[Dubed]

A valve not seating correctly will be a loss in compression but will not put more pressure into the block. If a valve seal is missing or damaged, this will cause burning of oil but still nothing inside the block

Thank you.

 

[Rick Stephens]

I feel for you. Have seen it before. Bore not to specification for the rings and or piston used (or vice versa). Always appropriate to double check dimensions during assembly, but most people don't. All it takes is a brain fart on the machine shop part, which we all have on occasion. Most home builders don't own snap gauges. But, you can and should at minimum insert and measure ring gap.

Bad part is any of those require dis and re assembly. But if the machine work is confirmed to size, re-hone and install of proper parts will fix it.

Best of luck!

Rick

 

[Dubed]

Thanks. Yeah it sucks. Boat sat for 6 years covered in the yard. I decided to restore with new custom fuel tank, new stringers, wiring, electronics, new trailer and on and on. Brought it to a very reputable race shop because I wanted it done properly and have no issues.
Ugghh! HOPEFULLY THEY TAKE CARE OF IT. But I'll lose Spring fishing season now

 

[76SeaRay]

Rings might not have been installed properly, that is, the ring gaps should be offset from each other and some of the rings have a top and bottom. Its good that you used breakin oil but did you do a run-in sequence on an engine stand or on the boat for the first 15 to 20 minutes to seat the rings before driving it?

 

[Dubed]

Yes broke in according to the shops instructions. In the driveway at first, then re-torqued the head. On the water three trips up and down the Rpm range (never over 3000). Avoided idling for long periods of time and tried to keep above 1800 rpms per their request

 

[Scott Danforth]

if you have 30 hours on it, the rings should have seated. however if you had any moisture in the bores, there would be corrosion which would lead to blow by.

compression test and leak down test will tell you what is going on.

for $4k, you could have bought a brand new 6.2 liter V8

 

[Dubed]

Ok, I ran home and did a compression test. #1=150+, #2=110, #3=150+, #4=140.
Obviously piston and or rings in #2 is burnt. Not sure why or what happened. Motor shop said I probably detonated piston... timing was set at 4°btdc. I have to pull it so they can take it down and look at it. Plugs were dry and black, probably from oil into carburator?
Very discouraging

 

[Scott Danforth]

running lean will burn a piston.

could also be stuck rings, etc.

I would drop a bore scope down the spark plug hole and look at the piston. I use the cheap ones you plug into your phone.

 

[Dubed]

I have a Milwaukee at work. I'll bring it home tomorrow. Shop told me to leave it the way it is and bring the carburator with it... not sure why. To see if it was running lean?

 

[alldodge]

30 hours and those plugs look oil soaked.
Oil is making it past the rings

 

[Dubed]

30 hours and those plugs look oil soaked.
Oil is making it past the rings

That's what I'm seeing. They were dry and cooked though. I would assume oil was going by #2 cylinder. I was also thinking it was from all that blowby dumping into the carb from the vent hose. Would that be a wrong assumption?

 

[Lou C]

Agreed those plugs look very oil fouled, kind of like my 35 year old Briggs and Stratton 3.5 hp lawn mower!

 

[Dubed]

Agreed. My question is.... would all that oil coming out of the vent tube and being pumped through the carb cause that? After all, it isn't a 2 stroke

 

[matt167]

I can't remember how the PCV is set up on a 3.7L but a stuck PCV would cause it to burn oil with blowby. I would look at it at least. But for all that oil to be from the breather it would be smoking like a 2 stroke. If uncle Buck's car was a boat, that is what you would have.

The other question is did the shop know what to do with the 3.7l? They were never used in a car. Few shops know they even exist