Oil out the exhaust
 

Ok, 496 BBC, I have oil blowing by and out the exhaust. On a hard run, about 2hrs I run through 4q of oil. It's not burning it, its raw dump out the exhaust. I have zero power loss, and the compression on each cylinder is above 150. Any ideas?

Oil cooler

I should mention that I do have signs of build up on my plugs.

Wrong dipstick?....causing you to overfill the pan....oil coming out breather into the intake....being blown out?

Since:
1. "no power loss"
2. oil build up on plugs
3. Good compression

It be looking for a leak down tester too.....

Is this a fresh build? how many hours? Details.

I like the oil cooler idea...but then you shouldnt have build up on the plugs.

Oil cooler

Are you running guide seals on exhaust valves? Excessive guide clearance?

pressure test the oil cooler.

Pressure test the oil cooler first very easy to do --- if that checks out good on the pressure testing - next place I would check would be --- possible intake manifold gasket - a engine serial number is a must.

Later 496 changed to a bigger intake manifold gasket for better sealing because the first gen was not holding up over time. As you mentioned plugs have build up but is that OIL OR GAS.

The motor is a built carb, 496, not a mercruiser, I run a larger oil pan so the boat actually takes about 10+ quarts. I have a guy that is going to do a leak down test for me but I have to wait 2-3 weeks. The oil cooler is a good idea, I never thought of that. It is not a stock one either of course, it is a much larger one. But that does not mean something didnt happen to it.

Cooler Check First. Keep it simple and rule that out. Might be a really small crack that only shows up after hard runs and oil is thinnest. I wouldn't run it much till figured out. If it gives way, you will have no oil and be in trouble in a hurry. Just my 2 cents. At least you have a large Pan.

Would a bad oil cooler cause oil build up on the back of the boat though. Would it get hot enough to soot up, I dont see a sheen on the water while it idles

496 does not have a PCV valve

YOU are 1000 % correct and I had no idea as from the start it was mentioned a 496. . I do not have the time to read all the posts but skim thru after the first post for most threads up here. . As boaters looking for help they should post all the important details and EXACTLY what they have so they can get the correct answers to help them out accurately from the get go. Thanks again for clearing up the exact engine

if i am reading this correctly this engine is a bored&stroked 454.that would be 4.310 bore with 4.250 stroke.if the oil cooler checks out and found to be good it could be a multitude of things,cracked block,blown head gasket cracked head,a lot of things to check.

Mike we all have to back up and ask was this boat properly winterized and by whom ? says compression good at 150 and NO power LOSS --- I take it then with those statements he can run full max RPMS and runs good as full power is felt thru out.

. I NEVER seen a marine engine with bad valve train components and or low compression and or very poor ring seal - blow by runs without having rpm issues and a true PIG thru out all rpms. - .

i agree,living in the midwest i get a few calls every spring from people with similar problems.cracked oil cooler was the first thing that came to mind but around here winter can cause all kinds of issues.

I apologize if my details weren't enough in the beginning. Yes I took the factory motor out and built a mark IV 454 block up to a 496. The mark IV blocks are the strongest blocks Chevy ever produced to handle the power I was trying to obtain. I also did not want the headache of fuel injection.
So, in a nut shell, the boat has no power loss up to the 5400 rpms the I run. Now, when I did the compression test I did it cold, not at operating temperature because I was hoping to find low readings. As far as winterizing the boat, I am the only one to touch my boat. I drain all the water out of the motor first, then I use a kids swimming pool with a pump to circulate the antifreeze through the motor while it gets up to operating temp. Then I drain everything out and leave it open all winter. As you can see I am very thorough on that for it's not worth losing a motor. The oil cooler is not factory mercruiser either, I have an extra large oil cooler. Problem is, I have to pull the motor to get it off. I need to try and pressure test it in the boat. It makes sense now that you have all pointed it out. I never thought of it.

My motor build was a copy of the old HP-i motor from mercruiser if any of you remember that one.

Bypass the water to the oil cooler and fire it up on the hose and look for oil if pressure checking is too difficult.

How is the cooler plumbed on the raw water side? If similar to Merc to where it is second inline after the power steering pump cooler could you not just remove the hose feeding either the circulating pump or crossover and dump the water into a container and look for a telltale oil sheen? If it is introducing that much oil into the raw water stream you would think it would show up in the water left in that hose or on the inside of the hose itself. Just a thought if it is that much of a beech to try and pressure test the cooler with the engine in the boat.

I have thought of just draining it and looking at the water through some Mason jars.

Pulled the oil cooler.... put 30 psi in it and it has held for over an hour... looks like a leak down test is in order on the motor.

Being this is a few years old, Im hoping you got this resolved. What did you find your answer to be? Having the same issue.. cooler tested okay.