1995 502 mpi blow by
 

I’m trying to figure out why I have a issue with oil puking out of my crankcase vents. Did a compression test today and the highest was 165 psi, the lowest was 145 psi. I’ve owned this boat for two years and the problem seemed to start at the end of the first summer. If I run pretty hard for any period of time I see where oil Is running down the valve covers from the vents and on a couple occasions I’ve even seen it splattered on the engine hatch. Any ideas?

Is it blow-by - meaning the crankcase is developing pressure or just oil splatter?

Sometimes the little holes in the top of the rockers will squirt oil right at the breathers. If there is actually blow-by there will be a constant flow of air/fumes out of the breathers with some pressure behind it. Like a big blow dryer. Or you could have both problems. Rocker is squirting oil at the vent, the pressure is carrying it out.

When you say vents are they the stock ones like here

https://www.mercruiserparts.com/bam/...31912/4854/160

Once you have a few more posts you can put up a picture of your valve covers.

I’ll have to do some more checking today to see if there is positive pressure on it or not. As far as the vents go yes they are factory according to everything I look at. It has a hose on the right valve cover that runs over to the left cover where they t in together then there is a small tube that that sits vertical and butts up to the bottom of the breather. Thanks

First thing I would do is get new rubber grommets for where the hose fittings connect the valve covers.

You have blow by, no testing needed IMO

Yours is the older style which doesn't have a PVC
https://www.mercruiserparts.com/bam/...31902/1614/210

All Dodge

Correct but mine doesn’t have the larger hose teed into the middle the way that diagram is showing. Mine has just one hose running from one side to the other then tees in under the breather. I guess what is throwing me off is the fact that the engine doesn’t smoke at all when running. I’m using oil but it all ends up in the bilge instead of being burnt.

The hose in the middle doesn't connect to the larger hose (tube), its just separated for drafting purposes. Note the dashed line, it shows that it is connected to the valve cover.

Ok yeah I see that now.

I would do a cylinder leak down test? Then post your results.

Leak down test
 

I just bought a leak down tester today. If the weather cooperates I’m going to try to do that tomorrow.