JCCRAZY

I have a 97, cool fuel 502mpi and am experiencing excessive oil blowout through the vent tube running to the breather. When the engine is run above about 3500 rpms the oil will even splatter on the bottom of the sun pad. The last run appeared relatively small amount while a 15 minute run a couple of hours earlier lost almost a full quart. I forgot to mention the tube was off the K&N when we noticed the excessive amount on the engine and the bilge. The engine is running a straight 50wt Kendall and will only pull about 4000 rpm at wot. Confirming fuel pressure readings is when we found this last problem. The pcv is clean and doesn't even have oil residue in the line and vaccum readings are normal. Temperatures appear normal and there is no evidence of blowby on the transom or from the other bank. Any assistance would be appreciated. Could it have excessive crankcase pressure due to oil weight? I operate in extremely high temps, above 90 degrees most days. Pcv seems to be original merc.

bobby daniels

Why anyone would run 50wt unless they wrongly thought it would stop blow by which is what it sounds like you have ,,do an compreeeion test and leak down ,and put real oil not grease in it .

JCCRAZY

Compression and leak down tests show no problems.

Most shops in this part of the country and many boaters running
50wt.

I personally would prefer 40wt but this is what is in the engine at this time.

blown1500

Your problem is caused by crankcase pressure. This is through the intake gaskets, the head gaskets(very unlikely) or through the rings. Boost can leak into the valley under the intake and cause this problem.
If you have gapless second rings, you can have terrible blowby when running and still have ZERO leakdown on a tester.

JCCRAZY

The engine is totally stock, so no gapless rings I would venture.
I would suppose an intake pressure might not show up with a vaccum gauge, or I would think it would at least fluctuate?

JCCRAZY

I am seeing a steady 14-15 inches. From what I have read this seems to be correct. No other evidence of of intake problems, such as a miss, but it does seem to be setting a MAP sensor code.
I plan to replace it and reset the computer.