7.4 Oil sheen

[High Cs]

I have a 7.4 liter MPI, closed cooling engine in a Bayliner 2556 that appears to be leaking oil showing up as a sheen behind the boat at idle. Engine is very clean and no oil/gas in the bilge. It has a new Bravo III outdrive and it is not leaking oil. I change the engine oil regularly and often and, up until this last oil change I never had to add any oil . I never see smoke of any color coming up. This time when I checked the oil it looked very good (yellow/new) but was down about 1-1/2 Qts. Although I haven't varified compression, it is very strong engine and at this point doubt I am burning oil.
I am no mechanic by any means but I'm thinking valve guides or seals? Any Ideas? expensive?
Thanks,
Don

 

[IdahoDoug]

Re: 7.4 Oil sheen

You need to clarify if this engine is FWC (fresh water cooled or an open system) as your signature says, or closed cooling system as your post says. Things point to an oil leak and it will be a big factor as to type of cooling system.

 

[High Cs]

Re: 7.4 Oil sheen

It is a "closed cooling system". Sorry for the missunderstanding in the signature. This was done a while back and I do not see how to edit this. Any direction here?
In my defense, the Mercury factory service manual uses the terms: "Seawater Cooled Models" and "Closed Cooled (Fresh Water) Models", in their headings, thus the "FWC" in my signature.
I think I might have an oil cooler cooled by raw water. Could this be the source?

 

[tpenfield]

Re: 7.4 Oil sheen

1-1/2 quarts is quite a bit to be down. It would depend on how long it took to go down. Your oil cooler theory may be correct . . . valve seals is a possibility, or maybe even head gasket.

Time for a compression test and maybe a leak-down test of all the cylinders. A quick check of the oil cooler theory would be to take the inlet hose off of the thermostat housing. . . if you see oil at the top of the hose, then it points to the oil cooler as the culprit.

 

[High Cs]

Re: 7.4 Oil sheen

This engine never used oil. I change oil at beginning of season, mid season, end of season (for storage) and start new season with new again. I never had to "add" oil before. I am at mid-season, due for change now, and was down 1-1/2 quarts. I first noticed the sheen on my last trip out. I suspect that this just started.
If the oil cooler is perforated in some way do I also risk water in the oil? Currently the oil looks good

 

[HT32BSX115]

Re: 7.4 Oil sheen

if you see oil at the top of the hose, then it points to the oil cooler as the culprit.

Howdy,


If you're losing oil and you don't have obvious leaks......you need to remove your oil cooler and pressure check it.

The engine (oil system) puts out FAR more pressure than the raw water system. The raw water goes from the oil cooler into the heat exchanger and gets dumped into the risers and overboard. Any oil from the oil cooler will get dumped overboard too if it leaks there.

Regards,


Rick