Burning oil

69fastback

2003 350 MAG - 380 hours 

This engine has never burned a drop of oil, and then mid season, it started using a fair amount. I tracked it, and in the last 9 hours its burned .5 quart. 

Today I warmed the engine up and did a compression check. 


Results: 

1- 158
2- 161
3- 150
4- 155
5- 154
6- 148
7- 151
8- 153


6 is a little low, but i dont see anything too alarming. Are valve seals a common issue in the 350 MAG?  Could also be the PCV, and i was going to change it but never got to it. I was going to do a leak down, but I’d have to remove the back seat to do it, so i didn't do it. I’m mainly just curious if people have had issues with valve seals. Also, all the plugs looked fantastic.  Any other ideas or common issues?

Handymans342

If you have the rubber cone type they do tend to crack with age. Do you see any smoke?

69fastback

Yeah I hate those cone type but I don’t know what’s on the 03s. I’ve never noticed any smoke,but the outdrive is also pretty far under the swim deck. 

Handymans342

Well you could see smoke running it in the driveway

69fastback

I didn’t notice any smoke sitting there at idle. I’m about as certain as I can be that it’s burning it though, because the engine compartment is as clean as the outside, and leaking 1/2 quart would be pretty messy. I wouldn’t think 1/2 in 9 hours would show much smoke at idle, would you? 

Handymans342

Rev it up several times and watch. I believe if it smokes when you let off the gas its the seals. Is an 03 Fuel Injected?

69fastback

I think you’re right. at least in a car, I know bad valve seals will smoke on start up because there will be oil pooled on top of the valve, and it will smoke on deceleration. It is a MPI motor, smartcraft, all that. I have VV mobile and it has no codes. 

Handymans342

So is it fuel injected or TBI?

69fastback

Multi port injected fuel injection. 

Handymans342

If you had a sticky injector, that could wash the cylinder walls and use oil and there would be minimal smoke.

69fastback

Am I wrong in thinking that would cause a performance issue, and would show up on the plug?  The plugs look fantastic, and and the boat runs normal. Just for reference, I run two nitrous cars and read plugs every pass. I probably read 100 sets of plugs per year and these plugs look great. 

Handymans342

I was just making suggestions. When my truck started using oil it was time to change it.

69fastback

Yeah I get it. I was just trying to think out loud about it. It doesn’t seem like a stuck injector for those reason, wouldn’t you agree?  It’s got about 50 hours on the oil, with a lot of towing tubes and skiers. One thought that has crossed my mind, is I always ran valvoline Dino oil in it before, and I had a shop winterize last year for the first time, and we forgot to discuss oil, so they put mercruiser semi synthetic in it. Maybe that’s a portion of the culprit. 

Handymans342

Could be.

Michael T

Even Valvoline "dino oil" has some synthetic in it. All oils do or they would not meet EPA mileage standards.  Today there should not be a problem going from "dino" to synthetic. There were problems years ago using super slick synthetics before the rings had seated but this should not apply to you. My guess if you were burning enough oil to show up on your plugs the volume of depleted oil would be higher and the oil control ring wear would show bigger compression discrepancies. My guess is your valve stem seals are to blame. I doubt you would see burned oil on the water - maybe on the muffs? Best wishes for the least expensive fix!

69fastback

I haven’t seen any oil anywhere on anything. Lol

I was leaning towards valve seals too, but the more I think about it, it’s highly unlikely that multiple seals started leaking at the same time, meaning all the oil would be going out one cylinder and that would show up on the plug. I’m thinking it may actually be a crankcase venting issue and it’s being distributed enough that it doesn’t show up on the plugs because it’s a small amount going out all of them. 

aero3113

Is it possible they put too light of a weight oil in? 

69fastback

They said it’s 25W40. 

zaverin1

That’s normal merc or quick oil 

zaverin1

Oil pressure will drop from
lets day when 30 weight is ran 

Michael T

I had two stem seals fail on a 302 in a Mustang and thought someone was stealing my oil until it finally turned-up on a couple of plugs - I figured one or more of the seals just broke up. and really let the oil loose.

MarkB

Handymans342 said:

If you have the rubber cone type they do tend to crack with age. Do you see any smoke?

When my original 350 MAG was burning about a quart every 5 to 10 hours ... no smoke.  In the end it was one of the rings. Merccruiser replaced the entire engine.

Michael T

I remember that Mark!

69fastback

MarkB said:

Handymans342 said:

If you have the rubber cone type they do tend to crack with age. Do you see any smoke?

When my original 350 MAG was burning about a quart every 5 to 10 hours ... no smoke.  In the end it was one of the rings. Merccruiser replaced the entire engine.

Did it show up on a compression test?

seadogs

Intake gaskets if it has the plastic gaskets. The silicone shrinks and allows oil and a vacuum leak of idle. Not enough on any single cylinder for a real plug color change. 

seadogs

That was off idle. 

69fastback

I finally found time to work on it again. I pulled the PCV and it's definitely gum'd up some and I had to shake it pretty hard to get it to free up. It still doesn't move freely like a new one, so I replaced it and changed the oil. I plan on taking the boat out tomorrow to see what happens. Hopefully I can get 4-5 hours of run time. 

MarkB

69fastback said:

MarkB said:

Handymans342 said:

If you have the rubber cone type they do tend to crack with age. Do you see any smoke?

When my original 350 MAG was burning about a quart every 5 to 10 hours ... no smoke.  In the end it was one of the rings. Merccruiser replaced the entire engine.

Did it show up on a compression test?

Yes, one cylinder was low on the test. It wasn't crazy low, but you could see it was the oddball when comparing to other cylinders.