Low hour Mercruiser 383 stroker/scorpion burns a lot of oil

[wildk]

Hi all, first post here. Honestly I should have joined this forum a while ago. I have a '97 Eastern 27' Downeast boat with a mercruiser 383 stroker (scorpion?) that burns alot of oil. Sorry for the marathon post, but here goes:

Here's the backstory:

I bought this boat last April primarily for tuna fishing and it had a 1997 Mercruiser 350 with a bravo 2 with ~4000 hours on it. About 10 hours into my use of it, the motor failed. It seemed one of the exhaust manifolds was cracked and I hadn't noticed, and the engine ingested water and hydro-locked. Bummer, but when I bought it i figured it would be up for a repower sooner rather than later due to the high hours.

So I found a 2006 Mercruiser 383 motor with less than 100 hours on it for a somewhat OK deal. The motor had perfect compression, and was generally really clean, so I picked it up, changed the thermostat, water pump etc, and got to installing it in the boat. It ran great, and produced significantly more power than the old motor, and the MPI fuel injection system was a welcome improvement over the carburated motor.

Now:

I soon realized my low-hour 383 burns alot of oil. No warranty from the shop i got the motor from so I am stuck with this problem now. It seems to burn about 1 quart every 5-7 hours. We put the boat away in the fall, and said we would deal with it in the spring and, well... here we are. Aside from burning oil the engine runs great. The compression on each cylinder is still exactly what it was when I bought the motor -185psi, and not a huge difference between hot and cold compression - seems about perfect. When each spark plug comes out, though, it is absolutely drenched in oil.

The prevailing wisdom last fall was that the valve seals had degraded from sitting. I don't know how long this engine sat to be honest. It's possible it sat for the better part of 12 years, but it is just not known. The guy I fish with has a friend who builds chevy motors for old muscle cars, and this guy is now telling me that I've got stuck rings, and that it needs a full rebuild (that of course he would be happy to charge me for), because there's no way valve seals could lead to this level of oil consumption. I am not sure how i could have stuck rings on all 8 pistons to be honest, with perfect compression and no other adverse effects.

My theory is there is alot more potential for oil consumption on a boat engine like this because that engine is working way harder than it would in any car. It's pushing a fairly large, heavy, and inefficient boat, and running at higher throttles quite a bit of the time. My fishing buddy is pressuring me to just drop the money and have this guy rebuild it (of course he wants to go catch some fish, so do i), but i am hesitant to go that route because the service would likely cost what the motor cost to begin with.

Any thoughts, past experience with oil consumption? Any advice is much appreciated. Thank you

 

[Rick Stephens]

My 8.1L burns a quart every couple thousand miles of easy driving. Did that when new and still burns about the same with 100,000 miles on it in a 1Ton pickup. I'm pretty sure I wouldn't spend a thousand dollars fixing it if my boat motor did that. A case of oil in the locker is very cheap.

I might consider pulling and having the heads redone, but I'd be doing everything 'cept the machine work myself. No way I'd do the bottom end for that kind of cost against benefit. I'd rather spend it on fuel and oil and go fishing.

Sorry, not a lot of help. There are some who wouldn't argue with a big rebuild cost, others, not so much.

Rick

 

[alldodge]

The compression is so high (185) because of the oil, it should be a bit lower (150).

It may need valve seals, but all the plugs being oily I might also thing of rings. Only issue with rings would be the same thing as with valve seals, why every cylinder?

I would pull the heads and see what you find. This way you can make an educated plan on what is found

BTW what is the oil pressure?

 

[Scott Danforth]

check your compression gauge. compression for the engine is 150 psi

anything higher is either oil or water in the combustion chamber or a crap gauge

Mercruiser is using 1.6:1 rockers on the stock vortec heads. this is close to mashing the valve stem seals from the factory

your rings could also be stuck from the used motor sitting

a leak down test will pin point.

if its your rings...... pull your plugs and pour ring-free in the bores. let sit for a day, then turn the motor 90 degrees and pour more ring-free in the bores. let sit for a day then rotate 90 degrees......

then crank it over a few times to get all remaining stuff out of the combustion chambers. change oil, install plugs, then take the boat out and run it.....hard..... at about 3500 RPM for a good 30 minutes plus

then check. if you are still burning oil at that point, then you can either live with it or plan on rebuilding the 377 to a 383

if the issue is the valve guides leaking..... pull the heads, take them to a competent machine shop (find the old guy that does race car motors). have them change the junk GM umbrella seals to real seals. this will require cutting the valve guide bosses. its about $100 for this service. while at it, I would have the rocker studs changed to threaded.

 

[Grub54891]

I know this might not be what you want to hear, but if it were me I'd get a new long block with a warranty. Will the engine builder stand behind his work?

 

[Bondo]

your rings could also be stuck from the used motor sitting

a leak down test will pin point.

if its your rings......

Ayuh,..... Lotsa Good ideas 'bout possible causes,.....

I pretty much Agree with everybody, But,.....
Without any history on the motor, I'll throw out another possibility,.....

What if this motor has idled,... right from the get-go,..?? a dock queen,....
Cylinder glazin',..... It was never broke in,......
A motor has to be run, 'n run Hard for the rings to seat,.....

If cylinder glazin' is the problem, a tablespoon, at 'bout 15- 1800 rpms, fulla Bon-ami will cure it,.....
Then a good Hard run,........

 

[wildk]

Thanks for all the ideas. I would like to do a leak down test. Hopefully will continue to have warm weather here. I'm gonna do one as soon as possible and report the results. If I had glazed cylinder walls, would it pass a leak down test?

As for the compression guage, it might be faulty, it's not a particularly nice one. Also, What is a regular rate of oil use in a motor like this, in a well running/"normal" engine?

Also, I'm pretty sure it's already a 383, not a 377. This is one of those mercury reman motors, I ran the serial number last fall and it came up as a 383.

 

[Scott Danforth]

Virgin bore motora are 377s

Reman motors are 383 (.030 over) or 385 (0.040 over)

Glazed rings would show ring blow by with the leakdown

Bon ami works well if the rings are not stuck. Basically laping compound.

 

[wildk]

if its your rings...... pull your plugs and pour ring-free in the bores. let sit for a day, then turn the motor 90 degrees and pour more ring-free in the bores. let sit for a day then rotate 90 degrees......

then crank it over a few times to get all remaining stuff out of the combustion chambers. change oil, install plugs, then take the boat out and run it.....hard..... at about 3500 RPM for a good 30 minutes plus

then check. if you are still burning oil at that point, then you can either live with it or plan on rebuilding the 377 to a 383

When you say ring-free are you referring to this fuel additive product?

https://www.westmarine.com/buy/yamaha--ring-free-fuel-additive--2280683

 

[tank1949]

Lots of good ideas, but perhaps a can of Marvel Mystery oil? Don't laugh! It does work to a point. If 8 plugs are wet, maybe all 8 seals rotten or hard from sitting? Mystery oil will help free rings and soften seals. But it is not a panacea. I'd live with 1 qt ever 8 hrs, but wet plugs may cause you to be stranded. Good luck!

 

[Scott Danforth]

When you say ring-free are you referring to this fuel additive product?

https://www.***************/buy/yamaha--ring-free-fuel-additive--2280683

yes, you can use ring free or sea foam or straight naptha mixed with a bit of ATF

the active ingredient is naptha (industrial solvent)

 

[wildk]

Any chance this could have been caused by overheating? The impeller was pretty chewed when i replaced it, I didn't think too much of it at the time, but now I am wondering if it ran dry at some point. It still pumped water when I bought it.

I realize this is all speculation till we get some test results. I ordered a leakdown test kit last night and will do it as soon as possible

 

[QBhoy]

Naturally one might assume that with better than new compression and seemingly plenty of power, it goes against the norm to have oil pass by the rings and burn off. Especially at such an alarming rate.
spark plugs aside, you’d also surely really notice it throwing blue smoke out the back and smell it too.
is there no chance of perhaps either too thin an oil or perhaps an oil cooler passing and loosing oil into the cooling water ? I’d think you’d really notice it smelling or blowing blue smoke out it.

 

[Scott Danforth]

yes, an over-heat will egg the bores and can damage the head gaskets. that can lead to oil control ring issues and oil leaks from the head gasket

further, a significantly over-headed head will cook the stock GM valve seals leading to oiling from the valve stem. This will show coked oil on the back of the valves.

 

[wildk]

Naturally one might assume that with better than new compression and seemingly plenty of power, it goes against the norm to have oil pass by the rings and burn off. Especially at such an alarming rate.
spark plugs aside, you’d also surely really notice it throwing blue smoke out the back and smell it too.
is there no chance of perhaps either too thin an oil or perhaps an oil cooler passing and loosing oil into the cooling water ? I’d think you’d really notice it smelling or blowing blue smoke out it.

The smoke is hard to see in the day but visible at night under the bright deck lights, especially when the engine is idling and the boat is not moving

I am (and always have since buying the engine) using mercruiser branded oil from a marina. I don't have the bottle on hand but i think it is 25W40

 

[QBhoy]

The smoke is hard to see in the day but visible at night under the bright deck lights, especially when the engine is idling and the boat is not moving

I am (and always have since buying the engine) using mercruiser branded oil from a marina. I don't have the bottle on hand but i think it is 25W40

Ok. But maybe thinking that losing a litre every few hours would be maybe quite obvious on occasion.
anyway, if not...what about the new engine...does the oil smell of fuel and thin perhaps ? Is there a chance it’s losing oil through a cooler interface maybe ?
Not totally familiar with that particular engine but I know there is a similar 350 version of it often used in tow boats like mastercraft type boats. Some are fresh water cooled. If it’s defo an mpi, does it have the right plugs in it ? (Trying to account for the condition of the plugs). The mpi needs the iridium plugs or at least platinum at second best. The usual BR6FS or similar don’t work as you’ll likely know.

 

[kenny nunez]

I have an odd question, does your engine have the original valve covers and not some kind of aftermarket style. The after market ones usually do not have baffles where the PCV and vent to the flame arrestor hose,
Is the PCV hose to the manifold full of oil? From the compression readings I am guessing that there is no excess “blow by” coming out of the valve cover vent,
If none of the conditions exist that I described then the only thing that you may need is to do as Scott said about removing the heads and having the better valve guide seals installed.

 

[muc]

Might be nothing wrong with it. I didn't see a serial number posted, so can only guess what you have.
Here is a copy and paste from one of the engines listed in the Mercury Repower Product Guide. Have no idea if this is your engine or how many of the engines have this in their description.

SmartCraft ready, 10‑Pin Harness. Flame cover not included. By design, oil consumption of one or more quarts every 5 hours may be typical. Fresh Water Cooling not recommended ‑ call for further application assistance.

 

[gr8shot]

By time you take the heads off, you'll want them milled before putting back on. All new gaskets for everything you took off, the machine work etc, you're probably going to be out a minimum of $500. Could be more depending on what the machine shop finds in the heads.

A lot of people are suggesting advice that involves you spending a ton of money. Your money. It's easy to suggest spending money when it's someone else's money! I'll play the other side of the fence.

If you figure the minimum you could spend to maybe fix the problem is $500, (assuming it's not rings or something that requires an overhaul) you could buy 100 quarts of oil for $500. Thats 500 hours of boating, when you'd otherwise be sitting with the boat torn down, waiting on parts or a machine shop, or maybe even an engine builder.

Go enjoy the boat, keep a few extra quarts of oil in the boat with you, and check the level often. It could be a lot worse! Keep an eye on it, and if the oil consumption does get a lot worse, then you'll know it's time for some serious action!