High oil consumption on one engine

I would check the transmission coolers also.
When they fail you will see an oil sheen in the water.
 
Wow! $10K will buy a lot a oil!
My first two cars burned and leaked oil but ran for many years. As long as it cranks, runs reliably, and your not polluting a sheen behind you, run it!!!
That’s just me though.
 
There is a fuel/oil treatment by Schaeffers called Neutra Fuel Stabilizer 131. You can run this in your fuel but also in your oil to help break down carbon deposits. IF if it's a stuck ring or valve, this could help free it up. Read online. The stuff works.
 
You didn't say what the actual oil usage was after you replaced the PCV valves. Even though one cylinder is lower than the others, it is still within spec (within 70% of the others). You need to do another compression test with a leakdown test to determine where that cylinder is loosing compression. Reworking the heads is a lot cheaper than a long block. So it's using some oil - the engines are 23yrs old - nothing at all wrong with running it while you figure this out. And personally I would think twice before I put any significant $$ in a 23yr old boat - if you like the boat and intend to keep it then do it, but don't think you will ever get the $$ out of. For the $10k you mechanic is talking, you could buy a complete reman.
 
You didn't say what the actual oil usage was after you replaced the PCV valves.

Same....replacement of PCVs didn't impact the oil consumption.

Realistic about not getting the money back on sale of the boat. It's like spending 85 grand putting in an all new kitchen or roof on your house before selling it...doesn't mean the price of the house went up 85k. But it can help sell the house if the whole price is right.

While we dream of upgrading to a 37-40....that's not for a couple of years..so we'll have this boat at least another 2-3.
 
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So my next step would be to find out what is really wrong - could be you just need the heads reworked, if so that is not too bad. If you are getting into the bottom of the motor, I would really look at a complete reman like a Michigan Motorz. By the way, the older 300's with v-drives are nice boats, worth investing the $$ in to keep in my opinion.
 
5.7's are plentifull and cheap. Buy a long block for around 2k and bolt all your stuff to it.
 
Lucas makes some really good additives. I have seen them work miracles in many car engines, transmission and steering. Lucas used to be for the big truck industry years ago.
 
Just had to re-read this thread I made a couple of years ago....
Made a run to Mystic today from Deep River, was a 2.5 hour cruise...majority of it at 3,200 rpm/19-20 knots.
Departed with both engines at top of the dipstick line
Arrived...port engine down about 1/2 qt, starboard engine down 1x qt.

Have made several short runs this season, this was the first longer one. Will be repeating in 2 weeks and another run to Greenport LI.

She doesn't visibly smoke...looking behind me when on plane, no smoke. Traveled with a couple of other boats this weekend, one of which was behind me most of the way...asked him about smoke..he didn't notice any.

Even starting up, at idle, no puffs of smoke.

Suppose it's really time to do a proper investigation, that "leakdown test".
Approx 720 hours on engines.

They fire up quickly..and run well. Sometimes I do swear I hear a "whistle" sound when above 3krpm....someone mentioned that piston blowby will cause the PVC to whistle into the intake.
 
I thought 1 QT in jet boat with a 350 at 3,500 RPM as a lot after 30 hours of use. Look at it this way you spent $10 on oil and $600 on gas. I use strait 30 weight oil. Oil type and manufacture can make a difference in oil consumption. What does your manual say to use.
 
I'm inclined to go with the "oil is cheap" group, but I understand you're wanting to know why the port engine is better than the starboard and it'd be nice to find out what the cause is. Someone had mentioned earlier about the valve guide seals; definitely good advice. Most OE seals are rubber "umbrella"-style seals that merely divert the oil out and around the guides. Engines that old are prone to having their rubber parts go bad. I've found more than one seal split, allowing oil to pour directly down onto the guide where it then gets drawn into the engine.
As for working insitu; rather than relying on the leakdown tester to maintain pressure on the valves to hold them closed, you can use an old snowmobiler's trick to remove flywheel and clutch bolts:

1) Grab a length of limp nylon rope that can easily slide through the sparkplug hole in the head (3/8"?)
2) TIE A BIG KNOW ON ONE END!!! Yeah, you don't want the rope disappearing down the hole...
3) For the cylinder you're working on, bring it up close to TDC of the compression stroke (~90* BTDC)
4) Snake as much of the rope down the sparkplug hole as will easily go in
5) Continue rotating the engine towards TDC. You'll know you've been successful when you feel the crank lock up and can't turn it any further

At this point, you've essentially hydraulically locked the piston and valves in place against the wad of rope, allowing you to take the valve springs off with no worries of them falling into the cylinder.
 
The '97 260 in our boat has over 1000 hrs now. It goes about 30 hrs without the level changing. Add close to a qt at 40 hrs and dump it at 50. We run 15w40 in it as there is always a pile of it laying around the shop.
 
I thought 1 QT in jet boat with a 350 at 3,500 RPM as a lot after 30 hours of use. Look at it this way you spent $10 on oil and $600 on gas. I use strait 30 weight oil. Oil type and manufacture can make a difference in oil consumption. What does your manual say to use.
Not knocking what is working for you but have you tried 15/40? We used to run the straight 30 in our 5.7 Kodiak with a 3 stage Hamilton in our 20' Ali-craft. It seemed to be a common choice with jet boaters. The result was similar to yours. Switching to 15/40 greatly reduced it's consumption and extended the service interval.
 
Send off a oil sample, If its not making any metal and its running good I would add the oil and run it. I agree on maybe changing grades of oil. Could make a difference.
 
Old school here. You DO know Mercruiser recommends straight 40w in their engines? It aint a car in winter. Multi-grade not needed.
30yo 7.4 here, straight 40w. Dont use a drop between changes.
 
Recommended Mercury Quicksilver Marine Engine Oil
150HP%20FourStroke_OilDrain-wWrench.jpg


Which oil is recommended by Mercury? See our marine engine oil charts below to see which is best for you.

Find your recommended Mercury/Mercruiser marine engine oil and filters here...see charts below.



See our Oil & Gear Lube Capacity Charts for proper amounts.



2-STROKE OUTBOARD OIL APPLICATION CHART

OIL Low HP: 2.5 HP -
30 HP Mid HP: 40 HP -
115 HP High HP: Above
115 HP All HP
Carubureted or EFI Carubureted or EFI Carubureted or EFI Direct Injection
Premium Good Good Not
Recommended Not
Recommended
Premium Plus Better Better Better Better
DI Engine Oil Best Best Best Best


4-STROKE OUTBOARD OIL APPLICATION CHART

OIL LOW HP: 2.4 HP - 30 HP MID HP: 40 HP - 115 HP HIGH HP: 115 HP AND ABOVE CATALYZED/NON-CATALYZED ENGINES
10W30 Good Good Good Not
Recommended
25W40 Good Good Good Good
25W40 Synthetic
Blend Best Best Best Best

STERNDRIVE/INBOARD GAS ENGINE OIL APPLICATION CHART


OIL CATALYZED ENGINE NON CATALYZED ENGINE
10W30 Not Recommended Not Recommended
25W40 Good Good
25W40 Synthetic Blend Best Best


Buy Your Mercury/Quicksilver Oil Filters Here



Buy Your Mercury/Quicksilver Oil Here
 
1qt of oil in 2.5hrs is a lot, if it's not burning it it has to be going out through the oil cooler -- just no way that much oil disappears without leaving a trail - smoke, a sheen, smell, something.

No oil cooler on her...not on this year of 5.7 Bluewaters.

I have been running Quicksilver the 25/40 since I got her....I think once or twice I found some 20/50 and tried that.
I've also started using a bit of that Rislone treatment. Will use more.
Normally I don't mind just topping off oil after every longer run, BUT....these older Dancers...gotta pull the 2x big curved aft bench seats and then the 4x big deck plates. I'm in mid 50's with a pretty bad lower back (lots of smashed disks). So I can't do much longer.
 
No oil cooler on her...not on this year of 5.7 Bluewaters.

I have been running Quicksilver the 25/40 since I got her....I think once or twice I found some 20/50 and tried that.
I've also started using a bit of that Rislone treatment. Will use more.
Normally I don't mind just topping off oil after every longer run, BUT....these older Dancers...gotta pull the 2x big curved aft bench seats and then the 4x big deck plates. I'm in mid 50's with a pretty bad lower back (lots of smashed disks). So I can't do much longer.
This is very interesting to me. My 91 has a aft seat that folds against the transom. When we were shopping i looked at some 95-98 that had wrap around transom seats that were bolted in place over the engine hatches. We shyed away from them cause wife and me couldn’t figure out how to open the hatches.
 

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