Oil consumption at 3,500 t 4,000 RPM gas engine

[northern]

General question. If you run your boat at 3,500 to 4,000 RPM extended time do you use oil. Reason for asking I ran engine for 35 hours at high RPM over a 4 day period and it burnt 1 quart of oil. No smoke, no water in oil, no loss of power. Engine hours 120. Engine 5.7 year 2008.

 

[skolbe]

My 350 mags don't. When I had a 260 with a 7.4 mpi it would use a little. Not a quart.

 

[MiLoII]

My 350 mags don't use a drop and that's my cruise rpm around 36 to 3800

 

[hottoddie]

When the crankcase is full or just overfull an engine will consume more oil than when it is 1/2 to a quart below the full mark. Is the boat propped correctly meaning can it reach full rated rpm with normal load? If it can't the engine is working too hard to do its job will also be more prone to using oil.

 

[Brite Idea]

I don't think you have a problem. It sounds like you were working the engine and the engine used a little oil. Look at it this way I usually run about that many hours in a season and use about a half a quart per engine(one uses more and the other less). You basically ran my whole season in four days. Did you go on a long trip? Was the boat heavily loaded? Did you pull skiers or tubes? Were you jumping the boat on plane from a dead stop a lot? These are things that will consume oil and if so you did it all in fours days.
Is this the 380 Aft cabin boat? It says 1989 boat year but you say 120 hours on the engine so while that's low hours how old is the engine we're talking about? Valve seals age by time not hours so if anything the seals could just need replacing.

 

[Pirate Lady]

General question. If you run your boat at 3,500 to 4,000 RPM extended time do you use oil. Reason for asking I ran engine for 35 hours at high RPM over a 4 day period and it burnt 1 quart of oil. No smoke, no water in oil, no loss of power. Engine hours 120. Engine 5.7 year 2008.

No oil, but anything over 3400rpm I’m opening the 4 barrels and burning >25gph, drain a tank in under 3 hours. You must be in a hurry.

 

[northern]

Not the 380 just single engine jetboat. Boat was loaded with 1000 pounds fuel (180 gallons) 700 pounds gear and tools and 400 pounds of people. Boat took about 10 seconds to get on plain and felt heavy. We went 550 miles on a northern river. We have never run it this long and heavy in the past. This was the first time we have had to add oil between oil changes. Have not been able to use the 380 since fall of 2019 as we live in Canada an keep the bot in US. Marina is looking after it so not concerned. the 380 is stored on land. It was put away full with all oils changed and there are heaters on it. Costing me $35 a month for electricity to keep it warm.

 

[sbw1]

We have owned three boats with 5.7 liter engines and none burned any oil. We were propped properly and cruised at 3350 rpms. I was told that at that cruising speed we would never wear out the engines. We sold one boat with 4000 hours on it and it still ran very strong with no oil burn. Every year we would run a ten hour leg to northern Michigan with just one stop for fuel. Again, no oil burn. Having said that, using a half a quart of oil is probably considered normal. You may want to check the accuracy of your dip stick. My port side Crusader 8.1 routinely blew a half a quart of oil out with in 15 hours of running when it was new. I filed a warranty claim. The tech drained all of the oil out of the engine and refilled it with the recommended oil quantity and the dip stick registered a half quart low. He then remarked the dipstick as full at that level. From that time forward the oil stayed at that level until it was time for an oil change.

 

[hottoddie]

I agree with others not to be concerned but just keep an eye on it down the road.

 

[scoflaw]

No problemo, oil is cheap

 

[El Capitan]

Normal at high rpms for extended duration. I wouldn’t recommend it.

 

[northern]

Normal at high rpms for extended duration. I wouldn’t recommend it.

Jetboats have 2 speeds 1200 to 1400 RPM when you are floating down a river. The other speed is fast when you are going up a river. Jetboats are not fuel savers. The jet is just a small propeller in an internal housing that is like an aircraft jet engine. They are designed to idle or fly. Fuel consumption on flat water is almost the same at 1400 RPM as it is at 3,500RPM
Thank you all for your input now I know I do not have a problem.

 

[ttmott]

Isn't there only four ways that oil levels can change?

• External leaks
• Valve Guide seals failing
• Piston oil control ring compromised
• PCV system malfunctioning.

Higher RPM will only increase the volume of oil flow through the engine; oil pressure does not increase. Other conditions that manifest (detonation, crankcase pressure, PCV malfunction, etc.) at higher RPM can be the cause of oil loss however engine speed in it's self isn't a cause.
Poor fuel quality can result in detonation or pre-ignition in higher load conditions and can unseat the oil control piston rings; this is really bad for the engine. The oil control rings are the bottom thin oil scraper piston rings. Detonation or pre-ignition causes the piston to rock in the cylinder, that knocking sound. Besides being really hard on the pistons and rings crankcase pressure goes up due to blow-by and can contribute to oil consumption through the Positive Crankcase Ventilation (PCV) system.
So, an engine just doesn't start loosing oil for no reason....
With all that said - a quart is a lot for only 35 hours of run time in my opinion. I would run it hard again and immediately pull the spark plugs to look for signs of oil; if oil is seen on the spark plug(s) start by looking at the PCV system then move to replacing the valve seals. Both of these can be done by a mechanic in a day.

 

[Stee6043]

I typically cruise at 3,500-3,600rpm and burn no oil with my 7.4's. 1,300 hour motors. It would worry me a bit to use that much oil but the consensus here seems to suggest it's no big deal...

 

[El Capitan]

It all depends over what amount of time. It is a normal function to have a gap between rings and cylinder walls thus the burning of oil. No such thing as burning “no” oil. Eventually consumption must occur. The rings are designed to allow oil to flow along the entire cylinder wall. That includes the combustion chamber. Which burns oil.

 

[ttmott]

It all depends over what amount of time. It is a normal function to have a gap between rings and cylinder walls thus the burning of oil. No such thing as burning “no” oil. Eventually consumption must occur. The rings are designed to allow oil to flow along the entire cylinder wall. That includes the combustion chamber. Which burns oil.

What? Maybe in a 1908 hit and miss engine. There should be no gap between the piston rings and cylinder wall; the ring fit must be circumferentially tensioned to the cylinder wall including the oil control rings. The end gap clearance when hot should approach zero. The only oil on the cylinder after the piston travels downward is that in the micro-pores and cross-hatch which is literally molecules thick. A healthy engine should have no discernable oil loss between oil changes.

 

[MonacoMike]

What? Maybe in a 1908 hit and miss engine. There should be no gap between the piston rings and cylinder wall; the ring fit must be circumferentially tensioned to the cylinder wall including the oil control rings. The end gap clearance when hot should approach zero. The only oil on the cylinder after the piston travels downward is that in the micro-pores and cross-hatch which is literally molecules thick. A healthy engine should have no discernable oil loss between oil changes.

Some of you guys should read the Mercruiser owners manual for the old-school 8.2 engines. It explicitly states in the owners manual those engines burn a quart of oil in 5 to 10 hours. It also states the reason is because it is a high performance high temperature engine there are larger gaps in the pistons, rings, and cylinders.

I have heard from truthful owners that those engines did indeed burn that amount of oil when they were brand spanking new, especially if they were ran hard.

I believe most people when they say that their engines don’t burn a drop of oil about as much as I believe those running around saying their boat doesn’t have a drop of water in it. LOL

P.S. After three seasons I finally do have my boat dry. I wonder how many weeks before the next leak appears. LOL

 

[Stee6043]

Some of you guys should read the Mercruiser owners manual for the old-school 8.2 engines. It explicitly states in the owners manual those engines burn a quart of oil in 5 to 10 hours. It also states the reason is because it is a high performance high temperature engine there are larger gaps in the pistons, rings, and cylinders.

I have heard from truthful owners that those engines did indeed burn that amount of oil when they were brand spanking new, especially if they were ran hard.

I believe most people when they say that their engines don’t burn a drop of oil about as much as I believe those running around saying their boat doesn’t have a drop of water in it. LOL

P.S. After three seasons I finally do have my boat dry. I wonder how many weeks before the next leak appears. LOL

Man you learn something every day around here. But what a strange way to get there.

So I pulled my manual and what do ya know...the 8.2L is an oil hog, says so right there in the fine print.

But this means nothing to the OP since he's running the tried and true 7.4's. Soo.... @northern I'm going back to my original opinion up there - I feel like you're burning too much oil in a 7.4. You don't get a "burning lots of oil pass" from Mercruiser with the 454's...just a "little"?

 

[Smokeio]

I had 425HP 8.1 Crusaders and was always adding oil to them. It seemed to vary but I would say 1/2 to 1 quart every 6 hours.

I also had a 320 with 6.2s and I can't remember ever having to add oil!

 

[juggernaut1]

Oil quality and viscosity will also play a part in oil consumption.

I use to use the Merc 25/40 oil and had to top up from time to time during the year. A few years go I switched to our locally made Penrite 15/50 100% pao/ester oil and now I don’t need to top up during the year.

FYI my brother an I used to have a rock lobster fishing business and ran an aluminium jet boat with a Cat 3208 425 hp and Hamilton jet unit. The jet boat was preferred around the reefy shallows compared to a conventional prop boat which doesn’t handle a reef strike too well and doesn’t have the manoeuvrability of a jet boat.