Mercruiser 454 Oil Burn

Mike

Mike

New Member
May 29, 2007
62
Middle River, MD
We've put about 10 hours on the engines so far this year and yesterday after checking oil levels I noticed that they were burning some. I added approximately 1/2 quart. Is this normal for a Merc 454 with 1000 hrs. I use straight 40w, should i use a different oil?? Most of the use has been at low rpms so far but i never run them over 3300 anyway.
 
Sounds like they are starting to show their age a bit. 40W oil is good. It's not unusual to see higher hour 454s (2000 hours) that use little or no oil, but your situation is not usual either. A lot has to do with how they were broken in and how they are run. Keeping rpms at or below 3300 is good if you are able to achieved the spec'd WOT with your boat. You are fortunate not to be running 502s. Most and maybe all of them used a quart of oil every 7-10 hours no matter how they were broken in and run. What kind of SR boat do you own?
 
Mine burn less than that: about a qt every 25-30 hours at 3400-3500 RPM. (also with 1,000 hours) BUT, they'll consume more if run hard while still cold.

1/2 Qt in 10 hours wouldn't keep me up at night, but I'd continue to monitor it.
 
I have a 1993 454 with 1104 hours. Last year I put about 150 hours on it and didn't have to add any oil. Using 15 40W same as in my truck.
 
sbw1 - I have a 1992 370 Express Cruiser with twin 454 / thunderbolt ign. with straight drives. Engines fire right up and run well. Consistently burn 30gal an our at a cruise speed of 20-22mph. No matter what the seas I burn the same.
 
Hi!
Is your engine 454 330 or 370 hp?
If 370 (magnum) they use some oil, and in some manuals is advisable to check oil level every hour, the same is with 502 because engine concept is the same (forged pistons, higher tolerances etc.)
However, all of these engines, both small and big block can use oil due to the way they are build, keep in mind that most generations don't have valve stem seals...
So, if there is no blue exhaust smoke or blow by keep pouring oil and gas and keep 'em running "hard"
I would not worry one bit!
Best regards, Goran
 
Mike said:
sbw1 - I have a 1992 370 Express Cruiser with twin 454 / thunderbolt ign. with straight drives. Engines fire right up and run well. Consistently burn 30gal an our at a cruise speed of 20-22mph. No matter what the seas I burn the same.
Click to expand...
I had a 93 370 Express. We used to cruise it at 3450 rpms and would start out at 23 knots and end up at 26 knots as the fuel was burned off. We would get anywhere from .5 to .75 mpg depending on the seas (following by far the worst) and the state of the tune up. For some reason those 454s were hard on spark plugs. I could never get three seasons out of them unlike all of the other boats I've owned.
 
Large bore engines are more likely to burn oil than small bore ones just because the area of each cylinder that must be sealed off by the compression and oil rings is greater. An engine that constantly runs at 3300 rpm under load is more likely to burn oil than one that runs 2600 lightly loaded. As engines get older, more from calendar time than hours run, the valve stem seals get small cracks in them and they leak oil into the cylinder........and this is most likely what is going on with your engines.

A compression test will tell you quickly if you have a problem with engine wear in the cylinders add a leak down test and you can verify the condition of the valve seats. Rule out those to concerns and you can probably get a good mechanic to change the stem seals without pulling the heads off the motor........but I'd probably pull the risers, manifolds to check them and, since that is a major part of the labor in pulling the heads, I'd probably pull the heads off and have a complete valve job done given that you are at 1000 hours.

Or, you can continue to watch it and add oil as needed, and single viscosity SAE 40 is about as good as you can do for minimizing oil consumption.
 
Frank,
My new to me 1992, 7.4 454 (600 hrs) had compression numbers of 120s in all eight cylinders. Your impression of these numbers ? Thanks, Dave.
 
Quint4 said:
Frank,
My new to me 1992, 7.4 454 (600 hrs) had compression numbers of 120s in all eight cylinders. Your impression of these numbers ? Thanks, Dave.
Click to expand...

Those numbers are a little low (not abnormal though) but since they're all the same, that's a good thing. Normally they should be around 150 psi or so. Try a different gauge to see if that changes your readings.

Doug
 
Since the topic has come up about rpm's and gas engines, does anyone know what the WOT specs are for 7.4 Horizons or 454's in general? You always hear the diesel guys talking about getting the WOT rpm's that the manufacturer specs out but never on a gas boat. Wonder if its as important? I'd like to see what ours does.
 
Skuza said:
Since the topic has come up about rpm's and gas engines, does anyone know what the WOT specs are for 7.4 Horizons or 454's in general? You always hear the diesel guys talking about getting the WOT rpm's that the manufacturer specs out but never on a gas boat. Wonder if its as important? I'd like to see what ours does.
Click to expand...

I too noticed the same thing. I was always curious why the diesel guys make a big deal about 50 - 100 rpms less than manufacturer's spec?

Doug
 
Because it makes a huge difference in how efficiently a diesel burns its fuel charge. Running it at WOT when it cannot turn up rated WOT rpms causes overloading, the cost of which can be very substantial in engine damage and in $.

Diesels are rated at WOT rpms and their fuel delivery systems, cooling systems, etc are all designed to run at and do their jobs at that speed. If you cannot turn a diesel up to the rated rpms then you are injecting more fuel into the cylinder than it can burn, you are not moving enough cooling water thru the heat exchanger, you increase engine temperatures etc. Unburned fuel leaves the engine either in the exhaust or into the crankcase. One of the worst things you can do with a diesel is to allow unburned fuel in the cylinder because the excess fuel washes the lubricating oil off the cylinder walls and causes early and extreme cylinder wear. Incomplete diesel combustion also increases carbon deposits in the cylinder and on the injector nozzels. Deposits on the injectors causes them to spit fuel droplets instead of atomizing the fuel charge.........and that just adds to the incomplete combustion and increases cylinder wash out.

The above situation doesn't affect gasoline engines, even EFI motors, because they are ignition fired, whereas, diesels are compression fired. The ignition system and spark plugs on gas engine set a fire in the middle of whatever gasoline vapor that happens to be in the cylinder and it all explodes. Sure, the engine may run a little rich, but it takes a completely dead cylinder to was down the cylinder walls in a gas engine.

Hope that fills in the blanks...............

As far as RPM's for 7.4's, there have been serveral different configurations for this engine over the years so to be precise you need to check the specs for your engine......the late 80's 340hp carbureted version was rated at 42-4600 rpm @WOT. I proped my 390EC for 4400 and cruised them at 2900 rpm (rated cruise was 3200 rpm) and we put right at 1100 hours on them with nothing but routine maintenance over an 8 year period.
 
So what you are saying is that unless a diesel engine reaches specified top rpm, it will not see complete combustion even at lower rpms? In order to diagnose complete combustion on a gas engine they will determine the air/fuel ratio, seeking an ideal 14.7:1. Because they don't do this on a diesel, they go by specified rpms at WOT?

Doug
 
I said:

"Diesels are rated at WOT rpms and their fuel delivery systems, cooling systems, etc are all designed to run at and do their jobs at that speed. If you cannot turn a diesel up to the rated rpms then you are injecting more fuel into the cylinder than it can burn, you are not moving enough cooling water thru the heat exchanger, you increase engine temperatures etc. "

What happens is the the amount of fuel injected or air induced is determined mechanically (or electronically on newer engines) based on the engine producing rated hp at WOT. These are design decisions made by the manufacturer. If your engines cannot product WOT RPMs but you still run at WOT, then you inject more fuel than the engines can burn. Unless you reduce power at each throttle setting by the amount of short fall at WOT then you overload the engines. If you are 250 rpm under at WOT and your WOT is 2500, then your cruise rpms should be reduced by 10%......instead of cruising at 2300, then you need to cruise at 2070 so you do not overload your engines. Most of us choose to change props or repitch current ones in order to "fix" over loading.

There are a host of other factors involved in this process that vary by engine make and type involving turbo charging, boost, boost pressures, etc. that I'm not qualified to discuss. But I do know that the quickest way to get you know the diesel repair guys in your area on a first name basis is to run your diesels in an overloaded condition. described above.
 
Appreciate the response. Makes sense. I am not a diesel guy by any means (only gas) but I want to learn at some point.

Doug
 
I called Mercury today after a little internet surfing and the guy on the phone was able to help me with my question. I asked about cruise and WOT for our '98 7.4 Horizons and he stated as follows. WOT is 4,400-4,800rpm while cruise is 75% throttle or 3,300-3,600rpm. I should have asked about fuel burn rates but I'm sure that is dependent on size of boat and many other factors. Guy was helpful!
 
Try using the MERc recommended 25-40. I personally replace one of the quarts with Slick 50 every other opi change and make sure you follow the 50 hour or 120 day change oil rule.

Regards,
 
We have twin 454 330 HP in 1989 38 Aft cabin. Been out this year and one burnt 1 QT and the other 1/2 QT in 216 hours. We run them at 1200 to 2000 RPM most of the time but go to 3800 10% of the time. Use SAE 40 oil. Both engines have 1400 hours. One engine blows a little white smoke for a couple of minutes when started after that no smoke. Did compression check last year and all cylenders were within 5PSI. Do not remember the value it is in my log book on the boat. Both engines seem to use more coolent than oil.
 
captainbill631 said:
Try using the MERc recommended 25-40. I personally replace one of the quarts with Slick 50 every other opi change and make sure you follow the 50 hour or 120 day change oil rule.

Regards,
Click to expand...
Don't know why you revived this thread after more than a year, but slick 50 is horribly bad advice.
 
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