75 horse merc 2 stroke w/ excessive fuel consumption

[smoky okie]

I have a 2004 75 hp 2 stroke Merc that is using about twice the fuel as Merc says it should ( about 15gph @ full throttle)

It's been that way since it was brand new. I've had it in the shop2 or 3 times and spent $300-$400 trying to resolve the issue, but no luck.

The mechanic that has it now says that it doesn't seem to be coming up to full operating temperature, and that he thinks that's the reason. He replaced the thrmostat, and says it still won't warm up like it should.

He seems to think that it's a design flaw similar to the 1980's model Jonrudes in which the cooling system was too large for the motors.

I'm @ the end of my rope with it. The only performance issue I have with it is that it seems to run rich, load up,bang around and try to die sometimes when in gear @ idle. @ full or mid throttle, it runs great.

Does anyone know anything, or have any experience with this issue that might be of help?

I

 

[LubeDude]

Re: 75 horse merc 2 stroke w/ excessive fuel consumption

If you have it on a relativly heavy boat and you are having to run it at WOT all the time you are going to use a lot of fuel. If it`s on a smaller lighter boat you should run it at about 2/3 throttle and you will see a significant drop in fuel usage.

Oh, and welcome to the forums.

 

[Silvertip]

Re: 75 horse merc 2 stroke w/ excessive fuel consumption

What type of vessel are we talking about? I have a 75 HP Merc 2S on an Alumacraft Navigator and with two aboard, full live well, full of fuel, and gear it burns exactly 7.5 gallons per hour at wide open throttle as indicated on my fuel flow monitor. I run a 13-1/4 x 19 aluminum prop. As was suggested, if your engine is over propped (too much pitch) on too heavy a boat, or both, you can expect high fuel consumption but 15 GPH doesn't sound possible. If you are using your fuel gauge as a measure, that's the most inaccurate instrument on the boat and can't possibly provide a meaningful number. If you haven't already done so, use a portable tank to get more accurate consumption numbers.

 

[smoky okie]

Re: 75 horse merc 2 stroke w/ excessive fuel consumption

It's on a n 1860 grizzly aluminum boat. I measured it by filling a portable tank to the top, running it fwo for 15 minutes, then refilling the tank. It burnt almost 4 gallons. I also idled it about 10 minutes in and out of the oxbow(lots of stumps) to and from the nav channel.

My mechanic seems to think the excessive consumption is due to blow by @ idle because the motor is running cold, and the pistons aren't sealing right.

 

[DHPMARINE]

Re: 75 horse merc 2 stroke w/ excessive fuel consumption

I've got to question a few things ........

If you've been in the shop 2 or 3 times with this motor.....since new

Spent $300 to $400 ....since new.

And you're at the end of your rope .....

And the motor is 3 years old ....

Why hasn't Mercury done something for you ?

If you bought it used,you bought a problem.We can work on that.

DHP

 

[Silvertip]

Re: 75 horse merc 2 stroke w/ excessive fuel consumption

A compression would show any ring sealing problems. Should have been the first check. An actual temp stick check (hot and cold) would be in order. What prop do you have and what are your WOT speed and tach readings. I would also check the enrichment solenoid and have a look at the carb float levels. But before going into the carbs, with the motor in its full down position, squeeze the primer bulb. If it doesn't firm up, one or more of the floats is not sealing and fuel will simply run through the engine.

 

[Paul Moir]

Re: 75 horse merc 2 stroke w/ excessive fuel consumption

Compression would have to be very bad to affect fuel consumption that much. You should have serious performance and starting problems if that were the case. Same goes for the thermostat theory.

What has your mechanic checked? If it was me, I would be looking at the fuel delivery system. Bad fuel pump diaphragms, mis sized jets, choke/primer issues, float valve leaks, etc.

 

[smoky okie]

Re: 75 horse merc 2 stroke w/ excessive fuel consumption

The Mercury factory service rep got involved when I had it at Bass Pro. Neither he nor the mechanic noticed that it was running cold, and more or less blew me off because they couldn't figure it out.

I bought it new, but it's out of warranty now. I got in touch w/a merc CSM yesterday, he wants me to take it back to Bass Pro and have them get in touch with him.

I picked it up from my mechanic last night, and here's where things stand right now. After running @ idle for 20 minutes in a barrell of 60* water, the block temp was only 62*f. Though the water was clean at first, the lower unit was covered w/ fuel/oil when we took it out.

He thinks that being as it's running cold, the rings aren't sealing and the fuel is blowing by.

I'm obviously not a mechanic, but this makes sense to me being as the only time it doesn't perform right is @ idle and in gear. What do you guys think?

If that is the case, then the new question seems to be "Why isn't it warming up?". They just put a new thermostat in it.

Oh and yes, part of the $ I've spent was to replace the enriching solenoid.

Thanx so much for all the input guys. It's become obvious to me that Mercury isn't going to solve this mystery, I'm going to have to do it myself, and with the help of folks like you, I bet I can.

Tim

 

[Silvertip]

Re: 75 horse merc 2 stroke w/ excessive fuel consumption

For some reason, water is not being retained in the block so its not heating up. Running too cold can cause "cold seizure" because the pistons expand at a greater rate than the surrounding cylinders. Remove the thermostat cover, test the thermostat in a pot of water using a digital thermometer to verify its opening point and whether or not it is closing fully. Don't let the stat or the thermometer hit the bottom of the pan -- suspend it in the pot. Yes, we know its new, but that means nothing. You are in verification mode here. Examine the thermostat cavity and cover to ensure there are no casting flaws that can allow water to bypass the thermostat. Verify proper poppet valve (bypass valve) operation and installation. Check the thermostat cover to ensure the small vent hole has not been painted shut and that it is indeed drilled through. Water coming out that hole is an indication the poppet valve is leaking. If that hole is closed, the diaphram cannot function properly. Others may disagree but I would still do a compression check to see where thats at. It's an inexpensive test and easy to do. Any auto store can provide you with a compression gauge for $25.00 or so. Or use a loaner program at some stores. Keep us posted as to what you check, what you find. Others will chime in over time.

 

[smoky okie]

Re: 75 horse merc 2 stroke w/ excessive fuel consumption

Thanx Silvertip,
Thermostat has been confirmed. compression test was run about 10 operating hours ago.

How do you verify poppet valve operation, and can you tell me a little about it's function and how it operates?

 

[Silvertip]

Re: 75 horse merc 2 stroke w/ excessive fuel consumption

At high speed, water pressure forces the poppet or bypass valve open to allow additional water around the thermostat for additional cooling. At slower speeds that valve is closed and temperature is more closely controlled with by the cycling of the thermostat. So if the thermostat is stuck open, installed incorrectly, or if the bypass is open way too much cooling water is passing through the engine. Because the bypass valve is diaghram operated, it needs a vent hole on the non-water side. Otherwise it would be working like a piston compressing air. Its located under the thermostat housing. Likewise, if there is a casting flaw, neither component can do its job and water would simply bypass. There has to be an explaination for this cool running. Cooling system malfunction is the only logical answer. The thermostat is under the cover at the left of #1 (top) spark plug. Get a new gasket, remove the cover, and take a peek at the parts and casting. By the way, how was operating temperature measured? Details please if not done with temp sticks or infrared gun.

 

[smoky okie]

Re: 75 horse merc 2 stroke w/ excessive fuel consumption

As I understand, it was with an infrared gun taken on the outside of the block.

I've probably put 100-150 hrs. on this unit w/ about 46%-60% of it at idle. Do you think that's enough use to cause significant damage to the engine? Here again, there was a comp. check done on it about a month ago and it was OK at that time.

 

[Silvertip]

Re: 75 horse merc 2 stroke w/ excessive fuel consumption

It's not the idling that's the problem in my view. It's the improper cooling -- or for a better term, lack of heating. Running an engine too cool causes carbon build up, poor idle quality, and very likely overly rich condition. If the engine runs OK otherwise, there is no damage. Especially since you said compression is OK (although a number would be better than just OK). You indicated the engine has behaved this way since new so I doubt carbon is the problem. Since the engine burns fuel like a 150 HP motor, I also suspect its not performing as well as you think it is so it may have been down on power right from the beginning. You said it loads up and tends to die at idle. Although that may be as simple as an overly rich idle mixture screw adjustment, it won;t account for double the fuel consumption. The carbs have high and low speed jets so it may be the carbs on this engine are either jetted wrong, or perhaps were the wrong carbs entirely. You can get the numbers off the carb and then go to Mercury Marine Parts Express. Type in your serial number and look up the carb part numbers. I'm really "reaching" here but it appears you need to do some self-help with this engine. While at Parts Express, buy a service manual so you can really verify things are as they should be from an "as manufactured" standpoint. Doing a spark check might also be in order as weak spark may be a problem at idle. There may also be a number of issues rather than just one. As was mentioned earlier, a bad fuel pump can result in excess fuel consumption since fuel can be drawn right into the engine through the pulse line thus bypassing the carbs. Rebuild it or replace it. That doesn't help the cooling situation though and I really doubt the cooling problem is the cause of the high fuel consumption. I really believe it's the fuel pump or carburetion or both. Nearly four gallons of fuel in 15 minutes is simply not reasonable unless its being "forced" through the engine. That leaves fuel pump or carbs (e.g., float level - stuck needle & seat). I'm out of ideas.

 

[Silvertip]

Re: 75 horse merc 2 stroke w/ excessive fuel consumption

One other thought is that this engine is oil injected so fuel that is bypassing the carbs -- as in via the fuel pump and pulse line, means the oil mixture reaching the internals is also not correct (less oil for the amount of fuel) so that extra fuel is diluting the oil which may be creating a lubrication problem as well. Run the engine briefly. Then pull the pulse line off the fuel pump and shake the end. If any amount of fuel comes out, you found at least one of the problems. There should be no fuel in that line. It points to a leaky diaghram in the fuel pump.