Gas Mileage on 2006 340 Sundancer

[Mikey]

My Brother has a 2006 340 Sundancer with 2 MerCruisers and we have been getting different gas mileage on the port and starboard engines. We have had it serviced multiple times, getting the Props calibrated back to factory specs, New top of the line Spark plugs and cables, Full Tune Ups, Fuel Filter, Oil Filter, Transmission checks, Compression Checks and the list goes on.
After a day out on the water the fuel on one side will have used at least 30 gallons more than the other. We have done this without the generator on so we know that the geny is not sucking extra gas and we keep the RPMs on each engine the same. We know there would be some differences in fuel on each engine but this seems like it is way more than it should be.

Please help.

 

[Bucit]

If the engines are running correctly I doubt it's burning 30 more gallons. I would say the fuel gauge or the Smartcraft gauges are inaccurate. Have you checked this when filling them up to make sure it's that big of a differnece?

 

[Mikey]

yes we have checked the fuel gauge and been mindful of how much fuel is filling the tanks. We will top both tanks off and run both engines the same for a day and then fill them up later that same day and one engine will have used a lot more fuel. up to 30 gallons.
we are thinking that one engine has a problem, but are out of ideas what it could be.

 

[skolbe]

30 Gallons would very hard to burn additional while cruising, not sure what the time period is. What is the time period? Generator burns less than a gallon an hour.

 

[Mikey]

The difference really depends on the RPMs. At really low RPMs one engine will use about a gallon of gas more per hour. On average we are finding that one engine is using about 4 gallons ever hour more than the other. This is around 2500 RPMs on both engines. And this is with the generator off.

 

[Vince_nj1]

Sounds like you have been trying a lot of things, so this puzzle is going to be a tricky one. Are you certain that you are driving with the engines synchronized? My thinking is that one engine has more of a load and requires more fuel to stay in synch. Could a bad cutlass bearing do this? If you have smart craft try driving with one engine at a time and measure the fuel usage at various RPMs. They should be similar. If one requires more fuel per RPM, then I would start hunting for a drag on your drive train. Another theory to search for is a fuel leak. Any chance you have crack in a fuel line somewhere and your fuel is being pumped overboard?

 

[skolbe]

When I took delivery of my boat - both tachs were setup to be Starboard tachs. So I thought I was the master of syncing my engines. Took me a couple weeks before I realized the problem.

 

[Mikey]

skolbe, we can get different readings on the rpm gauge, so i think that is wired correctly. I will have my mechanic check it out to be sure.

Vince, I was thinking that it would not be a fuel leak because the problem gets worse with higher rpms, and there is no leak when the engines are not on. I suppose that if it was a fuel leak, it would have to be leaking somewhere close to the injectors and we would see fuel in the bilge. Please correct me if my theory is wrong? I can also have my mechanic check for this again.
After having multiple mechanics try and asses the problem I am thinking that it has to be a heavier load on one engine, like you said. I will have the cutlass bearings checked asap.
[SUP]
the compression on both engines are within safe limits. [/SUP] Rt: All cylinders 170 psi Lt: 3 cyl 170 psi 5 cyl. 155-165 psi. Also the right prop is 21 degrees and the left prop is 21.5 degrees. These are to factory standards and i would think this would cause a difference in fuel economy but do you think this would cause a difference so drastic?[SUP]

Thank you both for your comments! [/SUP]

 

[dyerstraits]

We owned a 2003 340 Sundancer for seven years and had the same problem...approximately 10% differential in fuel usage between the two engines..spent a lot of money diagnosing the problem to no avail. After trading the boat in on a 2001 450 Express Bridge in 2010 another 340 Sundancer contacted us with the same problem...recall finding a thread on this site about there being a left and right prop....i.e. it is important to have the probs installed correctly..a starboard prop and a port prop.