Uneven fuel consumption Port vs Starboard

Grec

Active Member
SILVER Sponsor
Sep 20, 2020
298
Warwick, RI
Boat Info
2001 310 Sundancer
Engines
Twin Horizon 357 Inboards (repowered in 2017)
I have a 2001 310 Sundancer with twin 357 Mercruiser Horizons (replaced in 2016 and now have about 300hours on them) and V-Drive transmissions. I have been getting a huge disparity between the fuel consumption of the port and starboard engines. Both engines seem to run fine, although the starboard engine does show somewhat higher oil pressure. My tachs and synch meter don't always agree, but I do my best to keep them synched at all times.
I filled both tanks last Saturday and went for a fairly long cruise. We only ran the generator for about 1 hour while we were anchored for lunch that day. When I filled my tanks after the cruise, the port tank took 42 gallons and the starboard tank took 68 gallons. That means that the starboard engine is consuming about 60% more than the port engine (minus whatever small amount the generator used). It would seem that anything causing that much additional fuel consumption would show itself in the engine performance, but everything seems to run fine.

It's become a real mystery to me, and if there is a problem with the starboard engine I'd like to address it. Any suggestions of how to troubleshoot this would be greatly appreciated.
 
that's a big delta. I assume they're running the same props and everything is in good order, not dinged, etc.?

I'm not positive it applies to the gassers but a common test performed in diesel inboards is to run one of them up to WOT to determine the stall RPM (while the other is in neutral). That exposes if one engine is making significantly more power than the other and possibly overcoming one side being underpowered. On gas engines, especially fuel-injected ones, it would make sense that the ecu is feeding more fuel to the engine doing more work resulting in higher burn.

Do you have to advance the throttle significantly further on either side to keep them synced?
 
So this is only based on 1 fill-up? I wouldn't rule-out false tank levels. Did they really begin even and are they both actually full now? A partial clog in a vent can not only fool a pump into clicking off, but can even spit gas from the filler or vent before the tank is actually full.
 
Thanks for the responses. Both props were in perfect shape when the boat was launched this year, and I've had no low-water incidents, so I can't imagine that there are any prop issues. I'll certainly take a look at them next time we anchor for a swim.

I'll check into whether the WOT test you described is also performed on gas engines. I'd reserve that as a last resort though to avoid putting that kind of strain on either engine.

The throttle handles are just about even when the engines appear to be synched. My tachometers don't always agree 100% with my synch gauge though, so I get everything real close on the tachometers, and then synch them by ear. The synch gauge seems to normally be right on. Usually all three gauges "agree" after running for several minutes though anyway.

This is not based on one fill up. I've filled up four times this summer and the starboard tank always took significantly more than the port. That's what prompted me to start documenting it. On the fill up before this last one, I made sure they were both topped off. This last time, I filled the port tank first (42 gallons), then after the starboard tank took 68 gallons, I went back to the port tank to see if it would take more if I pumped slower. I wouldn't.
 
My tachs and synch meter don't always agree, but I do my best to keep them synched at all times..
Grec, let me offer a suggestion on synching your engines. Cover your tachs so you're not tempted to look at them, then get up on plane and listen to your engines. Set one engine at whatever rpm you want then bring the other engine up slowly to what you feel is about the same rpm.

When the engines are really synched you should hear (and feel) a 'sweet spot' when the rhythms of the engines just sound and feel right.

Then take the cover off your tachs and see if they match. I'm guessing they'll be off by just a bit and that's OK.
 
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Does the boat have fuel crossover valves? If so make sure they are positioned correctly to the tank to be drawn from and not in some mid position.
You might try putting the gen on the other tank and try again also.
 
What @ttmott said, I have seen this before myself, especially with a new to me boat. I doubt this is your issue though because both tanks always need more fuel after running.

You can set up a independent fuel source as well for both (a 5 gallon tank for example) and test consumption.
 
I'll try covering the tachs as you suggest firstclass. I'm familiar with the "slow-rolling humming sound" of synched engines though, and constantly work toward maintaining that sound. It's not always easy to hear though, when you are as close to the engines as you are on this boat.

The boat is still fairly new to me. I never thought of looking for a cross-over valve. I'll check that next time I'm there.

Someone at the marina suggested a bad fuel pressure regulator, but if that were the case, it would send the excess fuel back to the fuel tank, right?
 
I'll try covering the tachs as you suggest firstclass. I'm familiar with the "slow-rolling humming sound" of synched engines though, and constantly work toward maintaining that sound. It's not always easy to hear though, when you are as close to the engines as you are on this boat.

The boat is still fairly new to me. I never thought of looking for a cross-over valve. I'll check that next time I'm there.

Someone at the marina suggested a bad fuel pressure regulator, but if that were the case, it would send the excess fuel back to the fuel tank, right?
As crazy as it sounds mercury does not return the fuel (gasoline) to the tank unless something has changed. The fuel returns to the cool fuel module which the regulator is mounted. It's a shame Merc doesn't make a retrofit kit to get rid of the cool fuel module BTW... Regardless, if the regulator failed low or high then engine would not run or run extremely badly.
 
The real shame is that the Coast Guard won't let them put the fuel pumps in the tank where they belong.
 
I struggle with this being an engine issue versus a leak issue. Dumb question, it's not leaking fuel anywhere?
 
I have a 2001 310 Sundancer with twin 357 Mercruiser Horizons (replaced in 2016 and now have about 300hours on them) and V-Drive transmissions.

If you are fuel injected, you may want to consider options to pull ECU data. Either vesselview, if smartcraft enabled, or Fox Marine engine gateways (which I use to pull MEFI-3 data. That will give you fuel flow and digital rpm data. You could also consider an engine scanner, but I like my data displaying on the MFD.
 
No leaks mrsRobinson, although I've seen a guy going up and down the dock with a drum on a hand truck (JK).

Thanks unsalted. My new Garmin display has a gauge page. I think it's time to buy the harnesses, get it connected, and take advantage of it.
 
No leaks mrsRobinson, although I've seen a guy going up and down the dock with a drum on a hand truck (JK).

Thanks unsalted. My new Garmin display has a gauge page. I think it's time to buy the harnesses, get it connected, and take advantage of it.

I'm experiencing similar issues on my 2005 rinker 360 twin 496 with bravo 3s. I am in the process of connecting my engines with cables to a jbox and a gateway to my simrad go9se. I will let you know how it works out. Please keep this post updated Thanks
 
Did you do the repower? If not, any chance a bad fuel tank was also replaced that has a different capacity when the boat was repwered?
 
Took a run around to Solomons from Potomac. Two hour run at 30mph. Port motor was burning at 15.3gph @ 3250 rpms. Stbd motor was burning at 16.9gph @ 3250rpms. Both motors seemed to run great, although when we arrived at Solomons the stbd tank for sure took more fuel, but not 20 gallons - that's a ton.. Changed the plugs in the stbd motor and found 2 damaged plug wires. New plugs and wires installed, next trip a day later to Annapolis, everything in sync. Not saying this is your problem, but worth a check. I change plugs every other year. The port motor plug were changed out last year due to a different issue. Changing them again at end of this season to get both motors back on the same schedule.
 
Took a run around to Solomons from Potomac. Two hour run at 30mph. Port motor was burning at 15.3gph @ 3250 rpms. Stbd motor was burning at 16.9gph @ 3250rpms. Both motors seemed to run great, although when we arrived at Solomons the stbd tank for sure took more fuel, but not 20 gallons - that's a ton.. Changed the plugs in the stbd motor and found 2 damaged plug wires. New plugs and wires installed, next trip a day later to Annapolis, everything in sync. Not saying this is your problem, but worth a check. I change plugs every other year. The port motor plug were changed out last year due to a different issue. Changing them again at end of this season to get both motors back on the same schedule.
I think you should check your spark plug wires and make sure that all are attached, I had a similar issue with the port engine burning 2 to 3 gallons more at the same RPM, check the plugs and found two wires not fully attached to the spark plugs. simple to check Steve E
 
I agree with others directing your attention to plugs/wires. Have you noted the engines rpm's at WOT while underway? With my last boat (twin 8.1's) if both engines didn't reach the same WOT it was a sign that the weaker engine needed new plugs. It was usually a 200 rpm difference with one bad plug, 400 rpm if two plugs were not firing properly. No other symptoms were noticeable other than some transmission "chatter" at idle speeds in gear from the engine not spinning smoothly. What are your WOT rpm's?
 
I agree with others directing your attention to plugs/wires. Have you noted the engines rpm's at WOT while underway? With my last boat (twin 8.1's) if both engines didn't reach the same WOT it was a sign that the weaker engine needed new plugs. It was usually a 200 rpm difference with one bad plug, 400 rpm if two plugs were not firing properly. No other symptoms were noticeable other than some transmission "chatter" at idle speeds in gear from the engine not spinning smoothly. What are your WOT rpm's?
To check, one engine in neutral and one in gear then slowly run the one in gear to full throttle and see what the stall RPM is. Then, do the other. This simple test will determine if the engines are performing equally or not.
 

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