Unequal fuel burn
 

1999, new to me boat has twin Detroit Diesels. I do not have fuel flow indicators, but the starboard engine seems to use more fuel than the port. First time I filled up (fall 2014) the starboard tank took nearly twice as much fuel as the port. Being unused to the boat, I was curious, but did nothing that year. This year I filled up again, 60 gallons starboard, 30 gallons port. Both engines start and run equally, smooth in all respects, top end, easy synch, smooth acceleration, no hesitation. Appropriate top end RPM. smooth at idle, and shifting in and out of gear. We rarely use the generator, so that does not draw down one tank. I am unaware of unwanted fuel in the bilge.

Any diagnostic thoughts?

not that it would use this much,
but is the bow thruster attached to the starboard tank?

Or generator?

Other equipment
 

Thanks for the ideas. I rarely use the generator. Less than an hour or two a year, probably.

I thought bow thrusters were electric, but I don't have one.

I was thinking more of a leak or bad injector, but a bad injector would run rough and a leak would put fuel into the bilge.

I'd love to have access to fuel flow meters like they use on magazine boat tests. I guess newer boats have them already, along with synchronizers. I demo'd a boat in Tampa a while back that had digital fuel flow meters. Fascinating to see the significant change in fuel use as you played with throttles and trim tabs.

curious to know if you are using that engine a lot more to turn
I don't see anything that would double consumption unless there was a leak
how is the idle on that engine is it higher than on the port main

Idle vs. leak
 

Idle is about the same. I'll have to look harder at the possible leak. The fuel gauges are also close (new senders this winter.) I also wonder about the flow between tanks. I don't have a transfer pump, but is it possible the balance flows mostly in one direction?

Looks like I'll have to spend some time next to the engines when we get a cool day.

I'd check the fuel return lines. Sometimes they are plumbed to go to one tank, sometimes directly back to the injectors. Maybe there's some funky plumbing going on.

check fuel line piping
 

Usually in a twin tank set up fuel is drawn from one tank and the other is connected to the tank being drawn from. as some fuel is drawn out of the tank by the engines fuel from the connected tank is siphoned over to the supply tank. Since there is a check between them the connected tank drains while the supply tank stays relatively full. There is also only one fuel gauge in the system and that is in the supply tank. See if that is not your set up.

^ For my own clarity: in other words, each engine doesn't usually have its own fuel tank but rather one supplies the main (drawing) tank?

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clarification
 

Yes fuel tanks are connected to each other and the engine is connected to one tank. In this type of system there would be only one fuel gauge connected to the tank the engines are connected too. Best way to tell with out tracing lines is if you have one fuel gauge you system is this way. If you have 2 gauges then each engine has it's own tank.

Do you have anything that indicates exhaust gas temperature for each engine? (Pyrometers) EGT is a pretty good indicator of load on a marine diesel, if there's a large difference between the engines that would help point to one engine actually using more fuel as opposed to leakage. At the amounts you mention though, I would think a leak of many gallons would be fairly obvious.

With diesels, there is (or should be) a low pressure return line. The system always sends more fuel than will actually be used as the engine runs. The fuel will recirculate either by sending the excess fuel all the way back to the engine’s fuel tank, recirculating it through the fuel injection pump, or on some engines via the engine-mounted fuel filter assembly (kind of polishing it).

Wondering if both engine's returns are going to the same tank. That could account for the difference.

Return systems
 

Some good ideas here. Thank you all.

I do have two gauges.

My son borrowed the boat this weekend, so it will be awhile before I check it out. I'll post if/when I figure it out.

In most twin tank set ups, you have a three way valve for each tank. This allows port to port starboard to starboard, or only drawing off one tank for both engines, or combining tanks to draw off both tanks for both/either engine. The valve may be not all the way closed or open for each engine to properly draw. I believe (without being at the boat to look right now) that you should have two three way valves. Check to see if you have them, and their respective positions.

I agree that the problem is likely the return line plumbing, but if you want to do a simple and quick injector test (that I recently learned from someone I met on this forum), get an infrared thermometer (the type with the laser pointer) and fire up an engine from cold and let it run for 30 seconds or so, then shut it down. Immediately measure and record the temperature of each port on the exhaust manifolds. They should all be the same, one that is substantially cooler indicates a leaky injector. You can do a similar test for compression by disabling the fuel system and cranking the engine over for 10 seconds or so. All the exhaust ports should be the same temp and cold one indicates low compression.