Fuel difference

I have twin 275 hp Ford Lehman Diesel engines . When I fill up the tanks they are usually about 5 gallons apart . Now what is going on is there is about a 28 to 22 gal difference in the tanks upon fill up .. we had to have the motor mounts replaced and props balanced.. ever since then there has been this difference . Also I am now running at 1900-2000 rpms where before I was going 1800 rpms . It is the port tank that is requiring the extra fuel . I am not running the generator.. your thoughts ?

Three questions
- is there a cross over line between the tanks?
- could it be that either fuel returns or fuel supply valves are set differently?
- are the tachs accurate?

No cross over .. don’t know about returns . Tachs I think are ok , cause when I put both at 1800 the boat stays straight, does not go to one side or the other

Greetings,
Mr. K. Could you clarify further please?

-There used to be only a 5 gal difference and now there is a 22 gal difference? Both tanks taking more fuel?

-Were motor mounts replaced on both engines or only the port engine?
-Is it only the port engine that is now running @ higher RPM?
Thanks.

Both motor mounts replaced.just port tank taking more fuel . I keep both engines at same rpms

Greetings,
Mr. K. With my very limited knowledge, the only thing that may have changed, is the prop pitch, assuming both engines have been aligned accurately.

Both were aglined

Greetings,
Mr. K. From your description and assuming no changes were made or damage caused to linkages/engine settings (timing etc.) AND everything is aligned as it should be, pretty well the ONLY thing it can be is the props.


Might it be that at some point in the past the props were re-pitched and the changes not stamped in the prop so when you had the props redone, the prop shop returned the stamped settings to "original"?



This IS a head scratcher...

The props were done to the original specs as per the docs that came with the boat .

Are gears the exact same ratio? It is common for gears on older boats to have a slightly different reduction ratio, so to balance load one prop will have slightly different pitch. If you tuned props to be exactly the same, now load will be not be balanced.

What gears do you have? What is the ratio on each?

Can also phototach both the engines and prop shafts and calc the ratio.

F.L.s only return a small amount of fuel, but then again, they don't burn much either. Could be that they return at least part of the discrepancy . There has to be a crossover there somewhere and one tank must be sitting slightly lower.

pete

I have been told FL 120s return little fuel but the other models of Ford Lehmans can return a lot more.

Different injection pumps and injectors...so again make sure you double check ANY internet advice and even pro advice including American Diesel should withstand a "second opinion".

KEVMAR said:

No cross over .. don’t know about returns .

Click to expand...

Chasing down how and where the returns are plumbed seems a worthwhile endeavor.

Do both engines reach the same WOT?

Some additional info would be helpful. How big are your tanks and how far down are they when you fill them up? A 5% delta in fuel consumption between the engines would not be a concern. 20% would. When you say 22-28 gal difference, I assume you have filled up more than once - same fuel dock? Clogged or constricted vent on your tank could cause the fuel hose to shut off prematurely, though odd coincidence (as would be the return lines, but agree with Sunchaser its a worthy exercise).

What prompted the motor mount/prop replacement?

psneeld said:

I have been told FL 120s return little fuel but the other models of Ford Lehmans can return a lot more.

Click to expand...

My understanding is Detroit's set the hi-watermark for returning fuel and is the defacto benchmark for comparison. Everything else is a far, far, distant second. Doesn't mean they return negligible fuel, just in comparison to the 2-stroke DDs, it's a drop in the bucket. That said, I've never seen any stats on exactly what the volumes actually are.

If you don’t have a crossover, are you drawing from both tanks simultaneously while underway? If so, and you’re drawing through a manifold, you may have a crimp in a fuel line somewhere that causes a slight restriction leading to the imbalance. (All this is total conjecture.)

I don’t know what the problem is or what exactly you are asking but the difference may be because one transmission is running in reverse to get the counter-rotation and the ratios will be slightly different. Your wheels should have been adjusted for this difference.

mvweebles said:

My understanding is Detroit's set the hi-watermark for returning fuel and is the defacto benchmark for comparison. Everything else is a far, far, distant second. Doesn't mean they return negligible fuel, just in comparison to the 2-stroke DDs, it's a drop in the bucket. That said, I've never seen any stats on exactly what the volumes actually are.

Click to expand...

OK...it IS helpful to know FACTS when eliminating troubleshooting issues.


If Ford 120's return almost zero, 225's return near consumption numbers (such as 2-3gph) and.....well ....whatever Detroits return....I would say finding out a more exact number might be more useful than worrying what Detroits bring to the party.


Over the course of a fill up/tank burn.... it only takes a small amount to account for 20/30 gallon difference.

I have SP225s in my President 41. Typically one of my tanks takes a bit more fuel but not enough to concern me. My boat does have a crossover but it is closed. I suspect that your President does have a crossover but mine isn’t very noticeable so you may have one and just not noticed it yet. I didn’t notice mine for about a year.

How many gallons are you putting in to get the 20+ gallon difference? If it is 100 gallons then I would be concerned. If it is the entire tanks worth then 20 or so gallons may not be a problem.

Have you talked to Brian at American Diesel? I would get his opinion and also ask him how much does the 275 return as compared to a 120. Bob Smith said that a 120 would return about a pint to a quart in a days running. I don’t know about the turbo charged engines as to how much they return. Maybe you are returning fuel from both engine to the same tank.

Keep us informed as to what you find out.

Getting back to the tachs - if the 22-28 gallon difference is a small percentage of the total, then the tachs only have to be off a small amount. I do not believe say a 50 rpm difference in engine rpm will make the boat turn noticeably. But over a long run, the difference in load between the two engines could add up to a small percentage fuel discrepancy. If your tach signals come off the alternators, a change in belts, belt tightness or more load on one alternator can easily affect the tach readings. Easily checked with a phototach.


Ken

How many engine hours are we talking about between filling the tanks?

I see you are running the engines at a higher RPM than previously. Maybe it is time to return to your previous 1800 RPM to see if the smaller difference in tank levels returns at fill up.

Have you any knowledge of injector service in the last thousand hours or so? Noticed any exhaust difference?

You really need to go through the system and be absolutely sure the engines are totally separate in regard to fuel supply and returning to different, isolated tanks.

You could establish a temporary fuel supply for an engine and also dump its return into a temporary container. Run around for a couple of hours and take precise note of the contents of both. Then switch engines.

Just based upon the history is that, before the props were tuned, one was more efficient than the other. Now they are both close to optimal. As a result, the fuel efficiency at the same throttle, and/or the throttle applied may be different in effect. And, it would only take a small difference to account for a fuel difference. Also, if there is a cross-over valve, and the boat starts to list one way or the other, that could draw fuel to the lower side, making the difference worse vs equalizing it.

Did you get a report with the prop work? Was one way more out of kilter than the other? Or were their pitches sizes off in opposing ways causing opposing corrections, etc?

Also, check your tachs against a phototach and get them calibrated at cruise speed. From there, see how things go. I have little faith in tachs to be super precise, especially when I don't know who calibrated them and when. They could be causing you to throttle differently. And, people do weird stuff. Someone could have intentionally set them off by a little bit so that they could run with the same indicated RPM without having the rapport synchronized and annoying. One just never knows.

Also, I like to apply Occam's Razor to these things. My first bet is that a valve got bumped or moved while people were working in the boat. My second bet is that a vent got clogged and so it is venting via a cross-over and supply so supply and/or return is favoring the other tank when netted out.

Don’t know about Lehmans but my Cats return significant unburned fuel to selected tanks when running. This excess fuel is used for cooling.

Could you be returning unburned engine fuel to a single tank, as opposed to each engine returning fuel to both their respective tanks? I had this type of problem when someone unknowingly and inadvertently changed a fuel valve direction.

Clogged fuel filter? I had a tank imballance last summer and this was the cause. Good luck

psneeld said:

OK...it IS helpful to know FACTS when eliminating troubleshooting issues.


If Ford 120's return almost zero, 225's return near consumption numbers (such as 2-3gph) and.....well ....whatever Detroits return....I would say finding out a more exact number might be more useful than worrying what Detroits bring to the party.


Over the course of a fill up/tank burn.... it only takes a small amount to account for 20/30 gallon difference.

Click to expand...

Exactly - For some reason, OP left the thread before sharing denominator in the equation, so grasping at straws. Sort of like saying "My stock went up $1/share today." Makes a big difference if it's a $5/share stock or a $500/share stock.

Kevmar,
I don't mean to hijack your thread but I have some questions about your motors. I just bought a boat with twin SP275s. We are having work done on her so haven't had the chance to really run them yet. What is your fuel burn rate at 1900? Why did you start running higher revs? The 1st thing we caught on our motors was leaky aftercooler cores. It took 3 weeks to get new ones from England. Is there anything else to watch out for on these motors?

Thanks,

I noted you did not offer any reply or i missed it but I cannot find an answer to Ski's question [post #10} about the gears and the ratios. Not that he needs my verification at all but he is correct that some gears use slightly different ratios when run in reverse vs forward to get reverse prop rotation.
If that was your case then as Ski mentioned the loading will be higher on one prop when used like that to correct loading..
The repitch may have defeated a slight pitch difference to allow for the ratio difference.

Second is your tachs. Analogue tachs can be off, usually are, and by a lot sometimes whether or not you realize it or accept it. Accurate tachs can be had from Ebay for ~$20. Not robust so no rough handling but accurate. Laser pointer digital phototach. You may find there is more rpm difference than you realize.

A friend had this on twins. He matched revs by his tachs. I don't remember why the discussion came up but I offered to help check his tachs with my digital unit. When he matched revs by his tachs, my tach said one engine was almost 200 revs low. That meant one lazy engine and one overloaded engine.

Once we sorted out a table of the differences several changes came to light. He ran it at the same revs as before but according to the digital tach table.

The boat actually gained about 1/2 knot, he no longer had to crank the wheel as much to run on a straight course, his fuel use actually dropped, the boat was a bit quieter. None of these changes was huge but they were noticeable. Especially the fuel use over some time.

The engine revs were far enough apart that the usual RRR,RRR,RRR of frequency differences were not discernible. Once the table was used then the RRR. RRR could be heard and used to finish match revs.

The reason I ramble about this is your mention of the pitch change matching side to side. Before that change the fuel use you say was close side to side. After the pitch "correction" you started noticing a fuel use difference. So take a look at the gear boxes, their ratios and find out if one has a slight difference when run in reverse. Many did.

Check the ratios AND the tachs.

My fuel rate is different between the two engines the port using about 4.5 gph and star about 3.5 gph. I have been running the port at 1800 and star at 1900 . Last fill up was star 37 gal port 54 gal .. we also ran the generator fir about 4 hrs so that may have been the reason fir the port bring higher .. I had to have. It’s transmission oil coolers changed as be failed and filled the Transmission with sea water .. was flushed a few times seems ok now .. water pump Aldo failed and was replaced on port engine ...

Was your generator running? It comes of one the 2 tanks