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Old 01-19-2014, 09:12 AM   #1
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Swivel elbow fuel fittings

I have had a fuel issue since I bought the boat several years ago. The problem is inconsistant and difficult to predict.
I have 3 fuel tanks, Port saddle tank that feeds the port engine and generator. Stbd saddle tank that feeds only the stbd engine. Aux tank that has two feeds, to either the port or stbd engine either one or both engines.
The boat also has installed a pump that can transfer fuel from the port to starboard engine and vis a versa, presumably to balance the tank after generator usage.

So here's the problem. It seems both engines rpm start to decay when fuel in any tank gets below about half full. I've checked all the filters and they are good. On one cruise we were feeding both engines from the aux tank and when it was about half full the port engine rpm started to decay from 2300 to 2000 rpm. I switched port engine feed to it's full saddle tank and the engine rpm came back to normal. 20 minutes later the same thing happened to the starboard engine and it's rpm came back after switching to it's full saddle tank. The same thing happens when the saddle tanks get below half full.
The last time we were out I was feeding from the aux tank which was only about a third full. We were cruising at 1400 rpm and suddenly the engine quit. I switched to it's saddle tank, and it came back.
However, I have not filled the aux tank and just for grins at the dock I started both engines and feed them both from the aux tank, no problems. I didn't go above idle.

Anyway as I said the problem is somewhat inconsistent. I have been told to check for air leaks and I am beginning to suspect the several elbows around the Racor and another filter I have before fuel gets to the engine filter. These elbows can't be tighten or they will not point in the proper direction. Last week I replaced the fuel lines with new, I was told to do that too, 17 yo boat. Tomorrow I'm going down to the boat and using a hand vacuum pump put 5" HG on the entire system to see if I can track down the air leak, if that what it is. If I notice a drop in vacuum, I'm not sure how I can find the leak, since it's a vacuum and no fuel will come out.
Anyway as I said before I suspect the several elbows and may replace them with swivel elbow fittings that will allow me to tighten them securely, at least eliminating these as the issue. Anybody know where to get these elbows? Any other comments about this are appreciated.

Below is a picture of the current installation with an elbow on each side of the filter head. They must point down and so are not particularity tight.
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Old 01-19-2014, 09:32 AM   #2
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Greetings,
Mr t. Have you considered a banjo type fitting?


banjo - JEGS High Performance
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Old 01-19-2014, 09:38 AM   #3
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I can't help wondering if the problem is not the opposite of air leaks.

Are the tanks invented in such a way that pressure can equalize during pumping?
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Old 01-19-2014, 12:40 PM   #4
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I'm skeptical about air leaks as why is it fuel volume dependent? Both engines? Look to where the fuel system becomes co-dependent. Not air leaks as they would likely stop the engines and you would have to bleed each engine to get it to go and it's unlikely to let you restart without cranking etc.

Partially plugged screens in the tank outlets? Have you tried removing the tank filler after the rpm drop?

Do those filters feed both of your engines?
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Old 01-19-2014, 12:53 PM   #5
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Are all three tanks vented to a common vent, or are they each vented separately? My first thought would be vent(s) may be restricted. Insects love those vent holes if they don't have a screen on them.
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Old 01-19-2014, 01:48 PM   #6
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I thought vents too, but wouldn't the effect of a plugged vent be more pronounced with full tanks?
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Old 01-19-2014, 03:07 PM   #7
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Quote:
Originally Posted by Xsbank View Post
I thought vents too, but wouldn't the effect of a plugged vent be more pronounced with full tanks?

The fuller the tanks are the less air volume there is above the fuel.
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Old 01-19-2014, 03:55 PM   #8
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Thanks RT F I'll check out banjo fittings, didn't think of that.

It's not clogged vent lines. I've checked that out. Took my compressor and blew air though the vent line outlet on the side of the boat. No clogs. Each tank has it's own vent. And like xsbank mentioned, full tanks with little air volume above the fuel would make a clogged vent more apparent with full tanks.
There are no screens on the fuel pick up tubes, I remove one of the aux tank tubes and it's just open on the bottom.

The reason I think it's an air leak is best described by Rick B in an earlier thread. As fuel decreases in the tank the engine fuel pump has to suck harder to pull fuel from the tank, thus increasing vacuum. If there is an air leak it may not be apparent until fuel decreases in the tank to where a vacuum is created that allows air to flow into a not so tight fitting.
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Old 01-19-2014, 05:20 PM   #9
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Rick was likely discussing net positive suction head (NPSH). Assuming your fuel pump is a positive displacement pump, it would indeed draw a vacuum on the line. A quick and easy way to help you troubleshoot would be to check the elevation of your pump relative to the level of your tank. Simplistically, (negating other system losses) if the fuel level is above the pump you have static head, if the level is below you have suction lift.

If your pump elevation happens to be at or near the half way level of your fuel tank, you may very well be ingressing air into your system.

If your dip tubes are higher than your pump, I would start there.
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Old 01-19-2014, 05:24 PM   #10
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I think the key is that the trouble occurs when the tank you are running from drops to a certain level. One though is that your lift pump isn't working on the engine. When the fuel level is high enough you have sufficient gravity feed to run the engine at load, but when the level gets too low the engine has no way to pull the fuel up.

One test would be to rig a temporary fuel line to a bucket and see if the engine runs under load that way, and if it works with the bucket held at different levels.
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Old 01-19-2014, 05:37 PM   #11
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Two engines, both behave the same.
The problem has to be in the delivery part but air in the fuel lines will stop the engines, not slow them down.
I am not familiar with the filters you are using, they look like on-engine canisters. Are they spec'd to be sucked through or pressured through?
Perhaps the medium is too fine, are you using 2 microns?
Perhaps the filters are not flowing enough when the head pressure falls?
What engines do you have? Perhaps the total flow from the tanks is too low for the engine requirements? I don't have my Cummins manual but the fuel flow is significantly higher than the fuel burn as most of the fuel returns to the tank.
Those fittings for the "in" look very small and restricted...my bet is those fittings and filters are too restrictive.

I'll think of something else...
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Old 01-19-2014, 05:51 PM   #12
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Fuel tank and engines (lift pump) are about the same level.
I don't think the problem is the filters, after all the same filters are used when I switch tanks. Racors are I believe 5 mic and next primarys are 10/20 mic.

Northern Spy, when the tanks are full under your definition I have static head and when they are about half full I start getting suction lift.

I have attached a fuel press gauge on one engine and it shows normal pressures throughout the rpm range. I have not had it installed long enough to notice the pressure when the engine rpm decays or stops.

Xsbank, you said even a small amount of air will stop the engines not slow them down. If true then I'm chasing a rabbit down the wrong hole.
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Old 01-19-2014, 07:21 PM   #13
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Rabbits are what we are all about! There are a few on here who think I'm full of hooey, they should be chiming in soon!

What engines do you have?
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Old 01-19-2014, 07:54 PM   #14
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I agree with Pete and those who believe it is not an air leak.
If an air leak, the amount of fuel in the tank and the weight of that fuel would really not matter, as it is always easier for a pump to suck air than fuel.

Sounds to me like a restriction of fuel flow. As the level in the tank goes down, the restriction is limiting the amount of fuel, therefore the reduced rpm.

ANd since it happens in either tank, it is probably after the saddle tanks, so between the Aux tank and the engine. Filters could be the culprit.

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Old 01-19-2014, 08:25 PM   #15
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Quote:
Originally Posted by timjet View Post
...the several elbows....These elbows can't be tighten...of the current installation with an elbow on each side of the filter head. They must point down and so are not particularity tight.
Fix the elbows (air leak (s).
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Old 01-19-2014, 08:30 PM   #16
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Well maybe it is a combination of what we are all saying…

*

Remember those system losses I negated?* Prolly shouldn’t have done that.* Essentially this could be a s simple as the NPSHa is less than the NPSHr by the pump.* That said, the gravity head is relatively small for diesel per inch. *The system loss in this case could be any of the things that Xsbank or Wxx3 suggested.* High filter DP, reduced diameter fittings, restrictive valving, etc., any type of frictional loss that would take away from the NPSH.* Discounting the full versus half full on two engines at this point is still premature.* Although it is possibly symptomatic, it is the only data point he has provided.

PS. Never looked at the pictures as I'm working on a cell phone.

What engine do you have?
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Old 01-19-2014, 11:24 PM   #17
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Thanks guys. Engines are Cummins 6BT 330's.
Fuel goes from tank to fuel tank selector valves to Racor filter to Fleetguard FS 1000 filters then to the engines.
If it was a filter issue, switching tanks should not correct the low rpm issue.
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Old 01-19-2014, 11:32 PM   #18
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Tim- A little bit of air entrained in fuel supply can certainly cause drop in rpm. It does not necessarily cause a stall. I think you are on the right track checking lift pump pressure and see how that matches up with loss of rpm. Good chance air is getting in somewhere, or a restriction anywhere in supply.
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Old 01-20-2014, 02:03 AM   #19
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Tim, you did say when the tanks get to half full and the switching of tanks makes them happy again, I would posit that your fuel flow is only adequate when the tanks are full. If the filters are good and there is no restriction in the tank supply, it has to be the plumbing. Are those fittings only 1/4?" I think with two filters inline and too small fittings, the fuel flow drops when the tank head drops. If you can't bring yourself to bypass the Fleetguards, relying on the on-engine and the Racor, I would change out the fittings. You can get larger ones.
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Old 01-20-2014, 02:34 AM   #20
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All I could find for Cummins installation specs were not for a "6BT 330" as I couldn't find that exact model designation but for the engines I checked the fuel connections and therefore the supply lines vary from 1/4" to 1/2." I'm saying this because I suggest you research the actual model of engine you have and check that the supply lines and the return lines, are to spec. If by 330 you mean the horsepower (my engine is a 6CTA8.3-M1and the horsepower is not part of the nomenclature) then the engine I narrowed it down to needs 61 US gal/hour and 41 gallons of that is expected to return to the tank. The high return rate is, among other reasons, required to keep the fuel cool so a restricted flow has other consequences.

Anyway, let us know what you find, off to bed.
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