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Old 01-20-2014, 07:40 AM   #21
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Sorry, engines are Cummins 6BT-A5.9-M3.
All Fuel feed lines are 1/2 " ID with 1/2" fittings. All new except hoses from aux tank which are very difficult to replace. I believe return fuel hoses are 3/8" ID.

I'm headed to the boat today. Will close stbd fuel ball valve on aux tank. I'll remove the fuel hose from the last filter before fuel goes to the engine on stbd side. Will connect my vacuum hand pump and introduce 5" of HG vacuum. Wait and monitor vacuum gauge on hand pump.

If system holds vacuum I'll take boat out and run both engines on aux tank closely monitoring fuel press gauge on port engine. I have a camera on it and can see the gauge at the helm. The last time we used fuel from the aux tank the stbd engine quit when running at 1400 rpm. No more fuel has been added to aux tank. It is about 20% full but will stick it to make sure.
Stay tuned.
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Old 01-20-2014, 07:51 AM   #22
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I would plumb an outboard squeezeball rig into the fuel line as close to the tank as you can get, then pressurize the system. That will find a leak if there is one. Then I would reverse the rig and squeeze fuel into the tank to make sure it was clear that way.
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Old 01-20-2014, 08:18 AM   #23
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Try splicing in a piece of clear tygon tubing just before the engine filters. If there is an air leak you will see it.
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Old 01-20-2014, 09:11 AM   #24
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Mine had similar symptoms( see the recent "fuel vacuum gauge" thread).
The pick up tubes are pressed into a 90 degree pipe fitting & threaded into the tank. Inside the 90 was a tiny wire screen, plugged by the gunk from the tank.
My vac. gauges helped me solve the problem, vacuum would build over time to a very high level and rpms would start to drop.

Any restriction from tank to pump( if great enough) would cause these symptoms, I think.

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Old 01-20-2014, 10:50 AM   #25
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Quote:
Originally Posted by jleonard View Post
I would plumb an outboard squeezeball rig into the fuel line as close to the tank as you can get, then pressurize the system. That will find a leak if there is one. Then I would reverse the rig and squeeze fuel into the tank to make sure it was clear that way.
Caution! If something leaks at vacuum it may not leak at pressure. The converse is true as well.
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Old 01-20-2014, 01:15 PM   #26
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Caution! If something leaks at vacuum it may not leak at pressure. The converse is true as well.
That may be true, however I have found every leak I ever had in a diesel fuel system this way. And it's quick.
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Old 01-20-2014, 03:23 PM   #27
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Check your ball valves if all else seems ok,.
Not all of them have line size aperture.
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Old 01-20-2014, 07:14 PM   #28
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1/2 npt inlet and 3/8 npt return.
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Old 01-21-2014, 05:50 AM   #29
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While you're at it, get rid of those sharp edged hose clamps and install these"
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Old 01-21-2014, 12:12 PM   #30
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I went to the boat yesterday and disconnected the fuel line at the engine. I closed the ball valve at the aux tank so no fuel will enter the fuel lines. I created a vacuum of 10" HG on the open fuel line that was connected to the engine. After 15 minutes the vacuum held steady at 10".
To make sure there was no air leaking into the dip tube that goes into the fuel tank, I opened the ball valve on top of the tank and using the hand vacuum pump, pumped until I got fuel at the pump. The gauge read about 2-3" HG. 15 minutes later the vacuum was the same.

I measured the fuel in the tank using a dip stick and there is only 2.5 " fuel in the 20" high tank, so the tank is almost empty.

So it looks like I can eliminate an air leak as the rpm decay.

I replaced all the elbows with swivel types anyway.

Unless anyone has a better suggestion I will continue to monitor the fuel pressure and when rpm decay happens again I will note the fuel pressure reading and perhaps determine if its a fuel restriction issue.

I can't imagine a situation that would restrict fuel to the engines when the tanks are below half full and not at any other time.

Also to make sure I wasn't having a vent issue, using my compressor I pumped air into each tank through the vent outlet on the side of the boat. Each tank filled easily with air.

Also note that I removed one of the dip tubes from the aux tank and it was clear. There are no screen filters on the intake sides. These engines suck too much fuel for screen filters.
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Old 01-21-2014, 12:15 PM   #31
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Greetings,
Mr. t. So what "swivel type" fittings did you use?
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Old 01-21-2014, 12:21 PM   #32
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What Goodyear had it stock. I'll get a pix next time I'm at the boat.
Incidentally, when I replaced the fuel lines from Goodyear at about $3.50/ft CG approved, the same stuff at West Marine was $5 something a ft.
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Old 01-21-2014, 12:31 PM   #33
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Tim- How long were the dip tubes compared to tank height? I've had boats that the dip tubes were a bit short and when fuel got low, aerated fuel was getting picked up. Results were much like your symptoms. Tanks must have been sloshing a good bit. Fixed by installing new tubes that went to within 1/2" of tank bottom. Some of these engines are REALLY intolerant of even a little entrained air.
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Old 01-21-2014, 12:46 PM   #34
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"Worst Marine."
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Old 01-21-2014, 02:13 PM   #35
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Tim- How long were the dip tubes compared to tank height?
I don't remember how long the dip tubes are because I took them out 2 years ago. But I seem to remember they were about half inch from the bottom.
There is about 15 gallons left in the tank that holds 120 gallons, or about 2.5 " of fuel in the tank. I created a vacuum on that fuel line in that tank with all the valves open and the vacuum pump connected to the hose that connects to the engine. I created vacuum of about 2-3" of HG when fuel started to enter the clear plastic tube that was on the vacuum hand pump. It held vacuum for about 15 minutes before I removed the pump. This would seem to indicate no air is leaking in through the dip tube. Correct??

I always look forward to your advise on boatdiesel.
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Old 01-21-2014, 02:21 PM   #36
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Yea. If it held vac for 15min, pickup plumbing sounds like it is ok.

Are you sure your return fuel path is clear from engine all the way back to tanks? One boat was stalling a lot and we found anti-siphon fittings in return lines at tanks. Kept air from getting purged, among other issues.
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Old 01-21-2014, 03:56 PM   #37
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Are you sure your return fuel path is clear from engine all the way back to tanks?
I have not checked the return lines. However if some restriction was in the return line or air was getting in the lines would the engine decay issue not always happen, not just when the tanks are below half full as in my case?

Do you know of any problem that would occur if I took the return line off from the engine and blew threw it with my compressor.
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Old 01-21-2014, 04:34 PM   #38
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You are correct, your symptoms do not really fit the blocked return scenario. But it is easy to check. As you posted, just take return line off engine and blow back to tank.
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Old 01-21-2014, 10:36 PM   #39
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Couple things I wondered about.

You tested the tank vents by blowing air. That may not show a problem. If the hoses are kinked shut , under air pressure they would open up and show good. yet not be. If they are collapsing blowing air will not give an answer.. Just be sure as that is exactly what happend to me years ago, they collapsed.

I think in another thread you said you have vacuum guages on the filters now. Telltales types? Or did you think to read them on the monitor? What do/did they read when you started having trouble last time?

Dip tubes too close to the bottom? If as the fuel weight drops maybe the tank bottom is bulging up and partly blocking the diptubes. Try cutting the dip tube ends on about a 45o - 50o angle. Blunt the sharp end. This creates a larger suction surface area resulting in less velocity [I've forgotten the specific term] right at the entry but also if the tank bottom is the problem the cut will prevent blockage.
Or put a 90o fitting on to prevent this if you can get it in.

If this is the case when the engine quits then the tank may drop from the dip tube to quickly for you to see it on your monitor unless you were watching it at the time. The telltale guages would be better for this.

Did I see that you have rigged a fuel pessure guage. That should help at the next occurence to see if pressure drops and filter vacuum remains low.
You've been fighting this for a long time. Great persistence..
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Old 01-21-2014, 11:42 PM   #40
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Could it be that you are relying on a siphon effect with the fuel pickup at the top of the tank and no fuel pump to get the fuel to the filters? The on-engine pumps may not have enough suck to get the fuel up and out of the tank when they are depleting. If the fuel came out the bottom of the tank you would have head pressure in your favour - I suspect that you need an inline pump.
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