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Old 05-16-2013, 06:22 AM   #41
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I've installed 4 vacuum gauges and a fuel pressure gauge.

Murphy can supply a differential pressure gauge that will monitor for you and alert you when your settings are reached.

Sounds like a tank clean out and new fuel lines are in your future.
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Old 05-16-2013, 07:10 AM   #42
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[QUOTE=FF;156811
Sounds like a tank clean out and new fuel lines are in your future.[/QUOTE]

I've cleaned out the tanks a couple of times. The problem is the issue is inconsistent. It seems to happen more often when the tanks are less than half full, but then again not always. And it happens on both engines and all 3 tanks. The fuel pressure gauge should indicate by decreasing pressure when a restriction is developing and allow me time to investigate before the engines loose power.

Right now I'm showing about 5" of vacuum and over 20 psi fuel pressure and the engines run fine. The tanks are all full, so I'll be monitoring the gauges closely over the next couple of months.
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Old 05-16-2013, 12:16 PM   #43
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Tim, do you have any problems with the vents while refueling? Almost sounds like a vent problem.
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Old 05-17-2013, 06:46 AM   #44
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I've cleaned out the tanks a couple of times

If you physically got your arm inside and scraped out all the gunk , and wiped as much clean with a rag , the feed , pickup , hoses etc is at fault.

If "cleaning" was a dock guy with a pump and a hose , you cleaned the fuel inside the tank , but not the tank.
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Old 05-17-2013, 07:37 AM   #45
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Thanks guys, this has been an ongoing problem and the inconsistency is whats makes it difficult to track down. I'm hopeful that with a vacuum gauge on the racor and after the second fuel filter right before fuel goes into the engine mounted filter in combination with the fuel pressure gauge this will help me track down the problem.

Al, I have taken my compressor and blow out the vent lines for all three tanks - they were not clogged and there was no restriction. However the next time this happens (engine loosing rpm) I will remove the fuel cap and see what happens.

FF, it's not possible to access the tank in a way you advised without cutting holes. I have viewed to the extent possible my aux tank and it seems relatively clean. The last time I had the tanks professionally cleaned they put some sort of wand down in the tank in an effort to loosen gunk stuck to the side of the tank. The guys said based on what they saw, the fuel and tanks could not cause my fuel restriction issue.

I am not 100% positive it's a fuel restriction issue. The engines suddenly decay in rpm and when I switch tanks the decay goes away. So my assumption is fuel. I think monitoring the fuel pressure gauge when this decay occurs will determine if this is a fuel restriction issue or something else.

Comments welcome.

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Old 05-18-2013, 06:48 AM   #46
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I would look for an air leak first , as it has no cost.

Disconnect the fitting at the fuel tank and cap the line.

Disconnect the fitting and pressurize with about 10psi and a gauge and a valve.

If pressure holds for 3 hours or more , start with renewing the fuel lines.

IF the lines can be replaced mostly with metal , and only short pieces of rubber to connect , that is more trouble free.
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Old 05-18-2013, 07:09 AM   #47
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Leaks from pressure are not leaks from a vacuum. A line that leaks under pressure may not leak under a vacuum and vice versa. Test for the problem you think you might have, not the opposite.

If you want to test a suction line you test it under a vacuum, test it the way it works. An air leak into a fuel system is a vacuum leak.
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Old 05-19-2013, 07:59 AM   #48
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Quote:
Originally Posted by RickB View Post

If you want to test a suction line you test it under a vacuum, test it the way it works. An air leak into a fuel system is a vacuum leak.
Thanks Rick that makes sense to me. I think most people assume that a vacuum leak would also show up as a pressure leak and since it's much easier to produce pressure on a system than a vacuum, hence that suggestion.

A couple of questions:
1. If I had an air leak would it not show up all the time. This decay in rpm only seem to happen when the fuel tanks are below half full? I've pulled one of the pick up tubes out of the aux tank and it is fine.

2. How would I create a vacuum on my fuel system to test it. I have plenty of vacuum gauges to indicate weather the system is holding a vacuum, but how do I create the vacuum.
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Old 05-19-2013, 08:36 AM   #49
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Quote:
Originally Posted by timjet View Post

A couple of questions:
1. If I had an air leak would it not show up all the time. This decay in rpm only seem to happen when the fuel tanks are below half full? I've pulled one of the pick up tubes out of the aux tank and it is fine.
If the height of the fuel in full tanks is above the lift pump, and the filters are in good shape, there will be very little if any measurable vacuum created. As the fuel level drops below the lift pump the vacuum required to lift it to the pump suction will increase about 3/4 of an inch of mercury per foot. Filter clogging and long or small diameter tubing will increase this considerably at higher flow rates.

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2. How would I create a vacuum on my fuel system to test it. I have plenty of vacuum gauges to indicate weather the system is holding a vacuum, but how do I create the vacuum.
Shut the fuel valve at the tank side. Disconnect the fuel line where it enters the injector pump, connect a clean piece of tubing and suck like mad to create a vacuum. Close the tube off and see how long the vacuum lasts.

OK, if sucking on a diesel line doesn't appeal, try one of these:
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Old 05-19-2013, 10:13 AM   #50
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Thanks Rick, that's a perfect explanation of how an air leak can create the situation I'm experiencing.

I just happen to have one of those hand suction pumps I use to pass lake water through a filter medium that the local university does tests on to record algae content.
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Old 05-19-2013, 02:09 PM   #51
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One last question for Rick, if I have good fuel pressure regardless or what the vacuum readings are can I eliminate fuel restriction as a cause for the engine rpm decay? Fuel pressure is taken at the fuel filter before fuel goes to the lift pump.
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