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Old 08-28-2022, 12:05 PM   #1
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Taking fuel sample from the filter

Seems easy to bleed off a small amount of diesel to check for water and particles.
But isnít it true that if the filter is above the tank fuel level, that air will enter the bowl, making up for the lost volume? And that air wont have an acceptable escape path?
So, then is it much preferred that the filter should be mounted lower than the bottom of the tank? My tank bottoms stay dry, but they are very low in the bilge.
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Old 08-28-2022, 12:54 PM   #2
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I think that the air would have to have a path to get into the filter. If there are no air leaks wouldnít the fuel just siphon from the tank to replace what you are drawing out as a sample? If you have any air leaks in the fuel system then air would be drawn into the filter.
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Old 08-28-2022, 01:39 PM   #3
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If you're worried about it, and it's a Racor separator, add a outboard motor fuel squeeze ball to the drain valve. The valves inside the ball will act as check valves.

Attach the ball. Give it a few squeezes until you have a vacuum. Then open the drain valve. You may have to give a few more squeezes to get your sample. Close the drain valve and remove the squeeze ball, or not.

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Old 08-28-2022, 02:08 PM   #4
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Quote:
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I think that the air would have to have a path to get into the filter. If there are no air leaks wouldnít the fuel just siphon from the tank to replace what you are drawing out as a sample? If you have any air leaks in the fuel system then air would be drawn into the filter.


I suppose my thought is that air would be drawn into the filter drain valve. The fuel in the filter would flow towards the tank. Unless the filter is below the tank fuel level, at that point, fuel is a higher pressure than the filter.

Here is why this is a potential problem: Lets say the tank fuel level is lower than the filter. And, the engine is higher than the filter. You open the filter drain. Fuel dribbles out, but air bubbles in to replace that space. Then the air migrates UP to the engine, where it is stuck until it gets pumped into the injectors. Forcing an engine bleed.

Do Racors and the like have check valves?
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Old 08-28-2022, 03:09 PM   #5
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I suppose my thought is that air would be drawn into the filter drain valve. The fuel in the filter would flow towards the tank. Unless the filter is below the tank fuel level, at that point, fuel is a higher pressure than the filter.

Here is why this is a potential problem: Lets say the tank fuel level is lower than the filter. And, the engine is higher than the filter. You open the filter drain. Fuel dribbles out, but air bubbles in to replace that space. Then the air migrates UP to the engine, where it is stuck until it gets pumped into the injectors. Forcing an engine bleed.

Do Racors and the like have check valves?
The Racor 1,000, 900, and 500 all have inlet check valves.

I believe it's an ABYC requirement as well, but inspected vessels are required to have a fuel shutoff valve between the tank and separator / filter.

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Another approach would be to close the fuel valve in the above picture. Loosen the lid and draw your sample. Them remove the lid and top up the Racor with a small container of fuel.

Personally, I would try the squeeze ball approach that I mentioned earlier.

If this is for your generator and it has an electric fuel pump, it's pretty easy to wire the electric fuel pump to run and prime both the separator and the filter after the lift pump. Thats what I do when changing filters. I have a jumper that runs the pump to prime the separator and filter. The air is pushed back into the tank through the return line. Never fails to prime the system after filter changes.

Ted
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Old 08-28-2022, 03:18 PM   #6
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On our last boat I installed the Racor priming pumps into the Racor filters. It isnít the cheapest way to do it but it was very nice. I hooked them to a push button switch nearby so you just pushed the button and waited while the pump pushed the fuel up and out the secondary bleed screws.
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Old 08-28-2022, 04:20 PM   #7
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Quote:
Originally Posted by diver dave View Post
Lets say the tank fuel level is lower than the filter. And, the engine is higher than the filter. You open the filter drain. Fuel dribbles out, but air bubbles in to replace that space. Then the air migrates UP to the engine, where it is stuck until it gets pumped into the injectors. Forcing an engine bleed.

Do Racors and the like have check valves?
Yes. In practice if the drain is above the level of the tank you're drawing from you'll crack the drain valve and nothing will happen. I think the drain valve is designed to not let air in.

My filters will drain themselves from a tank more than about 1/2 full. So scheduled filter work is done with a full tank. If both my tanks are below 1/2 I close the fuel supply and open the top as Ted describes above.

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Old 08-28-2022, 05:54 PM   #8
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If you're taking a fuel sample for testing, take the sample after the filter. The bowl is going to have water and debris the racor removes.
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Old 09-08-2022, 02:58 AM   #9
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If you're taking a fuel sample for testing, take the sample after the filter. The bowl is going to have water and debris the racor removes.
So true!
Your sample needs to be representative of what you are burning in your motor.
A sample taken from the bowl will be disproportionately dirty, even compared to an unfiltered tank sample.
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Old 09-08-2022, 07:55 AM   #10
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No, i have Separ filters, and they describe a process for alternating draining a bit of diesel with opening the bleeder, back and forth. I wanted to make sure that wasnt going to admit air into the feed line.
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