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Old 09-26-2013, 11:24 AM
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Join Date: May 2004
Location: Texas
Posts: 228
Fuel tank cleaning questions

Hello all,
I have been working, this week, on the fuel system of my 1978 6.9.

The fuel tank is removed and I am puzzled about a few things.

First is that the fuel sending unit has no wiring harness. (No wonder the gauge is not working) Can not understand what would cause someone to remove it?

Second question is about the swirl pot. The top is not on the pot, it is just rattling around inside the tank. How do I get the top back on?

Third question has to do with the cleaning process. I have blasted the tank at the car wash and then at home repeatedly. Then washed with kerosene twice and let sit overnight with kerosene then blasted with water again a few times today. It is now sun bathing with an occasional blast of air.
What is the consensus on pour in tank liner? And do you have any other advise on what I should do before reinstalling the tank?

Also I have a fiberglass reserve tank that was made for a Mercedes that I hope to install. It fits nicely in the trunk with a nice angle back to allow for luggage.
Is there a prescribed method to plumb the tank into the main tank?

Thanks in advance for your time and help.

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Old 09-26-2013, 12:06 PM
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Join Date: Jan 2002
Posts: 18,355
Never heard of a swirl pot. What is it?
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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Old 09-26-2013, 12:43 PM
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Join Date: Oct 2012
Posts: 4,979
I think he is describing the semi containment device for the electric fuel pump or its pickup if the pump is external. Without a semi containment device for the fuel when a car corners and the fuel is lower. It could all slosh to one side leaving the pickup dry. Running dry might even just be damaging to the electric pump. I just am not certain.

This has been a fairly common item since electric fuel pumps came into common use. With fuel injection allowing air that is compressable to get into a highly pressurized area of the system may upset the applecart somewhat as well.

I forget the right name for those pots but they are very common. I never knew one to have a cover of some type though as well. Just an open fuel containment device with hole or holes to allow gas in and not let it get away too fast.

The urethane liner kits seem to be working out for quite a time now.If the tank is clean and not leaking do not bother lining it. If the device you mention has to have the cap on it you may have to cut a small area out of the tank and weld it back in to address this. The pot itself is usually spot welded to one half of the tank before the halfs are joined together with a rolling seam welder. It is not unknown for a pot to be loose in the tank but I never saw one with any form of a lid on it.
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Old 09-26-2013, 02:51 PM
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Join Date: May 2004
Location: Texas
Posts: 228
Yes the "pot" is in fact so that the pump does not suck air. It is like a tank in the tank. It is not necessary in carberated engines because of the float bowl.

In the pot (some people call it a flower pot) the fuel return enters at the bottom and creates a vortex that sucks fuel in from the surrounding tank.
you can see pot in the attached photo and where the fuel return enters at the bottom at the top of the photo.
Attached Thumbnails
Fuel tank cleaning questions-swirl-pot.jpg  
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Old 09-26-2013, 07:34 PM
Join Date: Sep 2005
Posts: 664
I replaced the fuel pump in an Audi 5000 once and found the fuel pot to be just that. It looked like a steel frying pan with higher than normal sides.

Nothing special about it. It is just the place where the fuel pump sits to suck gas, but it would be harder to empty it with a hard turn.

And.... If there is something in your tank that should not be there, like dirt or scale, you would only suck up what was in the pot area and not from the entire floor of the tank. Since the sides are so high and crud normally falls to the bottom and stays there it can swirl around on the bottom and never get into the pot where it could clog the pick-up screen.
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Old 09-29-2013, 01:44 PM
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Join Date: May 2004
Location: Texas
Posts: 228
Swirl pot

It is simple but brilliant. The more that i learn about the pot the more i realize its importance. A problem with the design is that as the return line enters its size constricts so that like a garden hose it sprays into the swirl and causes a sucking action from the main tank but that constriction is a common clogging point and causes a number of bad running issues that is hard to diagnos and hard to fix.

Mercedes started useing them in the 50s and it was not untill 1980 that others "invented" the design and started using it. I understand that race cars (F1) use it and dragsters have a small secondary tank external to the main tank.
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Old 09-29-2013, 01:55 PM
Registered User
Join Date: May 2004
Location: Texas
Posts: 228
Electrolysis cleaning

Yesterday i started cleaning the tank using electrolysis.
It is the only method of tank cleaning that does not remove good material and score the tank.
I have attached a battery charger to a 4 foot piece of re bar that i found in the mud next to the barn. The positive is connected to the bar (sacricicial annode) and the negative to the tank.
It is working great.
The tank is full of water with one tablespoon of baking soda (should be washing soda but i didnt have any. I it possible to convert baking soda into washing soda in the oven) per gallon of water.
I found that most cheap chargers let a little ac through and causes the reaction to slow down so i have connected a battery in the line and that has dramatically incrased the speed of the reaction.
Every few hours i wipe the rust off of the bar and when it it re inserted into the tank the meter on the charger goes from 1 to above 4
It is awesome to see and hear. Cant wait to try it on rusty parts and tools and pans.

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