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Old 02-02-2003, 02:00 PM   #1
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1985 34.5' Airstream 345
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Electric fuel pump and fuel line routing

I just purchased my 1985 345 and I found that the previous owner had bypassed the electric fuel pump since it was defective. I replaced the pump but I have a question on fuel line routing. The current routing of the fuel return line is from a T fitting between the fuel inlet line at the carburetor and the carburetor. The return line outlet on the mechanical fuel pump is blocked off. Should I remove the "T" fuel line return fitting at the carburetor and use the outlet on the mechanical fuel pump? I am concerned that the electric fuel pump might cause damage to the mechanical fuel pump since the return is blocked off. Need some guidance from other owners on how their lines are routed and do you have a "T" fitting at the carburetor. Thanks.
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Old 02-04-2003, 12:01 AM   #2
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I looked in my service manual and owners manual and found nothing. I'll go take a look at my 325 tomorrow evening and let you know what I find.

I expect to find something like the way I set up the electric auxilary pump on my '86 F250 and a small diesel on a boat where the electic fuel pump is inserted in the fuel line between the tank and the mechanical pump. In both diesel cases the normal mode is to leave the electic pump off and have the mechanical pump draw the fuel thru the electric Autopulse pump. The Autopulse is not needed in normal operation. The diesel benefit of the electric pump is ease of bleeding air out of the system upon service or filter replacement.

My 325 behaves as if it is similarly plumbed. I can run the engine at idle indefinately without turning on the electic pump. It is only when I put the unit in gear that I need the electric pump. To me that means that at low power the mechanical pump is pulling fuel thru the electic pump and the problem when a quantity of fuel is needed is excessive fuel line length.

Your unit plumbing doesn't make sense to me as either of the two pumps will, when operating, attempt to create a backflow for the other pump (the mechanical pressurizes the outlet of the electic pump and visa versa). I suspect that the return line on the mechanical is plugged because without the plug the electric pump output would just run back into the tank thru the mechanical return.

I'm curious if you can run with mechanical pump alone and I can't because my electic pump creates a lift problem for the mechanical pump. Your mechanical pump is not burdened by any electic pump restriction.

It seems as though these electic pumps are add-ons. I don't ever remember reading about an electic pump in the owners manual and the switch is located and labeled in a way that suggests it was added later.

Where does your electic pump draw fuel from? Does it draw from a tee in the line between the tank and the mechanical inlet?
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Old 02-04-2003, 06:56 AM   #3
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If your service manual is for a 1985 model, it might not include the electric pump information. I had lots of fuel delivery problems with my 1985 345 when I bought in 2 years ago. The shop called Airstream and they found out that the 1986 and later models had the electric pump added to correct the fuel delivery problem. Once the shop added the electric fuel pump in the rear of my unit, the problems went away.
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Old 02-04-2003, 08:33 AM   #4
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The electric fuel pump should have a higher volume and lower pressure than the mechanical fuel pump. The return on the mechanical pump is after the diaphragm and will return excess fuel from both pumps. The two pumps are in series and the electric pump output should only have suction from the mechanical pump.

My guess would be that the PO didn't know the electric pump existed. Without the electric pump there probably wasn't much fuel flow, a long small diameter line and the resistance of drawing through a non functional pump. Raising the return closer to the carb forced the mechanical pump to get gas to the carb instead of just circulating it. A pretty ingenious solution, he just needed to look underneath a little harder (or maybe an on the road fix and he never got around to repairing it properly).

John
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Old 02-04-2003, 11:09 AM   #5
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1985 34.5' Airstream 345
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Fuel line routing

After I bought the unit I had to drive it home about 800 miles. I was starving for fuel in the hills of Kentucky and Tennesee. The previous owner had placed a fuel line that ran from the in to the out of the electric fuel pump. My guess is two fold, one to keep dirt out of the pump the other to fool anyone looking at it thinking it is hooked up. There was an inline splice at the fuel line from the tank to the mechanical pump at the location where the electric pump would have been connected. The electric fuel pump runs as soon as the ignition switch is turned on and the service manual list and shows the exact pump that I purchased. Based on the input I have received and talking with several mechanics I will remove the "T" fitting at the Carburetor and use the return line on the mechanical fuel pump. By the way the "T" fitting had a plunger device in it to regulate the fuel going back in the return line. I don't know what caused the plunger to function. I believe this will now solve my fuel starvation. I did read in the forum where some owners placed two electric pumps in line. I wonder what kind of problems they had to do that. I appreciate the responses I received and look forward to receive additional information as some of you look at your units and the routing.
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Old 02-05-2003, 12:01 AM   #6
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OK, I studied my '85 325 set up. I have three lines at the tank. One line goes to the generator and that makes sense because the manual tells me that you can't run the tank dry with the generator so you always have some "get home" fuel. Generator pick-up must be higher in the tank.

The second line goes to an electric fuel pump located just forward of the tank (just behind the tag axle mud flap). The pump looks like an add on based on feed wire being outside the wiring loom but attached to it with cable ties. I also have a electical tape splice in there (yech). The pump is attached to a steel plate that appears to be a chassis reinforcement associated with the tag axle.

The third line must be the return.

The mechanical fuel pump has three lines at the bottom. One is tubing going up to the carb and the other two are the supply from the electric pump and the return.

This all squares with the post above about fixing fuel problems on the other '85 345 by adding an electric pump and the electric pump being standard in '86 and after. It also squares with the post about the return being after the mechanical diaphram. It all squares with our being able to idle all day with the electric pump off but not being able to move the rig without flicking the electric pump on.

Sounds like you are on the right track with connecting the electric pump where it used to be and getting rid of the tee. I'm not sure why one would need two electic pumps unless you were just commited to backup -- sort of like the dual magnetos on an airplane.

Let us know how it all works.
Jim
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Old 02-05-2003, 11:31 AM   #7
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1985 34.5' Airstream 345
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Fuel line routing

I will reroute the fuel lines this weekend. Since I have to do this outside I am hoping it will get above 20 degrees. Most of my work has been done laying in the snow that surrounds the motorhome or in zero degree weather. One thing that I will check is when the electric fuel pump actually turns on since I read in one response that the fuel pump doesn't actually come on until you put it in gear or flip a switch. I don't have a switch but it will be interesting to see if it comes on as soon as you turn the ignition switch or after you start the engine. The reason being there should be an oil pressure shut off switch in case you have an accident and if the engine shuts off due to the accident and not by the ignition switch, the fuel pump shuts down due to no oil pressure. I will be doing some voltmeter checks for this.
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Old 02-06-2003, 11:16 AM   #8
The Hawk's Lair
 
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1985 34.5' Airstream 345
BACK WOODS , Minnesota
Join Date: Dec 2002
Posts: 922
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Fuel line routing

I decided to change the fuel line routing last night due to the snow that was to start today. I found the the "T" fitting had a screw in jet in the return side instead of a plunger. The jet reduced the inside diameter. The electric fuel pump comes on as soon as the ignition switch is in the on position and has not been moved to the start position. You can really hear the fuel flowing up to the pump and back through the return line when the switch is in the on position and the engine hasn't been started. Sounds as if everything is working, now I just have to find some mountains to climb. Thanks for all the replies.
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