Mechanical fuel pump return line

[Logbed]

I'm in a process of swapping the engine. My previous engine and fuel pump did not use the return line. Actually I just found the line was open, sort of a breather for the tanks? Now I could either put a pump without a return and just plug the line, or put a pump with a return and connect it to the line? I just wonder whether the return is leak free? Other potential problems from using the line that I seemingly did not need before. Mechanical pump is the choise but what would you do with the return?

 

[Ricko1966]

Are you sure you have a fuel return line,and you aren't looking at a vent line. If you have a return a 3 port pump with return would be preferable.

 

[Matt69olds]

The return setup would be preferred with today’s lousy gas. The 3 port pumps have a tiny orifice in the return fitting, basically a controlled internal leak. It’s constantly circulating fuel back to the tank which keeps the fuel and pump cool. The other advantage is hot restart. Imagine it’s a 90 degree day, the engine is hot, you have been idling in a parking lot with the A/C going. As hot as it is outside, imagine what it’s like under the hood. Now shut off the engine, imagine how hot it will get after about 20 minutes with zero airflow. Whatever fuel is in the pump and line will begin to boil. The pressure will eventually rise to the point where the fuel pushes the needle off the seat, resulting in a flooded engine. The tiny orifice in the return line provides a vent to bleed off excessive fuel pressure back to the tank making hot restarts much easier.

Since the vast majority of vehicles on the road today are EFI, with closed fuel systems and electric fuel pumps, the oil refineries long ago omitted the additive to prevent fuel boiling and evaporation. We old school carbureted guys make up a tiny percentage of fuel buyers, it’s just not cost effective to include the additive to their recipes.

 

[Logbed]

Are you sure you have a fuel return line,and you aren't looking at a vent line. If you have a return a 3 port pump with return would be preferable.

How should I tell the difference? In my '85 truck there is a smaller metal line right below the fuel line and it was not connected to anything as my truck already had a swapped engine with mechanical pump that had only 2 connections, in and out.

 

[Ricko1966]

How should I tell the difference? In my '85 truck there is a smaller metal line right below the fuel line and it was not connected to anything as my truck already had a swapped engine with mechanical pump that had only 2 connections, in and out.

Sounds like the return line

 

[AuroraGirl]

The return setup would be preferred with today’s lousy gas. The 3 port pumps have a tiny orifice in the return fitting, basically a controlled internal leak. It’s constantly circulating fuel back to the tank which keeps the fuel and pump cool. The other advantage is hot restart. Imagine it’s a 90 degree day, the engine is hot, you have been idling in a parking lot with the A/C going. As hot as it is outside, imagine what it’s like under the hood. Now shut off the engine, imagine how hot it will get after about 20 minutes with zero airflow. Whatever fuel is in the pump and line will begin to boil. The pressure will eventually rise to the point where the fuel pushes the needle off the seat, resulting in a flooded engine. The tiny orifice in the return line provides a vent to bleed off excessive fuel pressure back to the tank making hot restarts much easier.

Since the vast majority of vehicles on the road today are EFI, with closed fuel systems and electric fuel pumps, the oil refineries long ago omitted the additive to prevent fuel boiling and evaporation. We old school carbureted guys make up a tiny percentage of fuel buyers, it’s just not cost effective to include the additive to their recipes.

also even if the fuel wasnt vapor in the line, the moment it starts sucking gas , you will cause it to boil (under a vacuum, the boiling point is lower)
a small pusher pump by the tank apparently can help reduce this even in insulated/3port setups by making that vacuum pulled on the line much lower

 

[75gmck25]

If that 2nd metal line ends very close to the fuel pump, then is probably the fuel return line. The picture I attached shows my three-port fuel pump and all the lines attached.

I would have to take a look at the vent line location in my truck, but I think it ends a different location that is closer to the passenger side. In my truck the vapor canister for the vent is on the passenger side, so that makes send. If you have dual tanks it's also easy to figure out which one is the vent line, since it has a T fitting so it connects to both tanks. It doesn't connect to the fuel switching solenoid. Also, if you have a fuel return line and two tanks, you should have a six port tank switching solenoid.

 

[AuroraGirl]

If that 2nd metal line ends very close to the fuel pump, then is probably the fuel return line. The picture I attached shows my three-port fuel pump and all the lines attached.

I would have to take a look at the vent line location in my truck, but I think it ends a different location that is closer to the passenger side. In my truck the vapor canister for the vent is on the passenger side, so that makes send. If you have dual tanks it's also easy to figure out which one is the vent line, since it has a T fitting so it connects to both tanks. It doesn't connect to the fuel switching solenoid. Also, if you have a fuel return line and two tanks, you should have a six port tank switching solenoid.

not sure about 85 but there is a chance HD emissions without evap may have been offered in 85, if it was, there would be short little lines off each tank and then a vent cap and no canister, but im not sure it would be that way on his

 

[AuroraGirl]

You must be registered for see images attach

Heres my 3 port pump, for another example