Fuel tank sending unit - 3 port or 4 port?

7900_Blazer

Full Access Member
Joined
Jun 15, 2021
Posts
143
Reaction score
104
Location
Texas
First Name
Francis
Truck Year
1975
Truck Model
Blazer
Engine Size
350
I some of you know, I had vapor lock issues on my trip to the mountains last summer. I'm add a return line (so new fuel pump and sending unit) and have 2 questions:

1. Can I use rubber fuel line for the return - or must it be steel?
2. One option on a sending unit is from Oreilly's; but it is a 4 port sending unit... the one from LMC Truck is a 3 port...

Does it matter which?

Thanks for any advice you can give me.
 

Bextreme04

Full Access Member
Joined
May 13, 2019
Posts
3,052
Reaction score
3,032
Location
Oregon
First Name
Eric
Truck Year
1980
Truck Model
K25
Engine Size
350-4bbl
For a blazer, a three port is one without the return line. Blazer tank does not have the fuel filler vent provision built into the tank, so the sender has one more port than an equivalent function saddle tank. Four port sender has 1) Fuel filler neck vent 2)Fuel Feed 3) Fuel Return 4) Tank vent(vapor return to charcoal canister)
 

7900_Blazer

Full Access Member
Joined
Jun 15, 2021
Posts
143
Reaction score
104
Location
Texas
First Name
Francis
Truck Year
1975
Truck Model
Blazer
Engine Size
350
Second question, the 4 port sending unit I purchased will have an extra port… I’m assuming I can simply cap it off?

Port 1 will feed the fuel pump
Port 2 is the fuel filler neck vent
Port 3 will be the return from the 3 port fuel pump…

Again, I’m assuming I do not need to set up a charcoal canister…?

Bonus question… can I assume the vapor locking is most likely caused by fuel sitting in the pump… and NOT being vaporized between the pump and carb? Should I also wrap that steel line between the fuel pump and carb with say a rubber fuel hose to insulate it a bit?

Thoughts?
 

TotalyHucked

Full Access Member
Joined
Feb 24, 2020
Posts
678
Reaction score
1,841
Location
Hoschton, Georgia
First Name
Zach
Truck Year
1985
Truck Model
Sierra 1500
Engine Size
5.3
I would pick up some actual heat shield to wrap around the steel line. I've got DEI heat shield (should be able to pick it up at any parts store) all along my fuel line under the truck cuz it's near-ish to the exhaust. If you still have some issues after adding the return and adding heat shielding to the line, you might try a phenolic spacer or something to put under the carb to help keep the heat from the manifold from reaching the carb
 

Bextreme04

Full Access Member
Joined
May 13, 2019
Posts
3,052
Reaction score
3,032
Location
Oregon
First Name
Eric
Truck Year
1980
Truck Model
K25
Engine Size
350-4bbl
Second question, the 4 port sending unit I purchased will have an extra port… I’m assuming I can simply cap it off?

Port 1 will feed the fuel pump
Port 2 is the fuel filler neck vent
Port 3 will be the return from the 3 port fuel pump…

Again, I’m assuming I do not need to set up a charcoal canister…?

Bonus question… can I assume the vapor locking is most likely caused by fuel sitting in the pump… and NOT being vaporized between the pump and carb? Should I also wrap that steel line between the fuel pump and carb with say a rubber fuel hose to insulate it a bit?

Thoughts?
Mine just has a hose to one of those plastic wobble cap vent thingies that you put on your diff vents. It keeps your tank from pressurizing. For emissions vehicles that goes to the charcoal can and then into the intake so you aren't venting hydrocarbons. It keeps it from stinking like gas vapor too.

Vapor lock happens on the suction side of the pump. Once it gets pressurized it wont vaporize(usually). The reason you dont get this as much on a return line pump is because it is flowing MUCH more fuel through the feed and return lines and therefore the fuel doesn't sit in the line and get hot before getting sucked to the pump. As it sits in the line and absorbs heat, it gets to the vaporization point and then the pressure drops as the pump does it's thing and the fuel vaporizes.
 

7900_Blazer

Full Access Member
Joined
Jun 15, 2021
Posts
143
Reaction score
104
Location
Texas
First Name
Francis
Truck Year
1975
Truck Model
Blazer
Engine Size
350
One of the tests I ran to verify my vapor lock was happening IN the pump... is with it running - motor hot - and really just barely enough fuel in the glass fuel filter which lives between the pump and the carb (so I know it is in the process of vapor locking)... I took a water hose and ran water over just the pump itself... almost immediately the glass fuel filter filled back up with gas... so I'm pretty certain my vaporizing of fuel is happening IN the pump itself.. not in the lines leading to the pump, nor in the line from the pump to the carb...

So again, I'm assuming if I can keep fuel circulating through the pump via a return line (as suggested)... ideally that will solve my vapor lock issue...
 

sirweesarunch

Full Access Member
Joined
Nov 4, 2019
Posts
366
Reaction score
842
Location
Oceanside
First Name
John
Truck Year
1980
Truck Model
K30
Engine Size
454
Its easier to cap off a bung than to add one. I ordered the 4 bunger.
 

SirRobyn0

Full Access Member
Joined
Dec 6, 2019
Posts
3,305
Reaction score
3,495
Location
In the woods in Western Washington
First Name
Rob
Truck Year
1984
Truck Model
C20
Engine Size
350
I don't see where anyone addressed your question regarding using rubber fuel line for the return line. Yes, it's fine. There are however a few concerns using rubber over metal.
1. Rubber line will deteriorate (rot) and while metal can rust, metal will outlast the rubber.
2. Rubber line has to be carefully secured to the frame, and it has to be secured up in a lot of places, so it doesn't.
A. Drape down and hit the ground or get snagged on something.
B. Rub on something and cause a leak.

So back in the day I use to deal with Class C motorhomes built on the 70's Dodge chassis. Dodge did not do a factory return line, and filter was before the pump. Then drop a RV coach body on it and guess what. The odds are stacked against it, in almost ever way. And while we are not in a particularly hot climate here we have mountain passes and plenty of hills. So what we did was to cut the metal fuel line between the pump and carburetor, flare it and install a fuel filter with two out lets one designed for pressure output to the carb, the other one was return to tank. We always ran rubber fuel line back to the tank. We secured the line with... IDK what they are technically called, but they are metal bands, padded with rubber and you run a self tapper though the holder into the sheet metal. Don't use zipties, to much risk in them breaking after years in service. Never had a single come back for a problem with the line, and always took care of the vapor locking.
 

Latest posts

Forum statistics

Threads
36,142
Posts
763,447
Members
25,273
Latest member
juliannv07
Top