73 K20 Driver Side Fuel Tank Sending Unit - 1 Port or 2 Ports?

[iflyfisher]

Hi Folks,

I will be removing the old bed here soon and want to replace the fuel tank sending units on both tanks while the bed is off. This truck has single port sending units on both fuel tanks. Based upon what I have read, the truck may have originally come with a two-port unit on the driver's side. I presume the second port would be for a vent. Is there a reason the passenger side would not have been originally vented? And should I add back a vent line to the driver's side?

Thanks for help.

Ken

 

[Grit dog]

I don’t know the specific answer but my understanding is one port tanks vent thru the gas cap (and no return line of course).
Always thought it was 1 or 3 port, when they changed it in whatever year.

 

[legopnuematic]

Two port senders are for trucks equipped with a charcoal canister/vapor return. That is how my 76 is setup, no fuel pump return.

 

[75gmck25]

A ‘73 K20 might have a very simple system, but I’ll explain some of the things to look for. My ‘75 K25 is kind of unusual because it has HD emissions and 8400 GVW (no catalytic converter), but I do have a fuel return line and evaporative canister.

Your sending units, fuel pump, gas cap, and evaporative canister have to “match” for the system to work properly.
For example, a single port sending unit would need a vented gas cap, fuel pump with no return line fitting, and there would be no evaporative canister under the hood.
On the other hand, a 3 port sending unit would need an unvented gas cap, fuel pump with a return line fitting, and an evaporative canister under the hood.

If you have a fuel return line, then your fuel solenoid would also need six ports, so that it could switch both supply and return lines.

 

[Ricko1966]

I'd put a 2 port passenger side,3 port drivers side,put a short piece of hose with a bolt on it on any un needed ports. Any future upgrades would now be easy. Look hard before you jump into this you may not be able to get the senders out with the bed off. I forget which way it goes but depending on what size tanks you have you may not be able to remove the senders,they'll be under the cab.

 

[iflyfisher]

Thanks Todd, Spencer, Bruce, and Rick. I have poured over the 1973 service manual, and it notes vehicles designated as 'passenger carrying vehicles' would have had the EVAP canister. But in certain drawings, only the C models are shown with the canister. There is no canister under the hood, and I don't see any abandoned lines.

My fuel gauge only works on one tank. When I switch tanks, the needle rises way past full. I have read about trouble shooting the electrical for faulty tank readings, but senders aren't too much, and I thought I would drop in two new ones given the age of the old ones. I have a new wiring harness to go in eventually. I presume that would have connections for the senders. @Rick, the senders are basically centered over the gap between the cab and the bed.

So...given the K20 is currently not set up for an EVAP canister, the question is should I add one or carry on without one and use a vented gas cap. The truck has the original 350 with 100K miles and was well maintained. I plan on keeping the truck close to original. I don't expect to add a more modern engine with a different fuel pump or EFI. Maybe Rick has the answer in swapping out the single port senders for the multiport units and plugging the other ports for now. If I find a vented gas cap is not working well enough, I add the EVAP can. If I were to add the EVAP canister, I presume each fuel tank would have a vent line running to a port on the canister?

Appreciate the insight!

Ken

 

[Grit dog]

^Basically yes to all that.
You could drop in 3 port senders and plug the unused ports with no issue and the ability to do/redo it however you want in the future. Cost is negligible.
If I was thinking hot summer driving and possibly low speed like 4 wheelin I’d think hard about just doing the 3 port and put a return line setup on now. But to get it running you don’t need any of that. It’s a 73 man! Basically the same fuel system as my lawnmower. It works.

 

[iflyfisher]

Thanks Todd. As my mechanical fuel pump doesn't have a return line and I am running the stock Rochester carb, I don't understand, "put a return line setup on now". Where am I returning fuel from?

Another question. I don't see a vent line between the tanks, so I presume each is vented separately?

 

[Ricko1966]

Thanks Todd. As my mechanical fuel pump doesn't have a return line and I am running the stock Rochester carb, I don't understand, "put a return line setup on now". Where am I returning fuel from?

Another question. I don't see a vent line between the tanks, so I presume each is vented separately?

When you raise pressure you raise boiling point,when you create a vaccum you lower the boiling point. With a 2 line pump there is a vacuum and it is held in the line. If you run a 3 port pump you run a return line from the pump back to the tank the fuel never sits under vaccum it is constantly moving,lowering your of vapor lock. You could also put a TEE after the pump if you don't want to change pumps. The straight through section of the tee is from pump to carbureator. The side shoot off the tee is return to tank put a lawn mower fuel shut off valve in the return line. The shut off valve is a restrictor so you don't bypass too much fuel shut it almost completely off.

 

[iflyfisher]

Thanks Rick. I think I get that. What I am unclear on is how to do it given my fuel pump has no return line. Not sure I am ready to abandon the mechanical fuel pump, and if I did, how the change in pressure might impact the carburetor. I haven't researched this, so I am largely in the dark. I have a modern GMC half ton that is my goto truck. This old Chevy will get exercised, but it will be mainly just for kicking around. I'm kind of the opinion that if it worked for my Dad for 50 years, it would probably do fine for how I will use it.

 

[Ricko1966]

Thanks Rick. I think I get that. What I am unclear on is how to do it given my fuel pump has no return line. Not sure I am ready to abandon the mechanical fuel pump, and if I did, how the change in pressure might impact the carburetor. I haven't researched this, so I am largely in the dark. I have a modern GMC half ton that is my goto truck. This old Chevy will get exercised, but it will be mainly just for kicking around. I'm kind of the opinion that if it worked for my Dad for 50 years, it would probably do fine for how I will use it.

I told you how to do it using a tee in my last post. The tee provides fuel to the carbureator and a return to the tank. You restrict the return so you don't return too much fuel to the tank. But honestly if you aren't having vapor lock problems,I'd use the don't fix it if it's not broke approach. I'd still use senders that gave me the most options for the future,just in case.

 

[Grit dog]

Thanks Rick. I think I get that. What I am unclear on is how to do it given my fuel pump has no return line. Not sure I am ready to abandon the mechanical fuel pump, and if I did, how the change in pressure might impact the carburetor. I haven't researched this, so I am largely in the dark. I have a modern GMC half ton that is my goto truck. This old Chevy will get exercised, but it will be mainly just for kicking around. I'm kind of the opinion that if it worked for my Dad for 50 years, it would probably do fine for how I will use it.

You buy a new fuel pump with a return port. $25. Probably worth replacing anyway for preventative maintenance if it’s not newer. Same as Rick said except you don’t need to meter the return fuel with a separate valve.
Not broke don’t fix it is fine. But when it’s 100deg in Southern UT next summer and you’re chugging around real slow on a hot day on a trail of traffic, or shut off the engine when it’s good and hot and it heat soaks the fuel system to above boiling having that return line will help with vapor lock.

 

[iflyfisher]

What a total airhead! Sorry Rick. I had completely spaced on the second half of the earlier post where you mentioned the tee. Something must have distracted me, and I never fully read it. My bad.

Todd, the fuel pump on this old truck does look like it was replaced at some point not too many miles ago. But that could be 20-25 years ago as this truck wasn't driven much during that period. I pulled it off when I redid the top end. It seems to be working fine...for now.

I am going to replace the senders with something that will allow me to run a return fuel line now or later. Perhaps while I have the bed off and tanks out, I might just as well run that hard line. I can use the tee as suggested by Rick until I need to replace the fuel pump. Rick suggested a 3 port on the driver's side and a 2 port on the passenger. If I run the return to the driver's side and run off the passenger tank, I guess we still avoid the vapor lock, but fuel returns to the LH tank rather than the RH tank. What size fixed line should I run back to the tank? Can I use 3/16" brake line? Assuming I could find an appropriate T.

I may get something like the vapor trapper and vent both tanks through it to the atmosphere. Seems easy enough to do while I have ready access.

Thanks again, Gents!

 

[Old Guy Bill]

I put a whole new fuel system on my '78 K20.
New lines and new sending units, and pump with return,
System worked fine until the factory style pump died within a month..
The next pump the pressure was all over the place, it would bounce from 3 to 6 lbs..
Went with the Edelbrock pump, it has a solid 5-6lbs of pressure, but no return line provision. I just capped those lines.
It has ran fine since then, no hard starts etc...

 

[Ricko1966]

5/16th is return line size. And my bad earlier should have told you 3 port on both sides.