Fuel tank Sending Units

[Dukagora]

I have a 79 crew cab 454 with dual tanks.

I think I am a bit confused on the fuel system.

There is no valve to switch tanks, and I cant remember now but pretty sure the wiring isnt there. So the passenger tank was made the primary.

I dropped the drivers tank, mainly because of the substantial dent, and confirmed both tanks have a 3 port sending unit. Is this correct? Should they both have a 3 port sending units?

I have had poor experience with gauge accuracy on spectra sending units and was going with a Dorman. The Dorman I ordered is Part # 692-078. I picked it up off Autozone online and after ordering, realized I hadn't seen which side it is for specifically. Worked with the person online who really didn't seem to know a whole lot about the situation, then went in locally after trying Napa who only had spectra. Oreilly is the same way. Autozone online told me its for the "main tank" but dont know which tank is technically the main tank. I assumed the passengers side since my 77 half ton was the same way and single tank only. Then I read somewhere that 78 and 79, drivers side was primary? Summit Racing lists the same sending unit as passenger side, which wont be correct.

At the local Autozone, they seemed to believe that one side was 3 port and the other is 2 port. Does this make sense?

My understanding is 3 ports. One goes to mechanical pump, one is return line, and the other is for canister?

Pics under the hood....
Obviously left is fuel pump. Right is what looks like 2 canister. Am I supposed to have 2?

Right Canister:
Right hose goes to nowhere, but the top of canister says it should go to tank.
Left hose goes to the T in the right pic in post #2. Sorry the pics are broken up, it wouldn't let me upload all 4 at once.

Left Canister: Labeled as "Purge"
Right port is capped. says "Air"
Left ports...
Top goes down to across what I'm going to call a cross member, and routes underneath the mechanical pump and goes back to the passenger tank.
Bottom goes to the drivers valve cover into a 2 port, I am guessing here, PCV valve. Top port on that PCV valve. Once you rotate your head 180 degrees, you can see it in the pic in post #2.

[Dukagora]

2 more pics

I don't know why the right pic is upside down....I didn't take it that way.

Based on the info I provided, does this all make sense? Is everything routed as it should be? Should I reconfigure? Do I need both of those canisters? If so, what should be done differently to make both tanks work as they should? I plan to take this truck on vacation soon and will certainly need max fuel capacity.

[sweetk30]

spectra premium is oem supplier . its just the old school tech that makes them not 100% accurate .

i do NOT care for lots of dorman stuff for the super cheep quality level .

3 port is 3/8" = feed - 5/16" = return - 1/4" = vent line to cannister up in engine bay .

as to who says primary tank is left or right i dont think half of the parts places get it correct. i just look for left or right . and the sender is a little different from side to side . you cant just swap them .

when i do my trucks up i use good oem or if new spectra premium .

take this info for what its worth . just my first hand experience from 20+ years in the auto field .

[Dukagora]

Thanks for the quick reply. However, and maybe it was gremlins, but with a new spectra unit, at about half a tank, in a different truck I had, the needle would start bouncing back and forth. On the gauge itself, Id say the needle would move back and forth about a 1/4 of the gauge worth.

I will be returning the dorman anyway once it gets here. It came direct from them so I am stuck until it shows up. If the Spectra really is OEM Id prefer that route.

If the issue will always be with the older gauges, how do you correct that? Is the gauge old and simply needing replaced, or would I need to upgrade to a different gauge to address this issue?

[sweetk30]

make sure you get the ground wire to the truck frame . use a star washer between the frame and eye of the wire . good and clean metal .

and some senders you can see in to the level resistor and its tabs . i play with it a bit and watch for dead spots . and adjust the tabs as needed to hold contact .

gm spec is 0-90 ohms so you can test the gauge with a meter before you install it .

[wilkin250r]

Did the gauge bounce while the truck was sitting still? It's not uncommon for older gauges to get "loose", and the gauge will actually bounce due to the fuel sloshing around in the tank. When that happens, the problem is with the gauge itself, not the sending unit.

The passenger side is usually the default side when you have the single-wire solenoid switching valve (which you probably do, on the 79).

And yes, both tanks should have the same number of ports. If you have a 3-port on the left, you should also have a 3-port on the right.

[100%Chevy]

The ports on the sending units are supposed to point inward toward the frame.
The tanks are the same but the sending units are"Keyed" to the tank.
So,the right sending unit will will have a tab that allows it to sit in the tank and point to the frame.Same with the left sending unit.
"Main" tank is a useless way to describe which tank is the default tank.Usually means the right side tank.
Mike.

[Dukagora]

I thought the term "main" tank was useless too. Glad I wasn't the only one who felt that way! Thanks for the info so far everyone. I thought the sending units were keyed as you described so thank you for confirming!

Wilkin250r, I don't recall as I haven't had that truck in probably a year now, long gone, and lots of rust. So I cant recall but I don't think it was specific to idle vs driving.

SweetK I don't think I quite understand what you mean when you say "and some senders you can see in to the level resistor and its tabs". Unfortunately I am a rookie at almost everything I am doing with my truck but am very determined to learn.

Since I started this thread, I have been throwing the idea around of ditching the tank I pulled off and getting an 87 tank with sending unit and all of that.

Given that the truck currently has no hardware related to the valve that used to be on the truck, if I chose to just drop in an 87 tank with corresponding sending unit, would that cause a problem for my existing carb in any way? I plan to move to fuel injection in the future and figured since I am already doing all of this, probably should just do it once and be done with it instead of having to come back and do it again later. I am thinking that the sending units push fuel, and if so, I don't know how much psi that would come out to. Also, I looked at pics of one on Rockauto and can see the wiring is very different.

https://www.partsgeek.com/n1rcpmp-ch...Selector+Valve
Given that this selector valve seems to be all you can buy now, I will have that with coinciding connector and switch in the next few days, so hopefully if I did go the 87 tank/sending unit route, everything will work well together.

Also, this means all hosing needs to be able to take 60ish PSI then right? I know the fitech unit runs at 58, but not sure about others. I planned to replace a good bit of the hose while everything is off. Regarding the filler tube, I have 2 sections of hosing for mine. One of the sections is rather hard so I planned to replace it. Do I need to use fuel specific hosing for that also? I bought 1 foot of I think 1" 3/8" last night and it was $20!

[tucsonjwt]

I installed Spectra sending units in my 83 C20 454 dual tanks. They bounce the fuel gauge needle. The original GM (AC Delco?) units did not bounce the needle. When I bought the Spectra units, they were about $50, as opposed to the AC Delco units, which were about $150. So, I went with Spectra. I would probably buy ACD if I had to install them again. I drive the truck less than 1,000 miles per year, so I can live with the bouncing.

On my 3/4 ton 83, there is no canister, since EPA in that year considered it a "heavy duty" truck. (Strangely, I do have all of the other smog stuff - lots of valves, smog pump, lines to the exhaust manifolds, etc.) The third nipples on the sending units on my truck just have a short piece of fuel hose and a one way valve - same valve that is on the rear differential.

I got a lot of fuel smell in the cab since the sending units are close to the cab, so I bought 25 feet of fuel line and routed the lines back to the rear of the truck, and terminated the vent lines above the spare tire. I bought new vent valves and capped off the vent lines and I do smell the fuel when I am at the rear of the truck, but it is better than smelling fuel when I am in the driver's seat.

I don't know why you have a canister. My Tucson truck goes through emissions every year and they check over the engine compartment thoroughly. They have never questioned my lack of an evaporative canister. I have never seen any evidence that an evaporative canister was ever installed. I assume that your crew cab is at least a 3/4 ton. You have a 79, so maybe the rules were different for that model year.

Let me know if you want to see some pics of my set up, recognizing that I have a different model year.

[Dukagora]

My truck is a 79 c20 454/th400 3/4 ton. Its interesting because this truck has nothing you have except the canisters. The 77 I had, same engine, 3/4 ton had everything you have including the canisters. The 77 was a california truck so I assume had all of that because of emissions. I donr have emissions to pass here thankfully. I am going to go grab the 77 service manual I printed out and see if there is any info in there on it. Its sometimes hard to say whats what with the many decades of use these trucks have seen to know what came originally and what didnt.

[Dukagora]

So does anyone know if an 87 tank and sending unit would affect the PSI to the carb in any way?

How does that get wired up on a truck that never had the fuel injection?

I have the 6 port valve, connector and switch on the way. I did find a diagram of what each pin did, just not sure how the connector for that sending unit changes things wiring wise since I would do this for both tanks.

[nascarmikeb]

Hey dukagora,

its odd that I am doing the exact opposite as you. I have an 1987 gmc that I am converting to carb and mechanical block mounted fuel pump. I'm trying to figure out how to make the backwards conversion with electrical connection.

Mike

[Dukagora]

That is odd. Maybe your issue will be resolved as well! I have a gen 6 454 I intend to put in to the truck. I want to do some performance upgrades before I put it in but cant decide what to do for sure. The block on that one doesn't have a spot for a mechanical though so I have to change some things in the future. I think someone on this forum makes a kit for what I am doing, but I do not know for sure.

[tucsonjwt]

If you don't have to be concerned about emissions, I would just do what I did and route fuel hose to the back of the truck from the third port. I don't see any advantage to having the evap canister. I think it is just one more thing to go wrong. I know the evap canister absorbs vapor, but I really don't need another do-dad on my truck.

Let me know if you need some photos of my set up. I have a dump body so it is easy to view from the top.

[Dukagora]

Ok sorry was misunderstanding. Haven't dealt with a truck from this generation with anything but a carb. So if I swap tanks, I have to do sending units AND the pump that goes in the tank WITH the sending unit. So that accounts for one of the connectors I see. On top of that, I probably need to get a fuel regulator also to manage PSI for the carb.

So I guess I need to figure out how to get power to the sending unit/pump in each tank with a truck that never used these fuel delivery components. Probably doesn't have the right connectors to make this stuff work and isn't wired right.