Author Topic: Sending units  (Read 426 times)

Offline Deathscythe

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Sending units
« on: September 19, 2018, 10:41:17 PM »
While I'm repairing issues with my 86 GMC Sierra, I've been deleting most of the emissions systems under the hood. There had been previous suggestions to keep the charcoal canister for the fuel system. On the list of things I need to do is replacing a leaking gas tank. I plan on pulling the bed to get to it, and since the bed will be off I will be replacing both tanks, sending units and hoses, selector switch and valve.
While pricing parts my local guy asked if I needed a 2 or 3 port sending unit for each tank. Since the ports are for fuel, return and venting, I'm assuming the vent tube goes to the charcoal canister correct? If I can eliminate the canister, could I just put in 2 port sending units instead of the 3 port units that are probably in it and it still run correctly? It's a carbureted 305 if that matters.

Offline hatzie

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Re: Sending units
« Reply #1 on: September 19, 2018, 11:10:17 PM »
You won't get senders with Supply and Return...  Two line senders are Supply and Vent with no Return...  Can you say vapor lock? I knew you could...

The 73 & 74 1/2 -1ton trucks had vent lines with a vent similar to the plastic axle vents. 75 and later 3/4 & 1 tons didn't get carbon cans til near 1980.

I'd keep the vapor can.  Vapor cans have less than zero impact on performance but they generally keep the garage from smelling like raw fuel.  You can go to the three hose vapor can setups from the 1970's if you hate the hose farms from the 80's...   Three hose is Vapor from the tank(s), Purge valve controlled by ported vacuum, and 3/8" purge to manifold vacuum (usually teed into the PCV hose).
« Last Edit: September 19, 2018, 11:12:26 PM by hatzie »
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Offline Henry

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Re: Sending units
« Reply #2 on: September 20, 2018, 11:03:03 AM »
Hi DS:
You can convert from 3-port tank senders to 2-port tank senders, but as Hatzie has said there are a few things that need to be considered. One of the ports on the 3-port sender goes back to the fuel pump which has 3-ports...so if you convert to 2-port senders you now need to change your fuel pump to a 2-port fuel pump. That 3rd port on your fuel pump is a return line to the tank(s).  You also need to use the appropriate fuel selector valve with whichever way you go: 3-ports selector valve for 2-port tank senders and 6-port selector valve for 3-port tank senders. If you try to mix components and try to get it to work you are going to get yourself into trouble.

BTW, when the bed is off, this is a good time to replace ALL hoses as they are a pain to get to otherwise. Also is a good time to see if you have any part markings or serial identification on the frame as this is also something that you will never see again when the bed is on.

Regards,
Henry

Offline Henry

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Re: Sending units
« Reply #3 on: September 20, 2018, 11:42:09 AM »
Hi DS:
I almost forgot: another thing to consider is that the electrical connection at the fuel selector valve is different for 3-port valves and 6-port valves. If you have a 6-port selector valve and the corresponding fuel return and charcoal cannister/venting system, it is easiest to just keep this system...if you are dead set on removing that cannister, you can just make the tank vent line(s) that comes from the tanks to the cannister just short stubs with little filters on the ends of them and it will all work ok. This will insure your tanks vent to the atmosphere ok.

Regards,
Henry

Offline Henry

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Re: Sending units
« Reply #4 on: September 21, 2018, 11:33:05 AM »
Hi DS:
If all this is confusing to you just let me know and I can post a diagram of the hoses for the saddle fuel tanks.
Regards,
Henry