Does my 79 have a fuel filter in unit?

[boltbrain]

I replaced the fuel pump in my 79 K20 350 and it’s not pumping gas so I tried pumping frim both tanks separately with an electric pump but still got no gas, both tanks have gas, one is almost full. The service manual says to clean the filter on the sending / pickup unit if it’s out, but before I drop a tank I would like to know for sure if there’s a filter in the tanks. Also where’s a good place to get a unit that’s good quality? Thanks much

 

[Snoots]

Filter is outside of the tank. Start from the carb/throttle body/whatever and trace the fuel line back. Look on the inside of the frame rail.

 

[boltbrain]

Thanks, I looked again but there isn’t one anywhere. I’ll try a little compressed air from a line into sending unit but just remembered my gas gauge does not work so will drop the one tank anyway

 

[Big Ray]

If it's a q-jet still it's here....

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[Big Ray]

And here's what the sending units look like. The filter socks clog up with time, and sometimes the pickup rusts up real bad, sometimes breaking off.
Sometimes the rubber lines in the system swell up,, or dry out and crack causing air leaks.

You must be registered for see images attach

 

[SirRobyn0]

@Big Ray is giving you great information. If it's still running the Q-jet there is a fuel filter in the carb inlet, if someone has put another carb on the truck there is likely an added inline filter. Then on the bottom of the sending unit is a screen called a sock. The sock is not enough of a filter on it's own, it's designed to keep larger stuff from getting sucked up. If yours have never been done after 40 years you can bet they are not the best, and if the truck has sat they are likely plugged. Don't, remove the sock assuming the inline filter will be good enough. Guy do that sometimes and then can't figure out why they keep having switching valves go out, filters plug ect. Again just replace the sock or the entire sending unit.

At the shop we see quite a few squares and we have had good luck with spectra brand sending units O'riellys is a spectra dealer BTW. A new sock will come with the new sending unit. If your not getting fuel out of a tank, like with your electric pump it's either a clogged sock or a rubber line that has split so badly you can't suck though it anymore. There is rubber the switching valve roughly under the passengers butt on the frame rail, and also rubber from the frame to the sending units. If your dropping the tank to replace the sending units you should consider replacing those rubber lines while you are in there. You'll need 3/8" fuel hose for fuel line, 5/16" hose for the vapor (emissions hose, canister hose whatever term you like to use) and if your truck has a return line you'll need 1/4" for that.

 

[Rusty Nail]

Yes. ^

ETHANOL fuel WILL destroy the rubber line - that IS inside the tank.
The only fuel filter of a '78 is at the carburetor inlet.
You will need a one inch AND 5/8 open end wrench... don't blow compressed air through it - unless you're looking for trouble.
Next thing you know? You'll be rebuilding a Quadrajet.

 

[SirRobyn0]

Yes. ^

ETHANOL fuel WILL destroy the rubber line - that IS inside the tank.
The only fuel filter of a '78 is at the carburetor inlet.
You will need a one inch AND 5/8 open end wrench... don't blow compressed air through it - unless you're looking for trouble.
Next thing you know? You'll be rebuilding a Quadrajet.

I'll second the don't blow compressed air through it. Those lines and hoses are designed to either have suction in them or a small amount of pressure. IF your lucky the rubber was rated for 50PSI when new 40 years ago.....

 

[boltbrain]

Yes. ^

ETHANOL fuel WILL destroy the rubber line - that IS inside the tank.
The only fuel filter of a '78 is at the carburetor inlet.
You will need a one inch AND 5/8 open end wrench... don't blow compressed air through it - unless you're looking for trouble.
Next thing you know? You'll be rebuilding a Quadrajet.

Thanks, I just took the tank off, no rust in it. Where is there rubber in the tank? Never bothered blowing air through a line, I’m replacing all the hoses and both sending units. I sure appreciate all the great info guys

 

[boltbrain]

@Big Ray is giving you great information. If it's still running the Q-jet there is a fuel filter in the carb inlet, if someone has put another carb on the truck there is likely an added inline filter. Then on the bottom of the sending unit is a screen called a sock. The sock is not enough of a filter on it's own, it's designed to keep larger stuff from getting sucked up. If yours have never been done after 40 years you can bet they are not the best, and if the truck has sat they are likely plugged. Don't, remove the sock assuming the inline filter will be good enough. Guy do that sometimes and then can't figure out why they keep having switching valves go out, filters plug ect. Again just replace the sock or the entire sending unit.

At the shop we see quite a few squares and we have had good luck with spectra brand sending units O'riellys is a spectra dealer BTW. A new sock will come with the new sending unit. If your not getting fuel out of a tank, like with your electric pump it's either a clogged sock or a rubber line that has split so badly you can't suck though it anymore. There is rubber the switching valve roughly under the passengers butt on the frame rail, and also rubber from the frame to the sending units. If your dropping the tank to replace the sending units you should consider replacing those rubber lines while you are in there. You'll need 3/8" fuel hose for fuel line, 5/16" hose for the vapor (emissions hose, canister hose whatever term you like to use) and if your truck has a return line you'll need 1/4" for that.

Thanks, I’ll check that carb filter too. The sock looks bad, I’m ordering two Spectra units, need to decide if I want the passenger side one to have three ports. I’m going to a three port fuel pump so I can avoid vapor lock, but don’t know where the line goes to from the fuel pump, the extra one. I see the vapor lines from the tanks go to the carbon canisters, I forget the name of that system, but you know the big black canisters with charcoal. ( I can’t decide on whether to delete that stuff). Would rather go to the tank with it. Thanks very much

 

[boltbrain]

O’Reillys says they have two port sending units and three port. Does the third port go to the fuel pump? My old pump just had two, but I want to replace it with a three port to get the vapor lock protection. Thanks

 

[SirRobyn0]

O’Reillys says they have two port sending units and three port. Does the third port go to the fuel pump? My old pump just had two, but I want to replace it with a three port to get the vapor lock protection. Thanks

Yes, get the three port sending units, use the three port fuel pump. You will want to connect the extra 1/4" port on the pump to the 1/4" port on the sending units, but it needs to go though the tank switch valve so it returns to the correct tank, so you'll need to change the tank valve to if your going to go three port. Or you can simply plug off the third port on sending units for now, and then you'll have the ability to add a return line in the future.

Hopefully this helps.

 

[boltbrain]

Thanks, ordered.
I was going to return it to one tank, would it matter which tank? I’m guessing it’s got to do with vacuum.

 

[SirRobyn0]

Thanks, ordered.
I was going to return it to one tank, would it matter which tank? I’m guessing it’s got to do with vacuum.

You can't do that. If you run a return line to just one tank you'll fill the one with the return line when your running off the other one. Worst case scenario it over flows the tank, you don't know it because your driving, it then pushes it into the charcoal canister and then... We you have a mess on your hands and a ruined canister, assuming the resulting fumes don't cause a fire. This does not work, the return line needs to return to the same tank your drawing off of.

Pollak 42-308P 3 Port Fuel Selector Valve Kit, will have the switch and the valve you'd need. If you're interested in going this route let me know and I'll give you the rundown on what it would take to convert, but like I said you can cap off the return ports and install a return system if you need it.

​

 

[boltbrain]

THANKS, I appreciate the offer. I didn’t know that much fuel is returning from the fuel pump. Thought it was 99% fumes.
If I cap off the return ports on the sending units, I also have to cap the pump port? How much pressure does that return have at the pump, the same as it has to the carb?
I can run the lines, mostly steel. I’d appreciate the rundown when I get there. First need to fix my tank or find another one. This one has leaky pits n a small place under a strap. Been watching vids on de-fuming on YouTube. It’s a small spot.