Trying to get fuel to my engine, a novice's saga...

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dkraven

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Neighbor moved and gave me his 77 Sierra Grande last fall, told me engine runs great but thinks sediment from the fuel tank (truck sat a lonnng time) is clogging the fuel line because it only starts if you pull the fuel line off the pump and blow compressed air through it. Sure enough there's no fuel coming out of the hose and after he blows air into it fuel comes pouring out. He hooks it up to the pump and the truck started right up.

The truck stalled twice going around the block to my house and we did the air thing twice more because each time the fuel hose to the pump dried up. I decided to replace the fuel tank and sending unit since he thought sediment from the old tank was the problem. Few months went by as I waited for it to warm up, replaced the tank, learned a lot about cramped space work, put a few gallons of fuel in the tank, sprayed starter fluid in the carb ... and aside from firing twice from the starter fluid nothing happened.

Decided the mechanical fuel pump would be pretty cheap to replace and might be the culprit. Went ahead and replaced it. Sprayed starter fluid in the carb and the truck fired up! Decided I was a genius, drove the truck around the block, sounded pretty clean. There was some smoke from the left header but figured I could diagnose that later.

Two days later it wouldn't fire up. Pulled the fuel hose off the pump and it was dry, no fuel coming out. Tried my neighbor's air compressor trick again and fuel flowed from the hose and the truck started. Over the next two days I started it fine each day without blowing air, and then yesterday once again it wouldn't start.

I'm not sure where to diagnose at this point, or why blowing air with the compressor into the hose prompts fuel to start flowing again. Once started up the truck will drive without stalling, so fuel keeps flowing once it's running. My next idea is to disconnect the tank switch unit under the truck as it has dual tanks and maybe there's a bottleneck in there? The right side tank is still the original and I'm just not using it until I can get one tank working. I had one person suggest it has something to do with the carb, but I don't see why that would be since the problem appears to be on the other side of the fuel pump from the carb.

Want to do right by this truck, so anyone with experience or understanding of the mechanics of what's going on here, I'd sure appreciate some help. My knowledge only goes as far as what I've learned struggling with this truck and reading on the internet. Thanks!
 

PrairieDrifter

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Have you replaced the rubber fuel lines everywhere? It sounds like it can't hold its suction like there's a cracked line.
 

75gmck25

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The old rubber lines deteriorate inside (small pieces fall off) and also crack on the outside (develops holes that are hard to see).

It sounds like there is a piece of junk that drops into and plugs the line, but blowing air into the line dislodges it and lets the fuel flow.
 

squaredeal91

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I too think its Sucking air from bad lines. And sounds like something could be perpetually clogging inside tank. Happened to me with a 75 ford. Found some Teflon tape plugging it up. Would back blow and run for a bit and then Would start starving of fuel and die.
 
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75gmck25

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Is this a dual tank truck?

In ‘77 I think the main (or single) tank was still the passenger side tank (in later years they changed to the drivers side for the single tank). The switching valves are old, and you will find many posts here about trying to get them to work right.

Also, how many ports are there on your sending unit and fuel pump? GM started using a fuel return line to keep fuel cooler and prevent vapor lock, and its common for someone to make the wrong connections for a three port system (usually 3/8” fuel feed, 1/4” return line, and 5/16” vent line).
Or they might think the vent line is just “emissions junk” where it’s connected to the vapor canister, and disconnect or plug it without realizing the tank now cannot vent to let air in as fuel is pulled out.
 

dkraven

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Thanks for all the responses. I did find out later I should have replaced the passenger side tank, but I did the driver side. The tank is new so unless I messed up and blew some junk into the tank with the compressor it should be clean. I'm going to replace the flexible fuel lines and bypass the switching valve for now (I can always set that up again once I know the truck starts consistently). While I'm at it when I unhook the line from the tank I'll blow it all out pretty thoroughly in case there's something in the metal line. Also will check the metal line for pinches.
 

HotWheelsBurban

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Also, check the fuel filter (careful with the one in the carburetor inlet line, because it's VERY easy to strip those threads) (not one of GM's finer designs!). What Dad used to do with our squares, is install an inline filter (AC #GF62C) in the feed line under the truck. This has a removable metal canister and a replaceable paper filter element. This will catch pretty much all the possible contamination and keep you from having to mess with the little in carb filter.
That part number is what we used in the 70s and 80s, when we had our parts store and could get them through the warehouses. If it no longer is made, there should be something similar from other manufacturers.
 

PrairieDrifter

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Also, check the fuel filter (careful with the one in the carburetor inlet line, because it's VERY easy to strip those threads) (not one of GM's finer designs!). What Dad used to do with our squares, is install an inline filter (AC #GF62C) in the feed line under the truck. This has a removable metal canister and a replaceable paper filter element. This will catch pretty much all the possible contamination and keep you from having to mess with the little in carb filter.
That part number is what we used in the 70s and 80s, when we had our parts store and could get them through the warehouses. If it no longer is made, there should be something similar from other manufacturers.
i just use a standard metal wix replacement, 3/8 barbs on each end with a little bit of rubber line on each side. I believe it is a WIX 33033
 

HotWheelsBurban

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i just use a standard metal wix replacement, 3/8 barbs on each end with a little bit of rubber line on each side. I believe it is a WIX 33033
That works too! As long as you're running a carb system or something low pressure like TBI, it's not as big a deal what filter you use. Where it gets more complicated is on a high pressure system like my Vortecs. They like 55-60psi.
 

Ricko1966

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Just for fun,next time it acts up,pull the inlet hose off the pump,put a length of rubber hose on it dropped in a lawn mower gas can,see if the pump will prime itself and start your truck. If so we've eliminated everything from the pump forward.
 

dkraven

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Just for fun,next time it acts up,pull the inlet hose off the pump,put a length of rubber hose on it dropped in a lawn mower gas can,see if the pump will prime itself and start your truck. If so we've eliminated everything from the pump forward.
This is smart and what I should def do next to rule out anything from the pump on up.
 

Grit dog

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This is smart and what I should def do next to rule out anything from the pump on up.
Yup, start chasing it backwards from the pump.
And idk if the sending unit has a check valve but blowing air backwards through the lines, switching valve and pickup may not be the best idea.

However, presuming you know which tank it’s running on, disconnecting the main line at the pump and slightly pressurizing the tank (like 5psi tops) and collect what comes out in a bucket “may” produce something that is/was partially blocking the line.
 

Dryriver1

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A little more food for thought, I didn’t see what type of metered fuel leak you have on top of the motor but if it is a quadrajet the possibility of leaking fuel wells could be an issue. It has been covered on the site.

I agree with MHPLSX.

Also if you are going to place an exterior (out of the carb filter)

I would remove the filter in the carburetor itself if a Quadrajet.

If it is a Quadrajet you might want to do a carb kit on it.
Just in my humble opinion.

Best wishes to you and all of your loved ones always!

Dryriver1
 

Ricko1966

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Okay quadrajets fixed the leaky plug wells like 1969 you don't want to ditch the original fuel filter because it has a check valve that keeps the fuel from draining back. Have you tried running with a fuel line to alternate tank yet? I have a suspicion and this will confirm or eliminate it.
 

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