Is this vapor locking, bad pump, or something else?

[kkgp81]

I picked this truck up the other day and trying to drive it home I experienced something odd. It drove flawlessly from one station to the next 30 miles at 70+ mph. Killed it, filled it up and started to roll again. It pretty much died right out of the station and the line/filter were completely empty. Well loaded it on the trailer and drove on. Had to reset it down the road on the trailer and it fired right up. Took off and about 20 minutes in she died again acting like it ran out of gas. Loaded it up and headed home.

At home it started off the trailer and no issues. Yesterday I put a clear filter in place of the metal one so I could see what was up. It is barely pumping fuel when it gets hot. I made a trip around my country block out here and you can see it’s barely if at all pushing fuel.

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After sitting for 15-20 minutes while I was crawling around under it I popped up and the filter is completely full- not running



It has a new fuel pump but I’m going to pick up a new fuel pump and swap it.

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[AuroraGirl]

I picked this truck up the other day and trying to drive it home I experienced something odd. It drove flawlessly from one station to the next 30 miles at 70+ mph. Killed it, filled it up and started to roll again. It pretty much died right out of the station and the line/filter were completely empty. Well loaded it on the trailer and drove on. Had to reset it down the road on the trailer and it fired right up. Took off and about 20 minutes in she died again acting like it ran out of gas. Loaded it up and headed home.

At home it started off the trailer and no issues. Yesterday I put a clear filter in place of the metal one so I could see what was up. It is barely pumping fuel when it gets hot. I made a trip around my country block out here and you can see it’s barely if at all pushing fuel.

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After sitting for 15-20 minutes while I was crawling around under it I popped up and the filter is completely full- not running



It has a new fuel pump but I’m going to pick up a new fuel pump and swap it.

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is there two filters in this fuel circuit?
The carb has a filter in the inlet.. check for it.

if so, the one there is not serving a purpose. But a picture of your current fuel pump would be helpful. And whats your gas cap like

 

[Ricko1966]

Drive it with the gas cap off,see if the problem goes away. Do a fuel pump pressure and volume test cold,and when you are having the problem,again with the gas cap off.

 

[Sad Sack]

Or if after it dies, spray some starting fluid or pour some gas down the carb to see if it starts right up..

 

[kkgp81]

Or if after it dies, spray some starting fluid or pour some gas down the carb to see if it starts right up..

It does start off starting fluid

Drive it with the gas cap off,see if the problem goes away. Do a fuel pump pressure and volume test cold,and when you are having the problem,again with the gas cap off.

What you see is with both caps off. Will have to figure out how to get a fuel pressure gauge on this setup.

is there two filters in this fuel circuit?
The carb has a filter in the inlet.. check for it.

if so, the one there is not serving a purpose. But a picture of your current fuel pump would be helpful. And whats your gas cap like

There might be one on the carb, I am not familiar with them but I am not sure a second in line after the first one that is not getting fuel would be an issue. never know though.


Going to pull the pump and give it a good look as soon as I find a replacement and get it in hand.

 

[AuroraGirl]

It does start off starting fluid


What you see is with both caps off. Will have to figure out how to get a fuel pressure gauge on this setup.


There might be one on the carb, I am not familiar with them but I am not sure a second in line after the first one that is not getting fuel would be an issue. never know though.


Going to pull the pump and give it a good look as soon as I find a replacement and get it in hand.

the fuel pump is drawing a vacuum on the fuel line from the tank, and its pushing the fuel through the filters to the carb. if the one in the carb is there ,its possibily contributing to it because a harder time to push through would mean resistance and i feel maybe that could lead to increased vacuum being applied, even minorly would be a lot of difference. But thahts why I asked WHAT your gas cap is. since you wont provide a photo of the pump or the gas cap im asking for a description., is there a vent in the center of it , or is it sealed (meaning you have a vacuum canister most likely..

Also love guns and roses

 

[xm20k]

First thing I would do is pull the filter in the inlet on the carburetor. Circled here in red, probably plugged up. Put it back together without it and see what it does. you'd be surprised how many people don't know there is one in there, so it never gets changed.

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[kkgp81]

Had not been back around it yet-

Non canister and doesn’t look like it ever had one.

Gas caps were off in the video but here’s what they look like

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And bottom of the pump. Looks like it has 3 connections.

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Canister location- well where one would be if it had one. Great place to store the leaky fluid bottles.

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Truck is a 78 and I’m pretty sure they swapped to doors in 79 so probably not the original bed/gas cap setup.

 

[kkgp81]

First thing I would do is pull the filter in the inlet on the carburetor. Circled here in red, probably plugged up. Put it back together without it and see what it does. you'd be surprised how many people don't know there is one in there, so it never gets changed.

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Honestly, I wouldn’t be surprised because I usually pull those old carbs off and box them. I’ve never bothered to take one apart.

 

[AuroraGirl]

Had not been back around it yet-

Non canister and doesn’t look like it ever had one.

Gas caps were off in the video but here’s what they look like

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And bottom of the pump. Looks like it has 3 connections.

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Canister location- well where one would be if it had one. Great place to store the leaky fluid bottles.

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Truck is a 78 and I’m pretty sure they swapped to doors in 79 so probably not the original bed/gas cap setup.

thats a 78? you have sealed gas caps(DO NOT USE TILL WE DETERMINE MORE)

if your gas tanks are 1978 original they may not have return lines and there may have been the intent for you to have a vent line filter on the tank and then a unsealed gas cap with a vent'

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Ignore the ghetto nature of this ... this si what your cap should have looked like assuming your bed came with doors, but I think your original bed would have been flush mounted with a body colored gas cap and the vent aspect of it would be not to look like mine here. but your truck had a different (83 ish plus bed?)

 

[AuroraGirl]

Pretty sure your pump looks fine. its got a return line now you need to confirm the return lines run to the tanks in some fashion, so if your switching valve works, is yours a solenoid (which would require 2 of them to handle this) or motorized?
also the kinked metal hardline to the carb, thats going to restrict flow. how bad are those flat parts?

 

[xm20k]

The correct type of gas cap and the pump would not have a return, sending units would be pickup and vent only.

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[kkgp81]

The correct type of gas cap and the pump would not have a return, sending units would be pickup and vent only.

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I feel you a little there except for it has the factory lines front to back with the third line. Nobody in their right mind even out of boredom would run a third line. My 78, 77, 79 small block trucks are setup as you have shown though.

a couple of sites show that the 3-outlet pump is the correct one.

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Going to work on getting the bed off to see what we have back there.

 

[kkgp81]

So bed is off and it’s a bit of wtf.

There is a pull from each tank, then what I think should be the return is ran from tank to tank. The vent is done the same way.

Man I love these old trucks and the WTF stuff that is done to them.

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Pretty sure the culprit is in this photo. The vent and returns are going from tank to tank and nothing connects them to the return etc.

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[texdave]

For an additional data point: my 76 3/4 ton Suburban has a vented gas cap, a factory return line (& corresponding 3 line pump), and no evap canister. I believe most square body trucks and wagons above a certain weight (so most 3/4 and 1 ton) didn't have to have the evap system in place in the mid-70s but I could be wrong.

I would check:
- that the vent line is venting -- too much pressure can cause issues (no kinks, hooked up correctly)
- that the return lines look functional -- w/o a working return, vapor lock is more likely between the pump and the carb (hooked up correctly, soft lines aren't shot)
- that you're using the right fuel cap -- rockauto only shows vented caps for a 78 k30 for reference
- fuel volume when cold and again when it's hot/having issues -- you can disconnect the fuel line going into the carb and put it into a container then crank for 30s and measure
- the in-line fuel filter in the carb, making sure it's not clogged