Fuel pump, vapor lock? My truck died on the hi-way.

[AirSpeed]

Yesterday after a long three days of new car shopping I was headed home, about an 80 mile trip and my 72 GMC started cutting out and finally died on the side of the road. It's a 72 4X4 with 350/350. It was a hot day, about 106, truck was running cool but suddenly would run out of fuel and die. The fuel filter was empty, no gas. After a while I tried to start it and it fired right up, ran fine. I had a tow truck on the way so I decided to take the tow home just in case it happened again. When we took the truck of the tow truck I could smell gas really bad in the cab, I occasionally got a wiff of gas in the past but this was like someone dumped a cup of gas in there. I let it air out and the truck started right up, there was no gas on the bed of the tow truck or anywhere in the cab that I could find , nothing dripping under the truck, I'm at a loss. Any ideas what and where I should start looking?

[ho70]

Originally Posted by AirSpeed

Yesterday after a long three days of new car shopping I was headed home, about an 80 mile trip and my 72 GMC started cutting out and finally died on the side of the road. It's a 72 4X4 with 350/350. It was a hot day, about 106, truck was running cool but suddenly would run out of fuel and die. The fuel filter was empty, no gas. After a while I tried to start it and it fired right up, ran fine. I had a tow truck on the way so I decided to take the tow home just in case it happened again. When we took the truck of the tow truck I could smell gas really bad in the cab, I occasionally got a wiff of gas in the past but this was like someone dumped a cup of gas in there. I let it air out and the truck started right up, there was no gas on the bed of the tow truck or anywhere in the cab that I could find , nothing dripping under the truck, I'm at a loss. Any ideas what and where I should start looking?

My truck did in way to work yesterday as well, turned out the in carb filter was clogged solid. Some posts on here have mentioned big block fuel pumps shut down when hot. I would also clean the fuel gauge pick up sock in the tank

[geezer#99]

Do you have a vented gas cap?

[PowerdbyChevy79]

To me ..it sounds like you may have some sort of vapor lock going on...your truck has (should have) the vent line for the gas tank that runs up the passenger side of the cab..I would start there

[AirSpeed]

Originally Posted by ho70

My truck did in way to work yesterday as well, turned out the in carb filter was clogged solid. Some posts on here have mentioned big block fuel pumps shut down when hot. I would also clean the fuel gauge pick up sock in the tank

Thanks, I actually pulled the tank out when I painted my truck, cleaned the pickup tube screen too. The fuel pump is only a couple years old and seems to be working fine. I took the filter out of the carb years ago and replaced it with an inline filter which is clean. Seems odd that I had such a strong smell of gas in the cab but nothing's leaking. I'm really hoping I find the problem, it's running fine now but I don't want to get stuck on the side of the road again. Thanks again.

[davischevy]

Check the fuel line from the pump to the carb. If its touching anything, even slightly it can cause a vapor lock in hot weather, especially with ethanol gas.

[pwdcougar]

What kind of carb to intake gasket are you using? Had a problem with heat transfer from the intake to the carb and it caused vapor lock. A thick insulating spacer type carb gasket solved the problem.

Paul

[AirSpeed]

Originally Posted by pwdcougar

What kind of carb to intake gasket are you using? Had a problem with heat transfer from the intake to the carb and it caused vapor lock. A thick insulating spacer type carb gasket solved the problem.

Paul

It's a quadrajet, thick gasket underneath. The fuel filter was bone dry, no fuel coming through at all. Can a vapor locked carb stop fuel from flowing from the pump to the filter and carb? I have a transparent filter and after awhile could see tiny amounts of gas getting "sucked" through it but wouldn't fill up. I just got back from driving it and it's running fine. The gas smell in the cab is also gone.

[LockDoc]

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Did you check the rubber fuel line under the cab? It may be cracked or have a hole in it. There is also a vent line in the cab that loops up on the passengers side by the rear window. That may be where the gas smell is coming from.

LockDoc

[leftybass209]

Yeah, you shouldn't be able to smell any fuel in the cab. I swapped my 71 tank for the 69 tank to get rid of that vented hose as it's one more thing to deteriorate. That is a good place to start.

[AirSpeed]

Originally Posted by LockDoc

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Did you check the rubber fuel line under the cab? It may be cracked or have a hole in it. There is also a vent line in the cab that loops up on the passengers side by the rear window. That may be where the gas smell is coming from.

LockDoc

They looked in good shape when I put the tank back in but that was five years ago so I imagine they could be bad now. Do you see any reason that would prevent fuel from getting to the carb but not leak any noticeable fuel anywhere, other than the smell? Could the venting system allow air into the gas line but not allow fuel to leak out somehow? I've looked at a schematic of the vent and fuel line routing and can't picture how that could happen but I'm exhausted and might not be thinking straight! Thanks for the suggestion!

[AirSpeed]

Originally Posted by leftybass209

Yeah, you shouldn't be able to smell any fuel in the cab. I swapped my 71 tank for the 69 tank to get rid of that vented hose as it's one more thing to deteriorate. That is a good place to start.

Does it vent out the cap or?

[LockDoc]

Originally Posted by AirSpeed

They looked in good shape when I put the tank back in but that was five years ago so I imagine they could be bad now. Do you see any reason that would prevent fuel from getting to the carb but not leak any noticeable fuel anywhere, other than the smell? Could the venting system allow air into the gas line but not allow fuel to leak out somehow? I've looked at a schematic of the vent and fuel line routing and can't picture how that could happen but I'm exhausted and might not be thinking straight! Thanks for the suggestion!

A hole or crack in the fuel line under the floor could suck air and prevent the fuel from reaching the pump, and a crack in the rubber line on the vent tube could leak fumes into the cab but neither one would affect the other....

LockDoc

[AirSpeed]

Originally Posted by LockDoc

A hole or crack in the fuel line under the floor could suck air and prevent the fuel from reaching the pump, and a crack in the rubber line on the vent tube could leak fumes into the cab but neither one would affect the other....

LockDoc

I'll check them out in the morning, probably replace them anyway as I'm sure they're the originals. Now I have to drain 20 gallons of gas out of my tank! Hope I have enough gas cans.

[john]

Originally Posted by AirSpeed

I'll check them out in the morning, probably replace them anyway as I'm sure they're the originals. Now I have to drain 20 gallons of gas out of my tank! Hope I have enough gas cans.

Just unscrew the line from the tank sender and break the suction.

[slikside]

Originally Posted by LockDoc

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Did you check the rubber fuel line under the cab? It may be cracked or have a hole in it. There is also a vent line in the cab that loops up on the passengers side by the rear window. That may be where the gas smell is coming from.

LockDoc

I had some strong vapor smells in my cab once in a while. It was the original rubber hoses in cab for the vent as LockDoc says. Since I've replaced them, I haven't smelled a thing.

Can't see that causing fuel starvation though. Fuel pump going intermittent??

[AirSpeed]

Originally Posted by slikside

I had some strong vapor smells in my cab once in a while. It was the original rubber hoses in cab for the vent as LockDoc says. Since I've replaced them, I haven't smelled a thing.

Can't see that causing fuel starvation though. Fuel pump going intermittent??

I'm going to check the lines tomorrow. I don't know if a mechanical pump could work intermittainly, is that possible? I replaced it two years ago, I guess it could be getting worn but it seems like it would either work or not.
Thanks!

[AirSpeed]

I'm going to replace my rubber fuel lines in the cab and under my truck today, I'm seeing a lot of black particles in my filter, I'm assuming it's deteriorated rubber breaking away inside the gas line, I can't think of anything else that would be black in there, anyone know what size line was used on these trucks in 1972? I drove my truck about twenty miles yesterday, ran perfect till I got two miles from home then it died again, luckily it was downhill all the way to my driveway.

[BB72CHEVKT]

My guess is fuel pump.

[hotrod68c10]

the tank can pressure up due to heat and if your vent is clogged it wont depresurize until the pressure gets high enough then its probably blowing the pressure out of your vent line into your cab then the truck starts because the pressures gone

[midniteblues]

it sure does sound like the rubber fuel hose collapsing under suction.
the ethenol in the gas nowadays destroys the rubber quickly from the inside out.

also use a flashlight and check inside your tank for crud or the source of your black particals

[AirSpeed]

Well I pulled my tank, dumped it out, found the screen on the pickup tube had deteriorated, the lines were full of little pieces of it. I replaced all the gas line with new, added a big filter inline under the truck where the gas line comes through the floor, changed the filter and line on the carb and the fuel pump. I didn't see any real issues with the gas line other than it being old and starting to crack but it's all new now. After all that I fired the truck up, it ran for about 40 seconds and died. No gas getting to the carb filter. All I can do now is replace the fuel pump, of course my local NAPA didn't have one in stock so I won't know if it's the ultimate problem or not until tomorrow when it arrives! Any ideas what I could have missed? I'm hoping it's the fuel pump and if so I hope the new one last more than 10 months!

[LockDoc]

Originally Posted by AirSpeed

Well I pulled my tank, dumped it out, found the screen on the pickup tube had deteriorated, the lines were full of little pieces of it. I replaced all the gas line with new, added a big filter inline under the truck where the gas line comes through the floor, changed the filter and line on the carb and the fuel pump. I didn't see any real issues with the gas line other than it being old and starting to crack but it's all new now. After all that I fired the truck up, it ran for about 40 seconds and died. No gas getting to the carb filter. All I can do now is replace the fuel pump, of course my local NAPA didn't have one in stock so I won't know if it's the ultimate problem or not until tomorrow when it arrives! Any ideas what I could have missed? I'm hoping it's the fuel pump and if so I hope the new one last more than 10 months!

Did you blow all of the lines out with air pressure while you had it apart? Mine had a lot of solid gunk in the steel lines. I used an old speedometer cable (the inner part) and rodded my lines out then used the air hose and blew them out good. I got a lot of junk out of them....

LockDoc

[AirSpeed]

Originally Posted by LockDoc

Did you blow all of the lines out with air pressure while you had it apart? Mine had a lot of solid gunk in the steel lines. I used an old speedometer cable (the inner part) and rodded my lines out then used the air hose and blew them out good. I got a lot of junk out of them....

LockDoc

Yep, then I injected a half gallon of gas through the line starting at the tank to the pump, I disconnected the pump end and let it drain into a bucket, there was still some crap in the line but it finally cleared up. Tip to anyone doing this, don't try a funnel! I ended up using a big syringe stuck in the line, wish I'd thought of the speedo cable, I just happen to have one! I'll keep that in mind next time I need to clean a tube like that, great tip! I don't know if it's possible but I'm thinking that some of the rotten screen on the pickup tube could have plugged up the valve in the pump, that or the pump just wore out fast. I'll have a new one tomorrow. Either way it feels good knowing all the gas lines are new and safe! Now I have to remember I installed a second filter under the truck. Thanks!

[LockDoc]

Originally Posted by AirSpeed

Yep, then I injected a half gallon of gas through the line starting at the tank to the pump, I disconnected the pump end and let it drain into a bucket, there was still some crap in the line but it finally cleared up. Tip to anyone doing this, don't try a funnel! I ended up using a big syringe stuck in the line, wish I'd thought of the speedo cable, I just happen to have one! I'll keep that in mind next time I need to clean a tube like that, great tip! I don't know if it's possible but I'm thinking that some of the rotten screen on the pickup tube could have plugged up the valve in the pump, that or the pump just wore out fast. I'll have a new one tomorrow. Either way it feels good knowing all the gas lines are new and safe! Now I have to remember I installed a second filter under the truck. Thanks!

No problem. Keep us informed on the outcome for future reference.

LockDoc