Fuel issues
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Fuel issues
Hi all! I have a 1976 F100 that i installed a 351W into about 3 years ago. The truck and engine has always performed great until recently. The truck will run fine for 30 mins and then quit. The fuel filter between the fuel pump and carb will be dry as well as the fuel bowl on the carb. I replaced the pump because i thought the pump was getting weak and after several bad pumps i installed an electric fuel pump mainly for diagnosis. I will get fuel for around 30 minutes but then the filter and fuel bowl goes dry. I have replaced the sending unit, any rubber lines from tank to carb, and have blown out the steel hard line from tank to carb. Can anyone offer any input? Im sure i am missing something simple but im tired of banging my head on my nrw lawn ornament? Thanks in advance for any help!
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Thanks for the input. I thought that either the tank was pressurizing or pulling a vacuum so i have been running it without a fuel cap. This issue is mind boggling. I have noticed it seems to occur more when the outdoor temp is 70 or above.
Postmaster
Joined: Mar 2011
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From: Taxachusetts
Welcome to FTE. Lots of good people here as you will find out.
Any smell of fuel in the engine bay, lines or tank when she runs out?
I'm sure you rechecked all your feed lines for tightness....or air leaks....
Is there a fuel filter on the carb fuel inlet?.......(Don't know what carb you have).
Have you checked the floats for correct set up and the needle valve for obstructions?
Have you checked the oil (dipstick) for any smell of gas?
When you say the vehicle "Runs fine for 30 mins and then quit," are you in traffic or just idling?
Any smell of fuel in the engine bay, lines or tank when she runs out?
I'm sure you rechecked all your feed lines for tightness....or air leaks....
Is there a fuel filter on the carb fuel inlet?.......(Don't know what carb you have).
Have you checked the floats for correct set up and the needle valve for obstructions?
Have you checked the oil (dipstick) for any smell of gas?
When you say the vehicle "Runs fine for 30 mins and then quit," are you in traffic or just idling?
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Thanks again. I didnt think about the possibility of the filler hose possibly being pinched and allow pressure of vacuum to occur. Ill give it a try tomorrow with the filler hose diconnected and see if it makes any difference.
Cargo Master
Joined: Nov 2011
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From: Great American SouthWest
Perhaps it's vapor-locking?
How is the fuel line run to the carb-is it near the block or headers? Is heat soak causing the carb to boil the gas? Are you in a state running a winter blend?
You mention the outside temp being about 70, but is the truck parked when the problem occurs, or are you sitting in traffic, or running down the highway?
I'm one of the guys that had a new non-venting gas cap, and every so often, the truck would just die, and the pump, filters and bowls would be dry.
If it's consistently a half-hour, without a cap, after trying several fuel pumps, and verifying the lines and filters...I'm thinking heat-related
How is the fuel line run to the carb-is it near the block or headers? Is heat soak causing the carb to boil the gas? Are you in a state running a winter blend?
You mention the outside temp being about 70, but is the truck parked when the problem occurs, or are you sitting in traffic, or running down the highway?
I'm one of the guys that had a new non-venting gas cap, and every so often, the truck would just die, and the pump, filters and bowls would be dry.
If it's consistently a half-hour, without a cap, after trying several fuel pumps, and verifying the lines and filters...I'm thinking heat-related
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I have checked for tightness of all connections about 100 times. This thing is causing OCD. I was running a Holley 4180 (i believe thats the correct model) but i recently switched to the Summit brand equivalent. I have the issue with both carbs.
It first quit on me with the no fuel issue while doing 60 mph down a busy 4 lane. I had the truck towed home and realized i wasnt getting fuel. I replaced the fuel pump and all was fine for one week. The fuel pump was leaking externally through a weephole on the pump and was spraying gas all over the engine and exhaust manifold. It was 72 degrees that day after having been in the 50s for the last week. I returned the pump and replaced it with a new one and now it doesnt matter if you are driving it or if its parked in the driveway, 30 minutes is all she will run on 65 and up days and 45 minutes if it is in the 50s
It first quit on me with the no fuel issue while doing 60 mph down a busy 4 lane. I had the truck towed home and realized i wasnt getting fuel. I replaced the fuel pump and all was fine for one week. The fuel pump was leaking externally through a weephole on the pump and was spraying gas all over the engine and exhaust manifold. It was 72 degrees that day after having been in the 50s for the last week. I returned the pump and replaced it with a new one and now it doesnt matter if you are driving it or if its parked in the driveway, 30 minutes is all she will run on 65 and up days and 45 minutes if it is in the 50s
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Thread Starter
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New User
Joined: Mar 2017
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Perhaps it's vapor-locking?
How is the fuel line run to the carb-is it near the block or headers? Is heat soak causing the carb to boil the gas? Are you in a state running a winter blend?
You mention the outside temp being about 70, but is the truck parked when the problem occurs, or are you sitting in traffic, or running down the highway?
I'm one of the guys that had a new non-venting gas cap, and every so often, the truck would just die, and the pump, filters and bowls would be dry.
If it's consistently a half-hour, without a cap, after trying several fuel pumps, and verifying the lines and filters...I'm thinking heat-related
How is the fuel line run to the carb-is it near the block or headers? Is heat soak causing the carb to boil the gas? Are you in a state running a winter blend?
You mention the outside temp being about 70, but is the truck parked when the problem occurs, or are you sitting in traffic, or running down the highway?
I'm one of the guys that had a new non-venting gas cap, and every so often, the truck would just die, and the pump, filters and bowls would be dry.
If it's consistently a half-hour, without a cap, after trying several fuel pumps, and verifying the lines and filters...I'm thinking heat-related
I had wondered about a heat related issue. The only place the fuel line is close to any source of heat is the short rubber line from the main steel line to the fuel pump which is by the front cross member. I have ran this configuration since installing the motor 3 years ago without any issues. We are still currently using the winter blend fuel here.
Cargo Master
Joined: Nov 2011
Posts: 3,354
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From: Great American SouthWest
Could be the filler hose or a collapsing fuel hose. Depending on the tank setup, there can be several hoses. On my 79, the hard line for he front tank wasn't leaking, but it wouldn't draw a consistent vacuum, so I had to replace it.
If the lines up front are getting too hot, they could be boiling fuel also. If they're close to the block or headers, you might try heat-shielding or re-routing
I presume you have factory fan...is the shroud in place?
If the lines up front are getting too hot, they could be boiling fuel also. If they're close to the block or headers, you might try heat-shielding or re-routing
I presume you have factory fan...is the shroud in place?
Postmaster
Joined: Nov 2011
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I had a similar problem or at least symptoms and it was caused by old rubber fuel hoses on the suction side collapsing... getting sucked shut basically. You have already replaced those however. It sure would not hurt to make sure that every single one has been replaced.
That said, perhaps fuel quality has changed? More alcohol and higher vapor pressure causing vapor lock where it was OK before?
On my truck, fixing the fuel lines got it working right most of the time except in hot weather/harder running it would vapor lock. The robust fix for me was adding an electric pump on the rear frame rail. A low and cool location for the pump.
That said, perhaps fuel quality has changed? More alcohol and higher vapor pressure causing vapor lock where it was OK before?
On my truck, fixing the fuel lines got it working right most of the time except in hot weather/harder running it would vapor lock. The robust fix for me was adding an electric pump on the rear frame rail. A low and cool location for the pump.
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New User
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Could be the filler hose or a collapsing fuel hose. Depending on the tank setup, there can be several hoses. On my 79, the hard line for he front tank wasn't leaking, but it wouldn't draw a consistent vacuum, so I had to replace it.
If the lines up front are getting too hot, they could be boiling fuel also. If they're close to the block or headers, you might try heat-shielding or re-routing
I presume you have factory fan...is the shroud in place?
If the lines up front are getting too hot, they could be boiling fuel also. If they're close to the block or headers, you might try heat-shielding or re-routing
I presume you have factory fan...is the shroud in place?
All fuel lines are in the factory locations and i swapped from a 302 to the 351W HO so i even reused my exhaust manifolds. I have the fan shroud in place as well as an electric fan installed in front of the radiator pulling air in. The truck runs a consistent 180° except on those 100° Georgia days when sitting in traffic. It would heat up to 195° and since the addition of the electric fan it runs 182-185° sitting in traffic when temps are 100+
Thread Starter
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New User
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I had a similar problem or at least symptoms and it was caused by old rubber fuel hoses on the suction side collapsing... getting sucked shut basically. You have already replaced those however. It sure would not hurt to make sure that every single one has been replaced.
That said, perhaps fuel quality has changed? More alcohol and higher vapor pressure causing vapor lock where it was OK before?
On my truck, fixing the fuel lines got it working right most of the time except in hot weather/harder running it would vapor lock. The robust fix for me was adding an electric pump on the rear frame rail. A low and cool location for the pump.
That said, perhaps fuel quality has changed? More alcohol and higher vapor pressure causing vapor lock where it was OK before?
On my truck, fixing the fuel lines got it working right most of the time except in hot weather/harder running it would vapor lock. The robust fix for me was adding an electric pump on the rear frame rail. A low and cool location for the pump.
I only have 4 rubber lines. The first is coming from the sending unit to the hard steel line which is approximately 8 inches. The second is a piece around 4 inches long between the two main steel lines. It looks like maybe a filter would have gone here because bothe ends of the steel line are barbed and not cut. The third line is from the main steel line (barbed) to fuel pump inlet (barbed). The last one goes from the steel fuel line from pump (cut) to the filter. I have installed an electric fuel pump trying to diagnose this issue but it hasnt made any difference in the delivery time of fuel.
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Quick update... I dropped the fuel tank just to double check all connections. In the process I disconnected the filler hose and the over fill hose from the tank and tried to run the truck. I still get the same 30-45 minute run time. I am starting to believe i either have a hard line that has an air leak losing the prime or a heat issue. How can i tell if it is an issue with the line losing flow without changing the line? If the box store auto parts store nearby had the replaements in stock without having to order and wait on them I would be able to try a quick replace and diagnose method. What is the temp at which fuel will percolate or boil? I have a thermometer and want to crawl the truck check line temps to maybe see if it is a heat issue.
Thanks, Joe
Thanks, Joe
Cargo Master
Joined: Nov 2011
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From: Great American SouthWest
In my case, with the hard line from the front tank to the selector valve leaking, I just replaced it with high-quality (fuel-injection rated) hose. It's true that the book says not to replace sections longer than 12", but that seemed kind of arbitrary to me. The new hose works for me.
Fuel injection hose not required for testing, but I wanted the better hose for the long-term
Fuel injection hose not required for testing, but I wanted the better hose for the long-term