I recently bought a 1986 4.9L it runs well until it doesn't. I seems like its not getting fuel when it warms up especially on Hot days. It start to act like I am running out of gas, stalls and will not restart. After letting it cool down a couple hours it will start back up idle perfect. It is not a choke issue, which is working as it should. I replaced the mechanical fuel pump, it was leaking when I bought it. Rebuilt the carburetor. The emissions (CEL) light has never come on. It passes the KOEO and the only codes I get on the KOER test are 77 (I am pressing a full throttle) and 41 (HEGO Lean). After letting it sit and cool down it will start up idle perfectly drive great and then repeat the whole process over again. Anyone had experience with the fuel line/fuel pump vapor locking? It still has the original fuel line which hugs close to the engine all the way to the carburetor.

 

This thread may help, those experts are still around here and will chime in soon.

https://www.ford-trucks.com/forums/1...n-problem.html

 

Been there, done that, and got the T-shirt...

Welcome to the forum, by the way.

Over the years, I have had LOTS of heat-related problems affecting the fuel delivery on my '84 351W. Before I bore you to tears with verbose details, please be aware there are two scenarios commonly lumped together generically as "vapor lock". It's important to understand which fault is present (sometimes both) so you can take care of the problem. A fix for one scenario may not necessarily help the other, and visa versa.

1) Heat affecting fuel in the line between the tank and pump. The mechanical pump is way up front, trying to draw liquid fuel from way in back of the truck. On a hot day, mixed with heat from the exhaust, the line gets hot. This makes it easier for liquid fuel to turn into vapor under the vacuum from the pump. The pump will happily keep pumping vapor instead of liquid. Unfortunately, the engine needs liquid fuel to run, so you often get bucking for a half minute or so as the carb float chamber gets depleted, and then the engine quits.

2) Heat affecting fuel sitting in the carb. Fuel gets hot and kinda percolates, turning into a foamy mess that bubbles out into the intake manifold. This is more prevalent after shutdown, when underhood temperatures climb. It can also happen while driving, usually right after working the engine hard such as climbing a hill, and you've throttled back after reaching the top. While fuel flow was high, there wasn't much time for the fuel to absorb the ambient heat. But when you throttle back and fuel flow rate is reduced, there's now time to absorb that heat into the fuel. It's kind of like how a circus acrobat can quickly jump through a flaming hoop, but can't just saunter through.

For the first scenario, converting to an electric pump is one option. Here's how I did it. This method keeps the fuel pressurized in the feed line, with no vacuum present to turn it to useless vapor:

https://www.ford-trucks.com/forums/1...tallation.html


For the second scenario, you may want to see about adding an insulated space under the carb, to help reduce heat transfer. On V8 models, most have an exhaust crossover passage in the intake manifold that can cause trouble, but you won't have that on the straight 6.

Some more light reading:

https://www.ford-trucks.com/forums/1...e-vs-heat.html


For either scenario, you may want to try a different brand of fuel and see if that helps. Personally, I've noticed a BIG difference between brands, enough so that I will only run Chevron.

Strange as it may seem, lower octane is more resistant to heat, too:

https://www.ford-trucks.com/forums/1...g-and-you.html

 

I think you are making the assumption it is a fuel problem. It may indeed be a fuel problem, but your truck also used a TFI ignition module mounted on the distributor that was famous for heat related problems.

I would first get the engine to fail,, and then get out and pull the aircleaner off and see if fuel squirts down the carb when you press the throttle by hand. If it does and it does not start, it might be the TFI module. If you have no fuel squirting, then it indeed may b a vapor lock problem.

 

The 2 Dave's think the same as you need to check for spark / fuel.
I think easier than pulling the filter as it can be a PITA on a six, at least mine is.

Take a can of starting fluid with you and when it stops spray a little in the air in take and see if it will try and run.
No try to run no spark.
Try to run no gas.
If you find it is a no gas thing remove the fuel cap and see if it happens.
The vent may be plugged but the tank(s) should be vented thru the canister under the battery.
Dave ----

 

Thanks for the comments. It is not a spark issue as I am not getting any fuel into the Carburetor when it happens, I was assuming that the Vapor issue was happening after the fuel pump but it makes more sense that it would be happening prior to the fuel getting to the mechanical pump. Would it possibly help to put a heat sleeve on the fuel line from fuel tank splitter valve to the fuel pump? I will try the gas cap and then probably look at converting to a non-mechanical fuel pump. It is a project truck that I am working with my 13 year old son. It's a lot of fun (but at times frustrating) to figure out issues. I have really appreciated this group and have learned a lot. I purchased it from a widow, her husband was the original owner. She told me it didn't run very good. 103,000 original miles and only had 400 miles put on it since the last oil change in 2018.

 

Can you please elaborate how you determined this? As OOTD (One Of The Daves) mentioned, ignition and fuel problems often have similar symptoms, so it's important to make sure which is at fault.


That's a bingo! Fuel under pressure, such as between the pump and carb inlet, will typically behave itself in the presence of normal heat. Fuel at ambient pressure, such as in the carb float chamber, is prone to boiling/frothing with excess heat. Fuel under a vacuum, such as between the tank and pump, is the most prone to heat issues because the low pressure encourages the fuel to change from liquid to vapor.

Fuel injected vehicles have the entire fuel delivery system under pressure. "Vapor lock" is pretty much non-existent with fuel injection.

Won't hurt. Sometimes fixing this problem is a battle of minor increments.

 

Are you sure the fuel line doesn't have any blockage ?

Think about it, the truck ran fine the way it came from the factory, what has changed ?

Why would the fuel line be getting hot before the fuel pump, which is mounted on the engine ?

A piece of heat sleeve may help from the pump to the carb. Where did the fuel pump come from ?

 

If vapor lock is the issue it would be on the suction side of the fuel pump. You have two tanks. Are both tanks operating properly via the switch and valve? Do you have any filters on the suction side?

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I know fuel isn't getting to the carburetor because if I pour in a small shot of seafoam into the air intake is will fire and run for a brief moment.

 

The fuel pump came directly from the Ford dealership. I have replaced the fuel filter and checked it a couple times and there is not blockage. The only filter is the one that goes into the carburetor. I do not have an inline fuel filter between the tanks and the pump.
Why would the fuel line be getting hot? Million dollar question and the one I am befuddled by and trying to figure out. It doesn't completely make sense to me either.

 

Both tanks and the switch operate correctly It happens with either tank in the same way

 

Found this from "related threads" below,

https://www.ford-trucks.com/forums/1...problem-2.html

 

I'm curious about this too. I can see why the fuel may vaporize in the carb but between the tank(s) and the fuel pump? That seems odd but under suction, I guess it's a very real possibility. I've never experienced a vapor lock issue or an extended vapor lock issue that would cause the engine to stop running. So, this is interesting and educational.

 

Fuel chemistry.

https://www.ford-trucks.com/forums/1...g-and-you.html


Lots of ambient heat from the exhaust, especially if equipped with a cat. On a 90F day, any fuel in the tank is already at that temperature. Add a little heat from the pavement, too. It doesn't take much to push fuel temperature above the magic number where it turns to vapor under suction.