Vapor lock issue?

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David Eldridge

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Just wanted to see if anyone has had issue with vaporlock out there. I have a 1970 Dart Swinger with a 318 and a 4 barrel swapped on. Lateley I've had issues with fuel at idle after running on the highway. I can watch the fuel filter dry up at idle, then it really starts to sputter. Give it some gas and enough fuel gets pumped to fill up the filter eventually. Lines in the engine bay are well clear of heat sources. Only other thing I can think of even possibly causing a problem is the fuel line back by one of the tailpipes, but even that is at least an inch away. Any input greatly appreciated!
 
Yep exactly. But at least your's recovers. Mine empties and stays there. As soon as you throttle up a bit for a few seconds it starts to fill, so I know the pump is fine.
 
That, or
You know that lil jumper at the back, between the sender spigot and the hardline? Check it out. I have seen them installed with gear clamps, one on each side, and snugged up enough to crumple the rubber and then; since it is higher than the fuel level, it doesn't leak fuel, but the pump sucks air in there and eventually fills your bowl with air. If the jumper is also kinked, that just makes it worse.
Tip; when you install gear clamps on the suction side, you need to install 2 per side with the screws staggered 180*, and don't tighten thechit outta them.
The factory had the right idea with the spring type clamps.
 
AJ/FormS said:
That, or
You know that lil jumper at the back, between the sender spigot and the hardline? Check it out. I have seen them installed with gear clamps, one on each side, and snugged up enough to crumple the rubber and then; since it is higher than the fuel level, it doesn't leak fuel, but the pump sucks air in there and eventually fills your bowl with air. If the jumper is also kinked, that just makes it worse.
Tip; when you install gear clamps on the suction side, you need to install 2 per side with the screws staggered 180*, and don't tighten thechit outta them.
The factory had the right idea with the spring type clamps.
Click to expand...
Cool thanks. I know I didn't tighten anything crazy. And it only occurs on a hot engine
 
It's that time of year. Winter fuels have higher allowable RVPs.
Summer fuel should be coming in this month.

Also tilt the filter so the outlet is higher than inlet. That lets the vapor blow out through the carb bowls.
 
Mattax said:
It's that time of year. Winter fuels have higher allowable RVPs.
Summer fuel should be coming in this month.

Also tilt the filter so the outlet is higher than inlet. That lets the vapor blow out through the carb bowls.
Click to expand...
Ahh ok. My filter is pretty level. I'll give that a try
 
I put a pressure gauge on mine And the pressure stayed high as the filter emptied, then when it was almost empty the pressure dropped off from say 5 to 3 and it started filling. You can hear the engine slow down right when this is happening and then speed up as the filter refills.
 
David Eldridge said:
Ahh ok. My filter is pretty level. I'll give that a try
Click to expand...
That tip comes from the factory.
wink-gif.gif


Chrysler Master Tech Sevice Conf. 1963, Carburetion & Performance page 13
 
Air in the filter doesn't matter much, unless a checkvalve in the pump is bad, then the compressed air in there will unload after shut-down and blow the fuel backwards . No big deal if the bowl stays full.
But if the checkvalves are good, then the pressure will hold, and when fuel evaporates from the bowl, and the floatvalve opens, the trapped air will fly up the pipe into the bowl. Eventually, the trapped air will be gone.
Unless new air is coming into the pump......
and the most likely places for that to happen is at the rear jumper, cuz if it was at the front you would see the evidence wherever the fuel is leaking. Only the rear jumper is higher than the fuel level and does not leak fuel.
And remember; the fuel pump does not suck. It creates a low-pressure area in the system. And atmospheric pressure, in the tank, forces the fuel to move towards that low pressure area; in the very same way that you "suck" pop up a straw. Now; if the atmosphere cannot get into your tank, your pump is gonna stall. The fuel will boil in the low pressure area,becoming a lighter than gasoline gas, and it will rise up to the float bowl. Some of it may get trapped in your horizontal filter and "appear" to be air.
So make sure your tank is vented.
 
AJ/FormS said:
Air in the filter doesn't matter much, unless a checkvalve in the pump is bad, then the compressed air in there will unload after shut-down and blow the fuel backwards . No big deal if the bowl stays full.
But if the checkvalves are good, then the pressure will hold, and when fuel evaporates from the bowl, and the floatvalve opens, the trapped air will fly up the pipe into the bowl. Eventually, the trapped air will be gone.
Unless new air is coming into the pump......
and the most likely places for that to happen is at the rear jumper, cuz if it was at the front you would see the evidence wherever the fuel is leaking. Only the rear jumper is higher than the fuel level and does not leak fuel.
And remember; the fuel pump does not suck. It creates a low-pressure area in the system. And atmospheric pressure, in the tank, forces the fuel to move towards that low pressure area; in the very same way that you "suck" pop up a straw. Now; if the atmosphere cannot get into your tank, your pump is gonna stall. The fuel will boil in the low pressure area,becoming a lighter than gasoline gas, and it will rise up to the float bowl. Some of it may get trapped in your horizontal filter and "appear" to be air.
So make sure your tank is vented.
Click to expand...
Yep tank is all factory and vented. So maybe that's my problem is the air pocket is getting stuck and stopping flow. Ill give it a shot
 
It might be vapor lock, but real vapor lock is actually pretty rare. I would be looking for something clogging the fuel system first. Number one suspect would be rust and dirt in the tank.
 
any thoughts on the "fuel injection" style clamps?

(i will be using those on my jumper)

0eefc3ac513c48b4bc086c6e04bbc47a_1000x.jpg
 
How in the world can an air pocket stop flow when the pump is pushing the fuel; it cannot. Some fuel may flash to a vapor when it enters the filter, but it will move thru the filter and condense on the other side. The fact that the engine runs at all proves this.
The speeding up and slowing down of the engine, is probably a response to an unstable fuel level, proving that vapors are entering the bowl.
Whenever I get a vehicle with a fuel problem like this, the first thing I do is a fuel pump volume test. Next I check the pump for suction. and finally for pressure. and don't forget the mechanical aspect; the pump lever has to go up AND down , forced to do so by the spinning eccentric on the cam. If these are working as designed, the vapor in the filter will sort itself out.
 
diymirage said:
any thoughts on the "fuel injection" style clamps?

(i will be using those on my jumper)

View attachment 1715525846
Click to expand...
better than common gear clamps. I have had good success with those on new lines. As long as you don't bunch up the rubber.
Make sure your lines have a bubble on the end and that alone will help seal the joint. Then the clamp only has to keep the hose from slipping off.
 
AJ/FormS said:
How in the world can an air pocket stop flow when the pump is pushing the fuel; it cannot. Some fuel may flash to a vapor when it enters the filter, but it will move thru the filter and condense on the other side. The fact that the engine runs at all proves this.
The speeding up and slowing down of the engine, is probably a response to an unstable fuel level, proving that vapors are entering the bowl.
Whenever I get a vehicle with a fuel problem like this, the first thing I do is a fuel pump volume test. Next I check the pump for suction. and finally for pressure. and don't forget the mechanical aspect; the pump lever has to go up AND down , forced to do so by the spinning eccentric on the cam. If these are working as designed, the vapor in the filter will sort itself out.
Click to expand...
Sorry not air pocket, vapor. I think its just boiling all the way up the line to the carb.
 
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Ok so Ive insulated the lines. Gonna see what happens. Buddy of mine has a 68 340 Formula S that does the same thing even worse so I've insulated those too. His is the only car that has been tested with the insulation and so far it's helped some but fuel is still vaporizing in the lines up to the carb and causing sputtering. No where near as bad mind you. Any more ideas. I've heard the fuel pump getting hot can cause it too.
 
I did the Uncle Tony fix with the 3-nipple filter. Took the stock 318 hard fuel line, cut it in half, flipped it so the end that was connected to the carb instead connected to the fuel pump, and vice-versa, and with a little massaging and rubber fuel hose routed it all clear of the block, with the 3-nipple filter mounted to the fenderwell by the radiator overflow tank. Fuel filter (NAPA 3040 - for late 70s AMC) in line between carb and pump.

It seems to work for me. I'd drive the car for half an hour, and everything would be fine, I'd bring it in the drive way, it'd sputter and shut off. Since I installed the 3-nipple filter, it hasn't quit on me the handful of times I've driven it. What I'm not sure of exactly is what is actually the fix - giving the fuel vapor an escape route, routing the fuel line away from the block, or using large sections of rubber hose that doesn't absorb block heat like steel line does.

20200522_063533.jpg
 
72Dinger said:
I did the Uncle Tony fix with the 3-nipple filter. Took the stock 318 hard fuel line, cut it in half, flipped it so the end that was connected to the carb instead connected to the fuel pump, and vice-versa, and with a little massaging and rubber fuel hose routed it all clear of the block, with the 3-nipple filter mounted to the fenderwell by the radiator overflow tank. Fuel filter (NAPA 3040 - for late 70s AMC) in line between carb and pump.

It seems to work for me. I'd drive the car for half an hour, and everything would be fine, I'd bring it in the drive way, it'd sputter and shut off. Since I installed the 3-nipple filter, it hasn't quit on me the handful of times I've driven it. What I'm not sure of exactly is what is actually the fix - giving the fuel vapor an escape route, routing the fuel line away from the block, or using large sections of rubber hose that doesn't absorb block heat like steel line does.

View attachment 1715532925
Click to expand...

Never had that problem w/ elec fuel pumps , all my cars have had them , never had one fail either .
 
72Dinger said:
I did the Uncle Tony fix with the 3-nipple filter. Took the stock 318 hard fuel line, cut it in half, flipped it so the end that was connected to the carb instead connected to the fuel pump, and vice-versa, and with a little massaging and rubber fuel hose routed it all clear of the block, with the 3-nipple filter mounted to the fenderwell by the radiator overflow tank. Fuel filter (NAPA 3040 - for late 70s AMC) in line between carb and pump.

It seems to work for me. I'd drive the car for half an hour, and everything would be fine, I'd bring it in the drive way, it'd sputter and shut off. Since I installed the 3-nipple filter, it hasn't quit on me the handful of times I've driven it. What I'm not sure of exactly is what is actually the fix - giving the fuel vapor an escape route, routing the fuel line away from the block, or using large sections of rubber hose that doesn't absorb block heat like steel line does.

View attachment 1715532925
Click to expand...
It works because you are returning fuel to the tank which prevents fuel sitting in the pump and perculating. I gave up with my mechanical pump and pump gas and went to an electric return system on my strrrt/strip car. If I was only running a street car a mechanical pump with return line fixes the problem of running today's ethanol pump gas.
 
David Eldridge said:
Sorry not air pocket, vapor. I think its just boiling all the way up the line to the carb.
Click to expand...
You are correct. Its vapor and the vapor pressure can exceed the pumping pressure. Since its a flow situation, liquid fuel that contunes to be pumped into the local where vaporization occurs will continue to block the flow of liquid. This can happen on either suction or pressure side. (In fact you can measure the pressure in the feed line to determine where the problem lies.)
Hot Fuel Lines
One more form of vapor problems was/is pushing liquid fuel in front of it into the carb bowls. Chrysler dealt with this at several times including the use of fuel pumps that had a tiny return bleed built in.

The engine often can continue to run at low power because there is sufficient fuel in the bowl (and vapors going out the vent into the air cleaner). When more power is needed, it revs and dies until the throttle is closed enough for the engine catch.

Realize the fuel is not completely boiling. Unlike a simple fluid like water, there is a wide temperature range which vaporization occurs. Winter fuel is allowed higher Reid Vapor Pressure, which can be a real problem when the weather turns hot.
Comprehensive Test Regular vs Premium
 
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