air bubbles in fuel line after engine shut down

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destin

destin

10 Year 15 Year
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Is it normal to see air bubbles heading back to the tank from the fuel pump in the lines after you shut the engine down? I would imagine it would stay primed and steady. Does this mean a slight air leak in my lines?


you can see air bubbles headed back to the tank on the clear line south of the filter.

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Bump

I think this is what has been causing some surging on restarts ( heat soaked ) and on cold starts.
Possibly the vehicle sorting out the extra air and mixture of aerated and non aerated fuel , along with large pockets of air ?

Seems to go away after a few blocks under load.

I guess I ind out tomorrow when I find the air leak ( I think it is the old glass filter before the pump )

If anyone has thoughts please chime in :)
 
I say it's not normal to see this only because most fuel lines are not transparent, at least that I've seen.
After you shut the engine down there is a lot of heat in the block especially above the head / exhaust manifold.
That heat vaporizes the liquid gasoline in the fuel line to the carb and it also expands - has nowhere to go because there is a shut off in the carb float assy.
I think some gets thru the carb needle valve and that is what causes vapor lock.
What doesn't get into the carb has to go somewhere and back to the tank is the easy route.
Is it hard to re-start that engine soon after you shut it down on a hot day compared to when the engine is not hot?
Make any sense?
 
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yeah can be hard to restart on a hot day if it sits for a bit after running say an hour, sits for 10 minutes.
BUt that being said the carb is not so hot to the touch nor is the lines ( the base is a little more hot. )

And the temperature is like 80 degrees out, not in the 100s or anything. That is why i was wondering if a large amount of air was working in the system on shut down, and the system was trying to overcome that on restart, be it hot or cold... Of course I could be totally off my hat lol :D

I pop on a inline filter later today after a few work meetings, and see if it alleviates the problem.
I will def post back.

But yeah the heat soaked restart does have the "boil off symptoms" fluctuating idle rpms / vacuum, like 550- 800 rpms, about 2 second cycles or so. I am hoping this is just being caused by the extra air in the system creating a mixture or pockets of aerated air, making a lean rich senario until it burns off...

lsit of new stuff btw

new tank
new sending unit
new gas cap
new lines
rebuilt carb ( sorted out the accelerator pump issue, jet pumps a nice stream now )
new points ( gapped at .0016, dwell/ cam angle at 43 degrees )
timing at 7 degrees before top dead center ( stock on the needle )

I post back later!

Any insights welcome :D

afterthought, would not the mechanical fuel pump create a a prime that would prevent and fuel back flow?
pump is new too, only 10 thousand miles on it.
 
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The fuel pump is a diaphram pump, not a positive displacement pump that would prevent flow back.
It creates pressure when it's running on the discharge side - if there was a fitting that leaked, you would see it.
There's no "vacuum" anywhere that would suck in air.
I wrapped the fuel line from the fuel pump to the carb inlet with good insulation (like the stuff fro ac lines) to prevent vapor lock and it helps a lot.
 
Kinda sounds like your gas caps not venting... You could be creating a vacuum in the tank and it's sucking gas from the lines and air from the carb bowl

Just thinking out loud destin

Try removing the gas cap right when you shut down and see if it still does it... That type of thing ... You know where I'm coming from?
 
desertpat said:
I wrapped the fuel line from the fuel pump to the carb inlet with good insulation (like the stuff fro ac lines) to prevent vapor lock and it helps a lot.
Click to expand...



Vapor lock occurs on the low pressure suction side of the fuel line. The suction cause the fuel to vaporize at lower temperatures. The pressure in the line prevents vaporization at high temperatures. Most modern cars with the fuel pump below the level of the fuel tank do not have problems with vapor lock. EFI cars never have this problem because the pump is typically at the tank and the whole line is pressurized.
 
I agree about the suction side and location of the fuel pump effect on vapor lock.
Best is when the pump is located in the gas tank.

But, the extreme high temps above the exhaust manifold / head do vaporize gas in the line despite fuel pump discharge pressure.

My insulation fix is very effective and that's what convinced me.

As was said - the bubbles were flowing back to the tank from the fuel pump - therefore, the issue must be originating from reactions between the pump and the carb, causing the flowback.
 
Pin_Head said:
Vapor lock occurs on the low pressure suction side of the fuel line. The suction cause the fuel to vaporize at lower temperatures. The pressure in the line prevents vaporization at high temperatures. Most modern cars with the fuel pump below the level of the fuel tank do not have problems with vapor lock. EFI cars never have this problem because the pump is typically at the tank and the whole line is pressurized.
Click to expand...

good info man :)

would it be ideal to keep the fuel lines low ( frame level ) from the gas tank to the fuel pump?

I placed an inline filter as a replacement to the old style filter, and I am not seeing the crazy air bubbles any longer. I might have had a small leak that allowed air in, but not allowing fuel to weep out drastically.

I will give the vehicle a hard hour drive and try to replicate the fuel boil symptoms.
If it is just a fact of life then that is fine, but i want to be sure i am not ignoring a symptom of a larger problem :)
 
yeah still happens

heat soaked, sits for a while, then the idle surges until i load the engine and it seem to sputter through whatever is cause it to mess up. Is it jsut burning off the bioled gas / inner pressure?

Is it possible my old worn-out carb is just dumping gas into the intake on shut down, causing fuel gas to build up in the intake? would this cause the problem? seems like the carb it self is relatively cool, and it is only 73 degrees out today. the intake of course is hot.

Maybe sticky pvc valve hose? ( i checked this last year and the ball bearing moved when i shook it )

and input input would be appreciated, i want to solve this problem.

would a small window single barel carb with the solenoid idle shut off prevent this?
The truck diesels on shut off intermittently when hot as well.
 
JohnnyC said:
Kinda sounds like your gas caps not venting... You could be creating a vacuum in the tank and it's sucking gas from the lines and air from the carb bowl

Just thinking out loud destin

Try removing the gas cap right when you shut down and see if it still does it... That type of thing ... You know where I'm coming from?
Click to expand...

Might be :)

It is a new OEM cap, looks vented , by it never hurts to try.

But I am thinking it might be gas pooling in the intake on shut down which vaporizes ? I could be crazy too :-)
 
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