Air bubble in fuel filter

ZGjethro · Post by **ZGjethro** » Fri Apr 30, 2010 3:18 pm

I installed a Holley fuel pressure regulator on the bracket where the original fuel filter was installed. I have a clear glass fuel filter mounted in a vertical fashion below it. The truck runs well, but there is often a large air bubble visible in this fuel filter. The top 20% could be air. I would expect an air bubble to float up and out of the filter, but this is not the case. Does anyone have any info on such a thing?

Joe · Post by **Joe** » Fri Apr 30, 2010 4:30 pm

I've seen that in lots of different fuel filters but it never seems to be a problem.

ZGjethro · Post by **ZGjethro** » Fri Apr 30, 2010 5:54 pm

Joe, thanks for the reply. The truck is running well, maybe a little on the rich side though. The bubble stays there most of the time. I was not sure if it could air lock or have any related problems with air in the lines or carb.

Joe · Post by **Joe** » Sun May 02, 2010 2:34 pm

You're welcome. The bubble shouldn't cause any problems. If it gets to the carburator it will just shot through in an instant and be vented. It can't cause a vapor lock, that only happens on the suction side of the pump and not on the pressurized side. If the engine is running rich then it's most likely a carburator issue and you next to adjust the idle air mixture or use a slightly smaller jet, depending on what part of the throttle range you're in when it's running too rich.

Lifer · Post by **Lifer** » Mon May 03, 2010 3:39 am

Joe wrote: It can't cause a vapor lock, that only happens on the suction side of the pump...

Not true, my friend! The old flathead Fords were notorious for vapor-locking at or near the carburetor.

Chunk · Post by **Chunk** » Mon May 03, 2010 9:04 am

Lifer wrote:
Joe wrote: It can't cause a vapor lock, that only happens on the suction side of the pump...
Not true, my friend! The old flathead Fords were notorious for vapor-locking at or near the carburetor.

I thought "vapor lock" was caused by a fuel pump drawing in vapor on the suction side instead of a liquid,which causes the pump to stop pumping. The pump can't pump vapor,it has to be a liquid.This is caused by the fuel heating up (from engine heat,under hood temperatures,etc) and turning into a vapor BEFORE the fuel pump,not on the outlet side which would be a liquid. Am I missing something here?

Post by **Cal_Gary** » Mon May 03, 2010 11:51 am

No, vapor lock can also occur in the line between the pump and the carb, and even up into the carb itself. I had an old Ford that wouldn't run w/o the coolant-fed spacer under the carb because the carb got so warm w/o it that it vaporized the fuel as soon as it hit the float bowl.

That's also why some racers use the fuel can with the line coiled up inside, then the outer can is filled with ice-to keep the fuel cool before it hits the carbs.
Gary

Chunk · Post by **Chunk** » Mon May 03, 2010 12:07 pm

Same reason Chrysler used the phenolic resin bodied carbs in the late 70's

Sal · Post by **Sal** » Mon May 03, 2010 1:40 pm

I was under the impression that cool cans were used in racing to reduce fuel temperature for a denser air/fuel ratio.

Joe · Post by **Joe** » Mon May 03, 2010 1:46 pm

Chunk wrote:Same reason Chrysler used the phenolic resin bodied carbs in the late 70's

No, it's not! Chrysler went to the plastic carburators for one reason and one reason alone. They were CHEAP!! If they had been concerned about the fuel boiling then they would not have made the body out of BLACK plastic! The fuel inside a carburator is at atmospheric pressure so it would boil at it's normal boiling point so this has nothing to do with vapor locks. Chuck is right, vapor locks are caused by the fuel getting hot and boiling into vapor and the fuel pumps can't pump vapor so no fuel flow and the engine dies. I'm sure most of you are aware that as you raise the pressure on a liquid you also cause it to raise the boiling point. That's why we use pressurized radiators, so that we can operate the cooling system at 220 to 230 degrees instead of only 212 degrees. The opposite happens when you lower the pressure, at less than atmospheric pressure then boiling point is less than normal. That's what happens when you have a vapor lock, it only occurs on the suction side of them pump and takes only a moderate amount of heat to make the fuel boil and since the pump can't pump vapors all fuel flow stops until you cool off the fuel lines. With regard to what Lifer talked about, that is not a vapor lock. The fuel in the pressurized side of the pump may indeed get hot enough to boil away (but it would have to get very hot since the BP is higher under pressure) but it doesn't affect the pump and the pump will keep pumping as long as the motor is running and there's fuel in the tank. IF it's hot enough to boil away all the fuel in the line faster than the pump can replace it then the motor may starve and die but it's still not a true vapor lock since it does not affect the fuel pump.

ZGjethro · Post by **ZGjethro** » Wed May 05, 2010 7:01 pm

I was wondering more if the air gap could move to a high place in the line and form a blockage of sorts, kind of like what happens with some house heaing systems. The air seems to be contained in the glass body of the fuel filter, even though it is vertical. The filter used to be horizontal before I redid the fuel lines in the engine bay. My fuel pump is back by the gas tank, so air and gas are under about 4-5 psi from the pump.

Joe · Post by **Joe** » Thu May 06, 2010 2:21 pm

Air bubbles like this can be one of two types. The first is where the fuel line is mounted MOL horizontal and the filter is a larger diameter so part of it is above the level of the fuel line. In this case the air bubble is nothing more than an "air pocket" with a dead end above it so it can't move any higher. Think of it like an air pocket in a sunken ship or a submerged car, the water below it may be moving but as long as there is no opening at the top of the compartment the air is trapped and can't go anywhere. Since the pocket is only made up of air and nothing solid there's no danger that it will ever "block" the fuel line.

The second type is where you might have a fuel filter mounted vertical with part of the fuel line above it. In this case if the bubble was in a narrow area such as in a fuel line it would probably be swept away because the flow rate is higher but in something with a relatively large cross sectional area the linear flow rate decreases (because fluids are incompressible) so the air rises to the top as fast or faster than the flow rate. If you have something like a clear fuel filter mounted vertically with the fuel entering at the top and exiting at the bottom sometimes you can actually see the bubble rising in the fuel but it never goes anywhere except when you stop the engine and then it may go up into the fuel line but as soon as you start the engine, and fuel starts flowing again, the fuel flow will again push it down into the filter. But once it gets there it stays there because the flow rate is insufficient to push it out.

I hope that helps. Long story short, it can't block the line so you shouldn't have to worry about it.

ZGjethro · Post by **ZGjethro** » Thu May 06, 2010 5:01 pm

Thanks. That's kind of what I thought. The filter used to be horizontal and when I mounted it vertical I noticed it always has air in it. I think the bubble is above the outlet in the filter.