air bubbles from tank hose at fuel filter

[dlb]

I recently got a clear fuel filter and one day when I was under the hood while the engine was idling, I noticed bubbles coming from the tank hose in the fuel filter. I'm assuming this means there is an air leak somewhere in the fuel line coming from the tank, yes? I think a PO replaced all those fuel lines so I'm guessing there is going to be a loose connection somewhere.

[Paul]

Maybe it's an older rubber hose by the tank thats got some cracks... probably much better than the mess I had when I started reviving this '88 Terc.

20220901_143035.jpg

[dlb]

I removed the tank today and took a look. All the hoses were in good shape and properly connected. I also plugged two of the hoses and blew into the third hose and confirmed the tank itself doesn't have any leaks. Then I inspected the steel fuel line for the length of the car, and the hose between the steel line and the fuel filter. Couldn't find anything amiss. There are a few spots where there is the steel line is rusty where it is clamped in place but no wet spots or anything obvious. I'm not sure what to do next to try to find the leak.

Edit -- just realized I can disconnect the front fuel hose from the steel line and stick it in a small container of gas. Then I can at least see if bubbles in the air filter occur even with all the variables removed. But if anyone else can look at their car and let me know, that would be great too.

[Nordical25]

That is a good first step dlb. Second step can be to connect that gas bottle into rubber lines, which you see in Pauls picture.

Then you can check if you have still bubbles. If you do, then you know that they come from the main fuel lines under the car. I did swap them to both of my cars lately so I know that they will wear out eventually thanks to age and road salt.

[dlb]

I did my little test but there are still bubbles! I don't know how this is possible. I disconnected the front fuel hose from the steel line and placed the end in a container of gas, then started the car and watched it. Still bubbles. I removed the hose and blew on one end while plugging the other end with my finger, no leaks in the hose. The only place it could then be leaking was where it connects to the fuel filter so I put a hose clamp on there and snugged it up -- still bubbles.

So it seems like there is something I'm missing, like the bubbles I'm seeing are not coming from a leak in the system. Maybe it's just agitation from the gas being sucked into the fuel filter. I don't know but since it's doing it even with what seems to be everything accounted for, I'm going to consider this normal and not a problem. I'd still appreciate if others could take a look at their fuel filters and let me know if they see bubbles or not though because I'm baffled.

[Mark]

I'm not sure what the vapour pressure of gasoline + ethanol is at lets say 15 degrees C, but Since the pump is outside and above the level of the tank and drawing fuel through small lines/hoses (restrictions), the pressure in the line is less than atmospheric (slight vacuum). Given turbulence and small, localized pressure drops around bends, etc, I wouldn't be surprised if the vapour pressure of the gas is reached temporarily in certain areas of the fuel line (sort of like cavitation bubbles around a propeller). If this is what it is, I assume any "boiled" fuel would condense again when it reaches the discharge side of the pump where the pressure is higher.

[Paul]

Did you try another filter? I noticed these filter bodies are glued together. Maybe a tiny leak at the seam. You would think it would drip when the car is off, but with a vented tank and under pump suction when running, perhaps not.

[dlb]

Did you try another filter? I noticed these filter bodies are glued together. Maybe a tiny leak at the seam. You would think it would drip when the car is off, but with a vented tank and under pump suction when running, perhaps not.

Good idea. I haven't tried another filter yet but will look at that next. Thanks!

[Nordical25]

I did my little test but there are still bubbles! I don't know how this is possible. I disconnected the front fuel hose from the steel line and placed the end in a container of gas, then started the car and watched it. Still bubbles. I removed the hose and blew on one end while plugging the other end with my finger, no leaks in the hose. The only place it could then be leaking was where it connects to the fuel filter so I put a hose clamp on there and snugged it up -- still bubbles.

So it seems like there is something I'm missing, like the bubbles I'm seeing are not coming from a leak in the system. Maybe it's just agitation from the gas being sucked into the fuel filter. I don't know but since it's doing it even with what seems to be everything accounted for, I'm going to consider this normal and not a problem. I'd still appreciate if others could take a look at their fuel filters and let me know if they see bubbles or not though because I'm baffled.

I did check this today with my car. My tank was nearly empty and I saw also bubbles at fuel filter. However the fuel filter was also quite empty.

Therefore, I suspect that the bubbles show in air filter when fuel level of tank is low.