Air bubble of doom in gascolator!

[4BravoWhiskey]

Hi everyone, I have a problem that has stumped my mechanic (so far) so I thought I'd see if anyone else has had this happen or has any bright ideas. My stock '53 170B keeps developing an air bubble at the top of the gascolator during flight. If it gets big enough then fuel starvation sets in, which is no fun. Trust me. (Picture remote Utah, engine cutting out, few airports and no roads or fields…)

So far I've changed the o-rings on the primer plunger, checked vents, checked fuel flow from both tanks, checked all fittings for tightness and any signs of leaks, disassembled gascolator cleaned and lubed gaskets, … at first I thought I was running one tank too low, but then it happened with 3/4 full tanks, in just a 45 min flight there was an air bubble nearly 3/4" tall at the top of the gascolator. Each time this happens I have to drain the gascolator completely, disconnect fuel line to carb, and run fuel through until it fills up and the bubble is gone.

Apparently air is getting sucked into the system somewhere, somehow… I searched the forums and didn't find anything but if this has already been addressed, someone please point me to the thread. I really want to get this taken care of!

Jack

[blueldr]

Does your airplane have a fuel pump in the system?

[4BravoWhiskey]

No fuel pump.

The only modification from stock is a fuel flow sensor (EIectronics International) located inline between the gascolator and carb. Had it in for about 80 flight hours prior to this air bubble issue, and there is no indication of any problems with that. It is installed at the proper height relative to the gascolator and carb, and I've tested fuel flow both before and after the sensor.

[daedaluscan]

Is the tank venting all good and clear? I mean you probably do have an air leak somewhere but why is air going in rather than fuel coming out? Are you getting good fuel flow when you just open the gascolator drain?

[DWood]

Are there any hoses in the system maybe at the wing root? Are you seeing any traces of fuel leaking anywhere? I suppose there could be an air leak that isn't leaking fuel. The other possibility might be the fuel valve. However since that is low in the system I would think it would always have fuel. Any porosity in the gascolator? Check all fittings. This one is strange!

Edit

Can you add a picture of your gascolator as I assume the glass is down. I would think the pickup point would be lower than 3/4 inch. But I might be wrong as I have a 48 which has the glass up and there is air in the top. The pick up is on the bottom of the glass on mine. I would think the B also picks up at the bottom of the glass. How far does the tube go down?

[blueldr]

I can't immagine any way that you could keep the head pressure from the tanks from pushing the air on out through the carburetor, After all, when we pull the glass off and clean it the thing is full of air and when the fuel is turned ON the air just passes on through to the carbureor bowl until the float level closes the needle valve.The needle valve in the carburetor will open as soon as an fuelis consumed and if the needle valve opens, I don't see how any air could stay in the gascolator.
If one drains the gascolator with the selector valve turned off, the gascolator bowl will fill with air and the air will pass on through as soon as the fuel is turned ON.
Have you actually had the engine quit for lack of fuel when it is operating?
With the engine not running and there was air in the gascolator with the fuel turned on and the air was locked in, a stroke or two on the throttle would cause the accelleraor pump to lower the fuel in the carburetor bowl and open the needle valve to allow the air to pass on through.
It sounds to me like you have some sort of fuel line obstruction between the tank and the gascolator.

[GAHorn]

Of what material are your gascolator gaskets constructed?
If they are EPDM (silicone rubber) or rubberized cork...that could be your problem.
Order new gaskets by PN from any Cessna distributor.
The genuine Cessna Gascolator Gasket PN is: 33-199-6 Gasket

[flyboy122]

I'm with Blueldr. An air bubble such as you are describing should push through the reservoir in the bowl. Could it maybe be a starvation issue? I.e. an air bubble isn't forming and displacing fuel, it's forming due to lack of fuel. I would check to make sure you have good fuel flow TO the gascalator.

I had this happen in a PA12. A bubble in the fuel lines caused reduced (not stopped) fuel flow. It would run for a while, starve for a few seconds, then refill and run again. Chasing it down was a nightmare, as it only showed up when the lines were bent around and twisted and the bubble formed. It didn't do it all the time.

DEM

[beaverbill]

Is it possible that the gascolator is mounted / hooked up backwards? There is an "In" port, and an "Out" port. I have heard of cases with symptoms like yours which resulted from just such a reversal.

[4BravoWhiskey]

Thanks for the input guys.

daedaluscan and flyboy122 - I checked fuel flow from both tanks (separately and together) at the input to the carb, and it seemed plenty good, flowed fast a although I didn't actually measure it (gallons/min). I should do that. It flows fast and fine when I just open the valve at the bottom of the gascolator.

DWood and beaverbill - I haven't found anything that looks like a seep, but I haven't checked the pickups at the tanks. And yes the fuel valve is so low in the system, I would think it would leak fuel if it were leaking air.

I don't have a picture of the gascolator handy but will take one next time I'm out there. The input and the pickup are both at the top, as is the primer connection. The input has a stand pipe going down through the filter, so it is always lower than the air bubble. The output (pickup) is just a hole in the top assembly, no pipe coming down. This is how my mechanic said it should be. Does this sound correct? I can't tell from looking at the 100 series service manual.

blueldr - The engine doesn't quit, but I had to run near full rich and full throttle to keep it going good enough to get to the nearest airport 15 min away when this happened the first time. I've kept a close eye on it since, evacuating the bubble each time it starts to appear (by disconnecting fuel line to carb and running some gas through. I have to drain the gascolator completely and refill before I can get the bubble to go away though… this seems odd to me and possibly indicative of the problem, or one of them.)

I didn't try pumping the throttle to eliminate the bubble with the engine not running. Will do next time.

George - the gaskets looked like some sort of fiber, but I'd have to take it apart again to know for sure. I put a light film of fuel-resistant lube (silicone) on the gaskets when I cleaned them up and reinstalled while stopped in Utah. No signs of fuel leaking but I'll order replacements anyway. Thanks for the PN.

[sfarringer]

On my '48, the glass bowl is on top.

Not sure I totally followed your description, but I got the impression that yours is upside down, though it is not clear how your gascolator drain could work that way.

[4BravoWhiskey]

Mine has a metal housing on both the top and bottom. The glass is a cylinder rather than a bowl. I think the '48 has a different setup.

[Kurt Aichele]

Is it in fact an "air" bubble? Could it be fuel vapor? It doesn't make sense to me that air could be "sucked' into a fuel system that is pretty much gravity fed. Did something change? Could it be the fuel system is getting some sort of vapor lock from getting too hot at that point in the fuel system? I have a '48, so the fuel system is a little different than yours. Just throwing some things out there to try and figure this out. A real puzzler, it is.

[bagarre]

A few notes that might spark ideas. Some things mentioned aren't lining up in my head.

The fuel system is under head pressure from the fuel tanks being high in the wings. Any leak in the system would cause fuel to leak out, not air being sucked in. The gascolator is the lowest point in the system and as such would have the highest fuel pressure.

The fuel hose is higher than the gascolator so if air came in there, it would move upstream to the carb (which may be what's happening) and not down the line to the gascolator. But it would seem that any perforation that might allow air in would cause fuel to leak out or at least the fuel pressure would prevent air from being drawn in.

If you're fuel starved, why would it help to run full throttle and mixture rich?

Just thoughts. I have no answers for this one.

[4BravoWhiskey]

Fuel vapor instead of air: The gascolator is under quite a bit of pressure due to the fuel in the system above that point, and even if it could get hot enough to vaporize fuel somewhere in the system, the vapor wouldn't stick around for hours in the gascolator. It would condense into a liquid very quickly at these temperatures.

Hose: I haven''t had a chance to check and see if there is hose in the fuel lines somewhere. Other than in the vent interconnects, of course, but that wouldn't matter and they are all recent anyway. I'll get back out there tomorrow.

Connections: The output of the gascolator goes to the carb.

Air in = fuel out? I agree that if there is a leak somewhere it should allow fuel to go out as well as air in, but if it's related to something like a check valve in the primer, maybe that could allow for it?

As for throttle/mixture, if the engine is sucking air along with the fuel, it makes sense to me that a richer mixture and more throttle would help keep the engine running. Need more fuel to compensate for the air. Maybe "starved" is the wrong choice of word as historically it may imply lack of flow from the tanks. In this case I mean the air in the gascolator is causing the engine to not get the fuel it needs (which also sounds like starved to me).

Speaking of which, I am way overdue for some breakfast...