Gascolator Empty Mystery

[Oklahoma Mooney]

I recently purchased a 1963 M20D (Converted to retract) and have flown it a few hours. On a recent pre-flight I noticed the gascolator in the nose wheel well (original glass sump) was empty. I made sure the fuel selector was on, even ran the fuel pump, and it would not fill. I opened the gas caps thinking maybe there was a venting problem but no joy. Finally I started the engine to see if it would draw fuel, it started first blade. Engine made power ran up fine. I shut down and checked the gascolator and it was full of fuel. After sitting for a day I checked it again and to my surprise gascolator was empty again. 

There is no evidence of a leak I can see or smell, but I haven't opened the belly yet. There are only two variables from when I first purchased the aircraft. 1) The aircraft was purchased with full fuel tanks, and now there is about 7 gallons per side on. 2) It has been colder in the evenings (down to 30's). 

Can the fuel flow backward into the tanks and be drawn out of the gascolator making it empty? 

Any input is appreciated. 

 

[A64Pilot]

Heat after shutdown turning the fuel to vapor?

Gascolator should be the lowest point, gravity should not have fuel from it running anywhere, fuel tank is uphill, so are the tanks. Any leak with the fuel selector on won’t drain the bowl, it will refill from the tanks. Tanks are vented there should never be any vacuum or pressure in them, but check the vents anyway

[47U]

2 hours ago, Oklahoma Mooney said:

After sitting for a day I checked it again and to my surprise gascolator was empty again. 

Welcome to MooneySpace!  We need pictures!

You probably have a slow leak in the gascolator drain, or the bottom cork gasket.  There’s a gasket at the top of the bowl, too. The inlet line to the gascolator might be leaking, also, but you’ll need to drop the left side cavity panel next to the left nose gear door.

There’s about 1,000 C and D model serial numbers with a parallel-flow fuel system.  There’s a T fitting at the electric fuel pump inlet that sends fuel through the nose wheel well to the gascolator.  From the gascolator it the line enters the cockpit at the forward end of the nose wheel well to an elbow fitting in the pilot’s footwell.  Then a -6 hose to the engine driven fuel pump and the output of the engine driven pump rejoins the fuel from the electric fuel pump in anther T fitting at the lower left firewall.  And a single -6 hose to the carburetor.  That’s why running the electric fuel pump doesn’t fill the gascolator bowl.

This is the T at the electric fuel pump inlet.  The T supplies fuel to the gascolator.  The outlet of the electric fuel pump goes to the T at the firewall.

Fuel line in the cockpit from the gascolator to the elbow in the footwell.  This line is subject to cracking because no one uses a backup wrench when tightening the -6 hose on the other side of the firewall.  Erratic fuel pressure and fuel flow (if you have it) means the fuel system is sucking air… somewhere.

Hose from the elbow in the footwell to the engine driven fuel pump.  If this hose isn’t positioned ‘just right’ it will impede removal of the oil sump suction finger screen (right side of pic).

Your fix is probably pretty simple… congrats on your D/C ownership!

[Oklahoma Mooney]

That is awesome detail, thank you very much. I will look at the areas described. I was thinking of upgrading to the Steve's gascolator at annual, thoughts??

[A64Pilot]

Why replace something that’s been working fine for over 60 years?

[takair]

Good details in 47U post. Helped a friend recently with an RV-4 that developed similar issue. Only thing we discovered was that the carb had lots of excess fuel leaking especially when pump activated. Blue staining at the airbox drain. Basically carb float valve not sealing. I can’t quite explain why the gascolator is draining. It seems to be uphill, I suppose some kind of siphoning event. Anyway, carb is on its way to a shop and will see the results. Had some additional symptoms as well….

Edited November 25, 2022 by takair
Spell check issues

[47U]

3 hours ago, Oklahoma Mooney said:

I was thinking of upgrading to the Steve's gascolator at annual, thoughts??

I’m with A64Pilot… unless there’s an issue with corrosion on the gascolator bowl seats that cannot be cleaned up so that the top and bottom gaskets don’t leak, I don’t see a need to change it.  

[Oklahoma Mooney]

1 hour ago, takair said:

Good details in 47U post. Helped a friend recently with an RV-4 that developed similar issue. Only thing we discovered was that the carb had lots of excess fuel leaking especially when pump activated. Blue staining at the airbox drain. Basically carb float valve not sealing. I can’t quite explain why the gascolator is draining. It seems to be uphill, I suppose some kind of siphoning event. Anyway, carb is on its way to a shop and will see the results. Had some additional sump time as well….

Thanks for the input I will check the around the airbox and carb for signs of fuel staining. 

[Guest]

16 hours ago, Oklahoma Mooney said:

That is awesome detail, thank you very much. I will look at the areas described. I was thinking of upgrading to the Steve's gascolator at annual, thoughts??

One of the best updates you can make to your Mooney.  The glass bowl with the bale design is outdated and can fail leaving you as a new glider pilot.

[skykrawler]

37 minutes ago, M20Doc said:

One of the best updates you can make to your Mooney.  The glass bowl with the bale design is outdated and can fail leaving you as a new glider pilot.

What he said.    And you might consider turning the fuel selector to OFF after shutdown until you find your leak - at current gas prices.

[A64Pilot]

The gascolator is at the lowest point in the fuel system, with the fuel valve on, and no fuel in it, there can’t be a leak, if there was the gascolator would fill with fuel. To prove that loosen the line at the carb and see what happens, as the air or vapor escapes it will be replaced with fuel, try it. 

Even a small leak over time will leave a blue stain, no blue stain, no leak.

Other than vapor pressure I can’t explain it, but think it’s normal, I’ve seen empty gascolator bowls before.

Edited November 25, 2022 by A64Pilot

[takair]

46 minutes ago, A64Pilot said:

The gascolator is at the lowest point in the fuel system, with the fuel valve on, and no fuel in it, there can’t be a leak, if there was the gascolator would fill with fuel. To prove that loosen the line at the carb and see what happens, as the air or vapor escapes it will be replaced with fuel, try it. 

Even a small leak over time will leave a blue stain, no blue stain, no leak.

Other than vapor pressure I can’t explain it, but think it’s normal, I’ve seen empty gascolator bowls before.

As I think about this some more, the fuel tank pickups are lower, I think.  If the float valve in the carb is leaking, then I think that it could allow fuel to drain back to the tank.  The float valve would normally act as a check valve preventing this. (?).  On the other hand, when the pump is running, assuming fuel in the system, a leaking float valve can cause the carb bowl to overflow and leave stains at the airbox.  This is what I believe is happening to my friends RV.  He has had the plane for 20+ and it only developed the symptoms recently.   Will need to think through a little more, but could this be happening to the OP?  Other symptoms in the RV include difficulty priming and starting, and on most recent flight…. engine roughness on takeoff, which may have been from excess fuel….that was when we discovered fuel coming from the airbox when boost pump was on.  Generally speaking it has similar plumbing as a Mooney.

[A64Pilot]

Good scheming, but a float valve won’t hold much against suction, not many float valves seal that well to be air tight over time, my C-140 had as original equipment an all metal needle valve, later ones were sold with neoprene tips that hardened, so best is the metal needle and lap it. but even with a valve that leaks a little the gascolator is full, you can drain it at any time, but with fuel off of course you only get a dribble.

Just put a Viton tipped needle valve in my 100 yr old Ford Model-T Holley carb, but the OEM one wasn’t leaking, but I was going through the carb anyway.

Fuel pickups are probably lower, but an aircraft’s design is supposed to put the gascolator at the lowest point in the fuel system, but not all are.

I’ve seen this before, but never worried about it, seen it in Racors in boats and in the glass bowls on Cav filters on farm equipment.

I just don’t think he has anything wrong, but if it worries him put one of the new expensive metal bowled ones on and he never has to look at an empty bowl again.

My 46 C-140 one is glass and will stay that way, it’s really thick glass

Google my fuel filter is empty after a couple of days or similar and see how often this question is asked

I’m guessing heat vaporizes enough fuel to empty the bowl, because I can’t come up with anything else.

[Oklahoma Mooney]

Thank you everyone for your excellent comments. As an update, I went to the aircraft today and 1) Checked the fuel vents = clear 2) Looked around the airbox for blue stains or evidence of leaks = Dry, no stains. 3) Looked at the fuel line in the cockpit from wheel well to firewall = no leaks. 4) opened the left lower (chin) panel and looked at the fuel pump, and fuel lines = totally dry, no blue stains. 5) looked at the fuel selector lines = all dry. 6) looked carefully at the gascolator and did see some old blue stains near the sump and bottom. 

I flew the aircraft and again, no issues and when I returned the bowl was full, no obvious leaks. 

I did note that the the gascolator in my D model is higher than the electric fuel pump, fuel selector, and the wing tank pickups. 

My current theory is that a leak or bad gaskets (as 47U pointed out) in the gascolator is letting a small amount of air in and it's allowing the fuel to backflow toward the fuel pump and line to carb.  I will continue to monitor it and will have the gascolator repaired / replaced at annual (soon). 

Thanks again everyone

[A64Pilot]

If you think it’s leaking, fix it now, order the gaskets, their cost is minimal, but leaks don’t often fix themselves and it could start sucking air maybe and if it did that it could make the engine quit.

‘If it’s a paper gasket I put grease on paper gaskets prior to installing them, keeps them from sticking and I think helps them seal.

[Oklahoma Mooney]

17 minutes ago, A64Pilot said:

If you think it’s leaking, fix it now, order the gaskets, their cost is minimal, but leaks don’t often fix themselves and it could start sucking air maybe and if it did that it could make the engine quit.

‘If it’s a paper gasket I put grease on paper gaskets prior to installing them, keeps them from sticking and I think helps them seal.

Will do, thank you

[47U]

44 minutes ago, Oklahoma Mooney said:

My current theory is that a leak or bad gaskets in the gascolator is letting a small amount of air in and it's allowing the fuel to backflow toward the fuel pump and line to carb.

If there is air getting into the gascolator, I think the path of least resistance would take the fuel back to whichever tank was selected.  Next time you shut down, close the fuel tank selector valve and see what happens.

And you should know (since you’re new here) that advice from @M20Doc is the gold standard.  By comparison, 99% of my Mooney experience is limited to my airplane.  I’ve touched a couple of friends (vintage) Mooneys, but the Doc is an expert.