Fuel pressure drop in flight from right tank - left is fine

[Flying074]

I was flying from Olympia to Spokane WA in a 1967 M20F and was having an uneventful flight at 9000' when it came time to swap tanks.  I rotated the selector to the right tank and within 2 minutes my fuel pressure dropped to almost zero and the engine started to sputter.  I switched on the electric boost pump and the engine stabilized with good fuel pressure.  Concerned, I switched back to the left tank which I had been flying on for the previous hour and the engine ran fine with good fuel pressure from the mechanical pump only.  Seeing as I was over a large airport, I recreated the incident with a switch back to the right tank and the exact same thing happened.  I swapped back to the good tank and landed.   I started the engine in the morning on the right (bad) tank so I know the plane was getting fuel at low RPM and pressure from the right tank.  I swapped to the left for the run-up and departure.  At flying RPM, the fuel flowed perfectly from the left tank but did not flow from the right tank - only with the electric boost pump on.  

I initially thought this might be a clogged fuel tank vent so when I landed, I checked the right filler cap but did not get a noticeable indicator of a vacuum in the tank.  I also tested the fuel again and didn't see anything concerning in the drain.  The right tank was still full.  

My initial thoughts are:

1. partially clogged tank vent. I will test the vent tonight to see if I can blow air through it.  Would a partial clog cause enough of a vacuum to inhibit the mechanical fuel pump from delivering enough fuel at high RPM but allow it to be overcome by the boost pump?  

2. partial blockage of the fuel line to the selector valve.  Could a small piece of debris or sealant from inside the wing tank have broken loose and gotten wedged into the line?  It seems like a partial obstruction of the fuel delivery line could cause a drop in pressure.  

I keep the plane hangared and it flew great last weekend on both tanks.  So, whatever cause this happened over the week or during my fueling this morning.  

Any thoughts what I should check before I pull it into the shop would be greatly appreciated.

Thanks very much for any insight or experience with similar issues

[N201MKTurbo]

It is probably your fuel selector. It is probably sucking air on the bad tank. Laser has seal kits for about $50. 

I would just switch it back and forth a bunch of times. It may have a piece of dirt in it and switching it may work it loose.

[carusoam]

Lack of pressure... sounds like air getting drawn in...?  Or the valve not going to the detent...

Blocked tank vent takes a while to become obvious... depending on how much air is in the tank already.  The oil canning of the sheet metal makes it obvious...

You can always send a wire up the vent to see if it bumps into something.

signs of air leaking into the line include blue stains at the fuel selector.  Air in is often accompanied with fuel out depending on a few things....

Signs of wear around the fuel selector include no feeling of detents they are supposed to be there. My 65C no longer had them...

 

Focussing on what has failed around here before... the valve gets some attention because if it fails to turn and stops moving... that can be bad...   if it leaks it can be putting fuel under floor, or letting air in...

 

Got a fuel flow indicator...?  Air bubbles leaking in often give really erratic FF data... if it reads properly the numbers will be high compared to book value FFs...

 

PP thoughts only not a mechanic.

Best regards,

-a-

 

[jaylw314]

15 hours ago, N201MKTurbo said:

It is probably your fuel selector. It is probably sucking air on the bad tank. Laser has seal kits for about $50. 

I would just switch it back and forth a bunch of times. It may have a piece of dirt in it and switching it may work it loose.

On the ground while pulling the gascolator ring, not while the motor is running!!  

[KSMooniac]

It is probably your fuel selector. It is probably sucking air on the bad tank. Laser has seal kits for about $50. 
I would just switch it back and forth a bunch of times. It may have a piece of dirt in it and switching it may work it loose.

I'd start here too...and drop some oil into the selector and work to through the detents several times.

Sent from my LG-US996 using Tapatalk

[0TreeLemur]

Less likely, but it could also be a plugged fuel intake strainer in the right tank.  The finger strainer attached to the fuel intake tube in many of our birds suffers from an excess of tank sealant, which partially blocks the screen.   Add a little bit of debris such as flakes of sealant, corrosion, paint, etc. it isn't too hard to block the screen.  The loss of pressure through the screen would show up as a reduction in fuel pressure.

[Gary0747]

There have been several accidents caused by the improper use of red rubber sloshing tanks sealer which is essentially the same as the final protective sealant that goes on over the CS poly sulfide sealant.  (PR-1005-L). The stuff can come off in sheets if installed improperly or exposed to fuels with too many aromatics ie Auto  Gas 

[Prior owner]

26 minutes ago, Gary0747 said:

There have been several accidents caused by the improper use of red rubber sloshing tanks sealer which is essentially the same as the final protective sealant that goes on over the CS poly sulfide sealant.  (PR-1005-L). The stuff can come off in sheets if installed improperly or exposed to fuels with too many aromatics ie Auto  Gas 

 When I spoke to the FlameMaster tech rep he was adamant about never applying their 3600 protective coating over anything but a freshly sealed tank- once the sealant and /or the 3600 coating (same as PR-1005-L) is contaminated with fuel, there will be adhesion issues. 

[Gary0747]

I wonder if people applying this in patched areas are applying some overlap that may come off later?

[N201MKTurbo]

If you clean the surface with MEK before you put it on (as the maintenance manual says) it will adhere just fine. The MEK will mostly remove an older coat and whatever is left is softened enough that the new stuff will bond just fine. 

It isn’t the fuel that keeps it from bonding, it is the layer dirt that is deposited on the sealer that keeps it from bonding. 

[Prior owner]

58 minutes ago, N201MKTurbo said:

If you clean the surface with MEK before you put it on (as the maintenance manual says) it will adhere just fine. The MEK will mostly remove an older coat and whatever is left is softened enough that the new stuff will bond just fine. 

It isn’t the fuel that keeps it from bonding, it is the layer dirt that is deposited on the sealer that keeps it from bonding. 

I wondered about doing it that way, and asked the rep that very question. They wanted no part of it (understandably).  

What you’re saying makes sense to me for a small area repair, but it sounds like It would make a real mess to try to do the whole tank like that.

I’ve only stripped and resealed one tank, but When I used MEK to clean up around the inspection cover faying surfaces, it was a lot of work to get the 3600 coating off the aluminum. Haven’t tried use MEK to remove an aged coating yet though.... perhaps it’s a lot easier.

[N201MKTurbo]

1 hour ago, PilotCoyote said:

I wondered about doing it that way, and asked the rep that very question. They wanted no part of it (understandably).  

What you’re saying makes sense to me for a small area repair, but it sounds like It would make a real mess to try to do the whole tank like that.

I’ve only stripped and resealed one tank, but When I used MEK to clean up around the inspection cover faying surfaces, it was a lot of work to get the 3600 coating off the aluminum. Haven’t tried use MEK to remove an aged coating yet though.... perhaps it’s a lot easier.

Nope, not easier.

If you wanted to do a whole tank, you would be doing it one wall at a time. It would be a big PITA.

Just find the leak, fix it and put the top coat on the repair. 

[Gary0747]

What happens if you skip the top coat?  Cover plate sealing surfaces don’t get the top coat?  

[Flying074]

Thanks to everyone who replied to this thread. I went with the easiest solution first and alternated switching the fuel selector back and forth while pulling the strainer drain to see if I could shake the obstruction loose and get it to come out through the drain.  I didn’t see anything obvious come out but afterwards I did a run up and then took the plane for a test flight.  The blockage appears to have cleared and I had good performance for almost an hour on the right tank.  Great info on the tank sealant and I’ll will look inside the tank next to see if there is any indication of flaking or peeling sealant to confirm whether this might happen again.  

 

Appreciate the forum ideas