Irregular fuel pump pressure

[Mac80]

Had issue today with slight engine stumble after after takeoff and once more at about 3,000 feet. Started looking at all gages and noticed the fuel pressure seem to drop off occasionally especially on left tank. Applied electric pump and pressure went up and mostly stabilized. Kept watching looked like right tank always was little more stable. Thought could be gage? Seem to be higher on right tank.

It was freezing at airport upon departure and freezing at 10,000 feet. When got to Texas the outside air temperature got above freezing and fuel pressure on right and left tank where equal and appeared normal and stable for last part of flight.

Always check for water with sump and do a drain on left and right tank per manual before starting.

Guessing it could be some small amount of water in fuel line icing up causing low pressure ? Had high humidity in morning last several days.

ANY suggestions?

 

 

[A64Pilot]

Issue with sumping the tanks in below freezing temp is of course the water may be ice. I’ve even seen the drain frozen shut on a helicopter, pilots thought it was broken but when we changed it we found it frozen with ice

I had a C-210 and it had bladders of course, the left wings bladder had a wrinkle in it and water would sit behind that wrinkle and it flow over it, so you would sump that wing and get clear fuel. Go fly and sump it after the flight and get a little water, I had to pull the access panels to be sure. I could sump and get no water, rock the wings good and get a little. There may be places water can sit in our tanks too? Behind a rib maybe, especially if the drain hole has B2 in it from a repair?

Anyway it may not be a bad idea to drain the tanks after and before each flight especially if below freezing weather is coming, can’t hurt.

Oh before I forget the water was getting in the 210 from a bad fuel cap seal, airplane was left outside and if it rained, then water got in. So if you suspect water. take a good look at the cap O-rings. I’d just change them myself, likely they need it anyway.

Edited November 6, 2021 by A64Pilot

[N201MKTurbo]

I have found when it happens it is the fuel selector. The fuel selector seals are preloaded against the selector plate by the pressure of O rings. When it gets cold, the O rings get hard and will not push the seals against the plate and allows air to enter the fuel stream. This causes the fluctuation in fuel pressure. If the fuel pressure drops too low, the engine will stumble. 
 

New O rings work much better than old O rings.

[Hank]

6 minutes ago, N201MKTurbo said:

New O rings work much better than old O rings.

Rubber parts need to be replaced from time to time. As they age, the softeners, plasticized and other volatile slowly leach out and they harden up. Orings, gaskets, belts, hoses, pucks, tires, anything rubber . . . .

[Ragsf15e]

2 hours ago, N201MKTurbo said:

I have found when it happens it is the fuel selector. The fuel selector seals are preloaded against the selector plate by the pressure of O rings. When it gets cold, the O rings get hard and will not push the seals against the plate and allows air to enter the fuel stream. This causes the fluctuation in fuel pressure. If the fuel pressure drops too low, the engine will stumble. 
 

New O rings work much better than old O rings.

Are these O rings easy to change out during annual or is it better to send to lasar for “overhaul”?  My pressure fluctuates too and I’m planning to go through the fuel system at annual.  The selector was “overhauled 11 years ago.  Detents are good.  Thanks!

Edited November 6, 2021 by Ragsf15e

[N201MKTurbo]

On 11/6/2021 at 9:54 AM, Ragsf15e said:

Are these O rings easy to change out during annual or is it better to send to lasar for “overhaul”?  My pressure fluctuates too and I’m planning to go through the fuel system at annual.  The selector was “overhauled 11 years ago.  Detents are good.  Thanks!

It is not hard to take apart. If you are fast with screw in cap plugs, you can do it without draining the tanks. I find the hardest part is unbending the pull ring and bending it back. 

To get the fuel lines off, unscrew the B nuts while pushing the tube against the fitting, then quickly pull the tube away from the selector and push the end of the plug against the flair in the tube, then while holding the cap against the tube, screw the B nut onto the cap. If you do it right, you will only lose about a teaspoon full of fuel. if you do it wrong all the fuel in the tank will run onto the ground.

Edited November 9, 2021 by N201MKTurbo
Remove stupid statement.

[Ragsf15e]

On 11/6/2021 at 10:44 AM, N201MKTurbo said:

It is not hard to take apart. If you are fast with screw in cap plugs, you can do it without draining the tanks. I find the hardest part is unbending the pull ring and bending it back. 

To get the fuel lines off, unscrew the B nuts while pushing the tube against the fitting, then quickly pull the tube away from the selector and push the end of the plug against the flair in the tube, then while holding the cap against the tube, screw the B nut onto the cap. If you do it right, you will only lose about a teaspoon full of fuel. if you do it wrong all the fuel in the tank will run onto the ground.

Another trick if you have a helper is to put your shop vac on the fuel filler. Have the helper seal around the hose with their hands. it will suck the fuel up the line and you can mess with the fittings at your leisure.

Very helpful, thanks.  I went through the maintenance manual and the parts manual for my ‘68F and couldn’t find the o rings you’re talking about.  Is there a different place to look?  You don’t know the part numbers for them or the larger gasket around the bottom of the gascolater, do you?

[Skates97]

On 11/6/2021 at 6:52 AM, N201MKTurbo said:

I have found when it happens it is the fuel selector. The fuel selector seals are preloaded against the selector plate by the pressure of O rings. When it gets cold, the O rings get hard and will not push the seals against the plate and allows air to enter the fuel stream. This causes the fluctuation in fuel pressure. If the fuel pressure drops too low, the engine will stumble. 
 

New O rings work much better than old O rings.

I seem to recall Don Maxwell saying you could take and put a drop or two of oil on the shaft of the selector, if it was sucking air through there that would temporarily help. It is not a fix for o-rings that need to be replaced but a way to help diagnose if that is the problem.

[Guest]

On 11/6/2021 at 1:44 PM, N201MKTurbo said:

It is not hard to take apart. If you are fast with screw in cap plugs, you can do it without draining the tanks. I find the hardest part is unbending the pull ring and bending it back. 

To get the fuel lines off, unscrew the B nuts while pushing the tube against the fitting, then quickly pull the tube away from the selector and push the end of the plug against the flair in the tube, then while holding the cap against the tube, screw the B nut onto the cap. If you do it right, you will only lose about a teaspoon full of fuel. if you do it wrong all the fuel in the tank will run onto the ground.

Another trick if you have a helper is to put your shop vac on the fuel filler. Have the helper seal around the hose with their hands. it will suck the fuel up the line and you can mess with the fittings at your leisure.

I really hope that the shop vacuum is a non sparking type, otherwise it’s a bomb waiting to go off.

Clarence

[Hank]

46 minutes ago, M20Doc said:

I really hope that the shop vacuum is a non sparking type, otherwise it’s a bomb waiting to go off.

with a short fuse!! gas fumes are VERY flammable!

[N201MKTurbo]

1 hour ago, Ragsf15e said:

Very helpful, thanks.  I went through the maintenance manual and the parts manual for my ‘68F and couldn’t find the o rings you’re talking about.  Is there a different place to look?  You don’t know the part numbers for them or the larger gasket around the bottom of the gascolater, do you?

I've never found one, you will just have to match them.

[jaylw314]

On 11/6/2021 at 10:44 AM, N201MKTurbo said:

If you do it right, you will only lose about a teaspoon full of fuel. if you do it wrong all the fuel in the tank will run onto the ground.

I'm not sure why, but the way you worded that cracked me up 

[jaylw314]

On 11/6/2021 at 10:44 AM, N201MKTurbo said:

Another trick if you have a helper is to put your shop vac on the fuel filler. Have the helper seal around the hose with their hands. it will suck the fuel up the line and you can mess with the fittings at your leisure.

Wouldn't it make more sense just to close the fuel cap and stick a suction bulb on the fuel vent.  Also less chance of explodey stuff

 

[Ragsf15e]

2 hours ago, jaylw314 said:

I'm not sure why, but the way you worded that cracked me up 

It did seem to infer personal experience…

[Ragsf15e]

2 hours ago, jaylw314 said:

Wouldn't it make more sense just to close the fuel cap and stick a suction bulb on the fuel vent.  Also less chance of explodey stuff

 

Not enough power tools or fire in your method.

[Ragsf15e]

3 hours ago, N201MKTurbo said:

I've never found one, you will just have to match them.

Ok, last time I bug you (today).  Anyone know how to tell if the gascolator is an HE or a Dukes from the picture?  Lasar has a kit I’d like to use at annual but they’re different for HE vs Dukes which were apparently both used in the 60s…

Or do I have to take off all the damn screws again… 
’68 F.

[Yetti]

 

Do a leakdown test on the mechanical fuel pump

[N201MKTurbo]

42 minutes ago, Ragsf15e said:

Ok, last time I bug you (today).  Anyone know how to tell if the gascolator is an HE or a Dukes from the picture?  Lasar has a kit I’d like to use at annual but they’re different for HE vs Dukes which were apparently both used in the 60s…

Or do I have to take off all the damn screws again… 
’68 F.

That is a Dukes

[N201MKTurbo]

44 minutes ago, Ragsf15e said:

Ok, last time I bug you (today).  Anyone know how to tell if the gascolator is an HE or a Dukes from the picture?  Lasar has a kit I’d like to use at annual but they’re different for HE vs Dukes which were apparently both used in the 60s…

Or do I have to take off all the damn screws again… 
’68 F.

When you get the kit, take a picture of all the o-ring bags and post it so we all know the part numbers going forward.

[Ragsf15e]