Fuel Pressure

[Jim Peace]

1964 M20C O-360 ,  

 

During climb out yesterday and today.

 

Engine started fine

Runup fine

Takeoff fine

 

Selected the electric fuel pump off during climb and the pressure went from 5 ish to 2 ish. and bounced around a lot from 2 to 2.5.  I put the electric back on and it went right back to 5 to 5.5.

 

I turned it off and back down it went.  As I climbed it began to creep back to its normal 4.0 to 4.5 range over the duration to get to 8500/9500 msl.

 

Once there it was fine.

 

When I was low altitude for landing I went to climb power and to check it and it was fine.

 

Is this an indication that the mechanical pump is going to fail soon?

 

I do not recall fuel pressure ever dropping so low and bouncing around so much.

I took a video with my iPhone but have no idea how to get it on here.  I tried emailing to myself and then to transfer to desktop and attach here but no luck...

 

Thank you in advance

 

Jim

[Marauder]

Jim -- I'm not sure what units you are using. My F is in PSI and I will see around 26.4 to 27 PSI with the boost pump on. With the pump off, the engine driven pump will run somewhere slightly above 24 PSI.

I have never had an engine fuel pump fail but have replaced the boost pump. I would think that if the engine pump was failing it would fluctuate consistently and not get better.

[N201MKTurbo]

It could be the fuel pump or a loose fuel line between the boost pump and the engine pump. Turn on the boost pump when you are on the ground and let it run for a bit and see if any fuel leaks out. If you have a failing diaphragm in your engine driven pump it will leak from the pump drain. If you have a loose fuel line it will leak from the loose fitting.

Carbureted engines have about 5 psi and injected engines have about 25 psi.

[sleepingsquirrel]

Just for good luck check the vents are clear by gently blowing into them with a tube attached. Check for evidence of dirt dauber mud , or blockage.

[Jim Peace]

Thank you,

 

I just flew a round trip to MRY from VNY this weekend.  I have an annual coming up so I thinkI am done flying for a while.  Will pass along these ideas to my IA.....

 

I can forward the video to anyone if they would like to see what I am trying to describe.....

[FloridaMan]

It could be the fuel pump or a loose fuel line between the boost pump and the engine pump. Turn on the boost pump when you are on the ground and let it run for a bit and see if any fuel leaks out. If you have a failing diaphragm in your engine driven pump it will leak from the pump drain. If you have a loose fuel line it will leak from the loose fitting.

Carbureted engines have about 5 psi and injected engines have about 25 psi.

 

I don't know how it is on carbureted aircraft, but if you leave the boost pump on and the throttle/mixture open on an injected setup, it will eventually start draining fuel from the intake onto the ground as well. 

[Marauder]

I can forward the video to anyone if they would like to see what I am trying to describe.....

Jim - if you would like to share the video, one of the easiest ways is to upload it to YouTube or Vimeo.

You will need to setup an account, but you can also use the account to subscribe to other people's channels that produce videos that you may like. There are a number of people producing aviation related materials.

[N201MKTurbo]

On an injected engine, if you turn on the boost without the mixture at idle cutoff it will just pump fuel into your cylinders. The fuel will run down the intake tubes to the sump and drain out the sump drain.

On a carbureted engine it should only fill the float bowl of the carb and then stop.

[jrwilson]

Normal for mine and many other early C's. In fact, if u look at your fuel pressure gauge, there is a green arc notch/preferred area for climb which is down to 3.5 to 2.5. Don maxwell said early c's do that. In fact so do archers and rv's, planes with low wings and o360s.

[jrwilson]

I remember noticing it a few years ago, my fuel pressure gauge had been masking it, but the gauge was fixed and then I started noticing it. Spent thousands trying to fix a problem that wasn't. Finally my mechanic called don. He wasn't sure why the early c's do, but they do. My hangar neighbor's early c does it, it's in the piper FAQ section on the Cherokee 180s and it's mentioned in the vans forums often. Low wing/o360...

[jrwilson]

If it's really erratic, you could be sucking air so check the tightness of your lines but the values are fine.

[moodychief]

I had a similar incident on my 63 C model that kept getting worse. Mainly showed in the climb but was okay in cruise and descent. After months of replacing items we finally found the culprit. Inside the cockpit in front of the doghouse there was a small section of fuel line that goes from the gascalator through the cabin then through the firewall. The line had a small crack in the top where it had rubbed against part of the aircraft structure. You couldn't see the crack because it had abrasion tape around it. Anyway, with the electric pump on it would push fuel fine but when the electric pump was turned off and the engine pump was pulling fuel it was sucking air through the crack. I'm not saying this is the same thing but may be worth checking that small run of fuel line to make sure it isn't the culprit. This line is accessible through the pilots side at the front left corner of the doghouse.

[mooniac15u]

My 1963 M20D did the same thing. When I switched off the electric pump the pressure gauge would drop and then slowly climb back to normal. It did that the entire time I owned it.

[ArtVandelay]

On an injected engine, if you turn on the boost without the mixture at idle cutoff it will just pump fuel into your cylinders. The fuel will run down the intake tubes to the sump and drain out the sump drain.

On a carbureted engine it should only fill the float bowl of the carb and then stop.

Is this true of both Lycomings and Continental?

[bonal]

Just took a short trip and had 3.5 on climb and 4.0 in cruise with out boost pump with boost it's close to 8. If you have a leak you will suck air with the boost off and push fuel out with it on.

[jrwilson]

Just took a short trip and had 3.5 on climb and 4.0 in cruise with out boost pump with boost it's close to 8. If you have a leak you will suck air with the boost off and push fuel out with it on.

My top red line on fuel pressure is 6 psi. Is yours higher?

[bonal]

Now I am going to have to check it again I know my green arc goes to 6 and when I'm running down the runway with boost on I'm just over 7 but one has to consider this is likely an original gauge and I'm sure accuracy leaves much to be desired. The important thing is seeing a steady pressure and one that is normal for my aircraft.

[N201MKTurbo]

Is this true of both Lycomings and Continental?

 

I'm not an expert on the Continental injection system, but I think it is true.

 

As for any carbureted system the fuel flow will stop when the float bowl is full, unless you have a flakey needle valve. 

[Jim Peace]

OK.

 

Here is a video of my fuel pressure as soon as I turned off the electric pump and then you will notice when I turn it back on then off again.  Is this what other people see?  This was during climbing after the engine was cold.

 

http://youtu.be/Z4NqYTXpskU

 

Why is it that I have flow about 15 legs in this airplane recently and this is the first time it is happening?  

 

reminder, at cruise all is well with fuel pressure not to far below what the electric pump would provide and it is stable.

[gsxrpilot]

Mine doesn't bounce around like that, but I get the same 2 to 2.5 on the climb out but get 5+ with the electric pump on.  It comes up a bit in cruise and often gets all the way to 5 in the descent.  Mine is a 1964 M20C.  Don Maxwell has flown it several times and gives it a clean bill of health.

[carusoam]

I remember my C going through the pressure drops as described...

Drop 1 electric pump on/off

Drop 2 climb angle

What I don't remember, is the vibration of the needle that is being presented in this video.

Bob has some experience with something that may be similar. On a JPI system...

There is a device that smooths those pressure fluctuations out. Memory won't give me the name of that thing. Sometimes the device is part of the gauge.

Best regards,

-a-

[jrwilson]

I remember my C going through the pressure drops as described...

Drop 1 electric pump on/off

Drop 2 climb angle

What I don't remember, is the vibration of the needle that is being presented in this video.

Bob has some experience with something that may be similar. On a JPI system...

There is a device that smooths those pressure fluctuations out. Memory won't give me the name of that thing. Sometimes the device is part of the gauge.

Best regards,

-a-

Is it a snubber valve that smooths the fluctuations? Maybe the gauge needs an overhaul or you're sucking air somewhere.

[adamsboomer]

This seems low to me.  I push just over 6 with boost pump on, and 3.5 in the climb, 4 level.  I am flying at higher altitudes though with it leaned significantly.  Any chance you have moved to sea level and are having to run richer? Just $.02.

[Jim Peace]

This seems low to me.  I push just over 6 with boost pump on, and 3.5 in the climb, 4 level.  I am flying at higher altitudes though with it leaned significantly.  Any chance you have moved to sea level and are having to run richer? Just $.02.

This was filmed climbing through about 6500 feet on way to 8500.  Once at 8500 all was well....

 

I wrote Don Maxwell and sent the video...he wrote me back with this:

 

""I would check the gascolator gasket. It should be changed every year. Also the shaft on top of the gascolator may be leaking. You can check by dropping a few drops of oil on the shaft and see if that changes anything. The vent to the fuel tanks can have a dirt dobber nest in it. Check that the vents are clear. How old is the pump?""

[moodychief]

We thought my problem was the gascolator gasket because air was getting into the bowl. Spent a lot of labor checking and replacing it more than once. Check the fuel line mentioned in my previous post as it can mimic a bad gasket.