High fuel pressure

[N6758N]

I have about 130hrs on my O-360 since overhaul. When I did the engine, I also installed an overhauled carburetor, new fuel pump, and new fluid lines at that time. Consistently since overhaul I see high fuel pressure readings from the JPI, around 6-6.5 psi at low power settings (below about 1500 rpm), once airborne at higher power settings this problem goes away, although I still see a lot of fluctuations on the fuel pressure readings off of the JPI (from 2.5ish to 4 psi erratically). I have talked to both Tempest who makes the fuel pump and Marvel-Schebler who makes the carb, each ones seems to think its the other parties fault. I already have swapped out the fuel pump and this made no difference.  At this point I am thinking the carb might have an issue with the float causing the high readings. I do not think it is just an indication problem as the high readings at idle seems to make the engine run crappy during mag checks  ( aggressively leaned and I have checked mag timing about 3 times now). I'm an IA and I am scratching my head on this one, any one else experience something similar? 

[Oscar Avalle]

5 minutes ago, N6758N said:

I have about 130hrs on my O-360 since overhaul. When I did the engine, I also installed an overhauled carburetor, new fuel pump, and new fluid lines at that time. Consistently since overhaul I see high fuel pressure readings from the JPI, around 6-6.5 psi at low power settings (below about 1500 rpm), once airborne at higher power settings this problem goes away, although I still see a lot of fluctuations on the fuel pressure readings off of the JPI (from 2.5ish to 4 psi erratically). I have talked to both Tempest who makes the fuel pump and Marvel-Schebler who makes the carb, each ones seems to think its the other parties fault. I already have swapped out the fuel pump and this made no difference.  At this point I am thinking the carb might have an issue with the float causing the high readings. I do not think it is just an indication problem as the high readings at idle seems to make the engine run crappy during mag checks  ( aggressively leaned and I have checked mag timing about 3 times now). I'm an IA and I am scratching my head on this one, any one else experience something similar? 

I am experiencing the same situation with my JPI 900, specially with regard to the high fuel pressure. However, I don't see any low power fluctuations. I am monitoring the situation, so if you find out what is going on let me know please.

[carusoam]

T,

Are you seeing a change in pressure due to climb attitude and/or leveling off?

I recall seeing a change related to attitude in my M20C.  The fuel pressure scale is low and not very broad.  A one psi change can really show up.

The vertical distance between the pump and the pressure sensor is rumored to account for this oddity.  It would be a head pressure change as the head changes...

PP ideas, not explained very well....

Best regards,

-a-

[N6758N]

Just now, Oscar Avalle said:

I am experiencing the same situation with my JPI 900, specially with regard to the high fuel pressure. However, I don't see any low power fluctuations. I am monitoring the situation, so if you find out what is going on let me know please.

Oscar,

What are your symptoms? Does it only show a high reading at low power settings? I haven't tried calling JPI yet as their customer service isn't always great. 

[Oscar Avalle]

Just now, N6758N said:

Oscar,

What are your symptoms? Does it only show a high reading at low power settings? I haven't tried calling JPI yet as their customer service isn't always great. 

High pressure at low power settings. But again it is in the range of 6.1 to 6.5. 

[N6758N]

1 minute ago, carusoam said:

T,

Are you seeing a change in pressure due to climb attitude and/or leveling off?

I recall seeing a change related to attitude in my M20C.  The fuel pressure scale is low and not very broad.  A one psi change can really show up.

The vertical distance between the pump and the pressure sensor is rumored to account for this oddity.  It would be a head pressure change as the head changes...

PP ideas, not explained very well....

Best regards,

-a-

Anthony,

No changes due to altitude, these numbers are consistent no matter what phase of flight. You do bring up an interesting point about the length of line however, something I didnt consider yet. The fuel pressure reading is taken off the carburetor and mine has a pretty long hose running up to the firewall where the transducer is mounted. 

[neilpilot]

Just now, Oscar Avalle said:

High pressure at low power settings. But again it is in the range of 6.1 to 6.5. 

We see similar high pressure readings at low power, sometimes even when on the mechanical pump only.  Engine operation is smooth, and runs at the mid-to-upper green in cruise.

[N6758N]

2 minutes ago, Oscar Avalle said:

High pressure at low power settings. But again it is in the range of 6.1 to 6.5. 

That is where mine is at as well, sometimes it gets close to 7psi. I believe the book says 6 should be the max. 

[carusoam]

For consideration...

What is the accuracy of the original gauge that is used?

how many hash lines are there between five and six psi?

I am wondering if it may be a limitation written in the POH based on the accuracy of the pilot reading the gauge.

Now the gauge has been slightly moved and the gauge has been improved to reading tenths of a psi.

Also consider what the effects of increased pressure are on the system.... spray pattern, possible seal leaks, that kind of thing...

With such low pressure, it is unlikely to have a real or significant change...

PP thoughts with an engineering twist. Not a mechanic.

Best regards,

-a-

[Hank]

3 hours ago, carusoam said:

For consideration...

What is the accuracy of the original gauge that is used?

how many hash lines are there between five and six psi?

I am wondering if it may be a limitation written in the POH based on the accuracy of the pilot reading the gauge.

Now the gauge has been slightly moved and the gauge has been improved to reading tenths of a psi.

Also consider what the effects of increased pressure are on the system.... spray pattern, possible seal leaks, that kind of thing...

With such low pressure, it is unlikely to have a real or significant change...

PP thoughts with an engineering twist. Not a mechanic.

Best regards,

-a-

I can't tell if there are any hash marks between 5 and 6. Mine runs here, but sometimes drops to the bottom of the wide stripe. 

[N6758N]

25 minutes ago, Hank said:

I can't tell if there are any hash marks between 5 and 6. Mine runs here, but sometimes drops to the bottom of the wide stripe. 

Thanks Hank, yours is about where I would expect it to be. Mine does not usually run that high in cruise. Another interesting thing is that my carb appears to be leaking due to the high pressure. There are blue stains on the outside. 

[carusoam]

Thanks Hank for posting the photo.

the one hash mark it has didn't even get a 5 next to it...

Back in the day when I had memory and an M20C.  I calculated the height of a colum of 100LL need to achieve 1, 2, 3...6 pounds. 

I believe I explained the change in FP (to myself) from climbing to cruising this way...

 

This looks like a two part problem.  One part is the variation of pressure in flight.  The other is the pressure exceeding red line.

 

Is there some form of regulator that can be adjusted or exchanged for fuel pressure?

 

From my M20C POH...

Red line (minimum) 0.5 PSI 

Red line (maximum) 6.0 PSI

Green Arc -- Wide (Normal operating range) 2.5 to 3.5 PSI

Green Arc -- Narrow Operating range) 0.5 to 6.0 PSI               (I copied each character as it is in the POH, something is weird here)

 

Try to determine why the FP exceeds the redline and get it adjusted.  It is possible that the pump is too strong for some reason or the pressure regulator isn't adjusted properly or there is a restriction in the line that is different than it is supposed to be.

Would the pump get stronger as the diaphragm gets aged and more rigid?

Private pilot ideas only.  Not a mechanic...

Best regards,

-a-

[DXB]

I am curious about this issue.  My fuel pressure has always been high at low rpm. When turning boost pump on,  it gets pegged at high end of scale.  The analog gauge and the JPI that replaced it behaved identically so I am sure it is not the gauge.  My MSC thought it was insignificant at annual last year so I started ignoring it - the pressures in the carb'd planes seem to be really quite low overall regardless.  I don't have blue stains on my carb or rough runups though, even when full rich.  I'd be curious to know what you find.

[N6758N]

7 hours ago, carusoam said:

Thanks Hank for posting the photo.

the one hash mark it has didn't even get a 5 next to it...

Back in the day when I had memory and an M20C.  I calculated the height of a colum of 100LL need to achieve 1, 2, 3...6 pounds. 

I believe I explained the change in FP (to myself) from climbing to cruising this way...

 

This looks like a two part problem.  One part is the variation of pressure in flight.  The other is the pressure exceeding red line.

 

Is there some form of regulator that can be adjusted or exchanged for fuel pressure?

 

From my M20C POH...

Red line (minimum) 0.5 PSI 

Red line (maximum) 6.0 PSI

Green Arc -- Wide (Normal operating range) 2.5 to 3.5 PSI

Green Arc -- Narrow Operating range) 0.5 to 6.0 PSI               (I copied each character as it is in the POH, something is weird here)

 

Try to determine why the FP exceeds the redline and get it adjusted.  It is possible that the pump is too strong for some reason or the pressure regulator isn't adjusted properly or there is a restriction in the line that is different than it is supposed to be.

Would the pump get stronger as the diaphragm gets aged and more rigid?

Private pilot ideas only.  Not a mechanic...

Best regards,

-a-

There is no external adjustment for fuel pressure, the way it works is the engine driven pump is supplying fuel to the carb, the fuel pressure hose is tapped off of a part on the carb which supplies the transducer with its reading. An older pump would theoretically loose a little pressure if anything. I have already swapped the fuel pump so I can eliminate that variable. I understand why the pressure is higher at lower rpms as is there is less demand on the system so there is an overabundance of fuel. Maybe its all in my head since other seem to have similar issues. But the fact that mine is almost 1 psi over the max limit and my carb appears to be leaking due to that is what has me puzzled. 

[N201MKTurbo]

The pressure of the mechanical pump is set by the pressure of the spring in the pump and the pressure in the crankcase.

[N6758N]

11 minutes ago, N201MKTurbo said:

The pressure of the mechanical pump is set by the pressure of the spring in the pump and the pressure in the crankcase.

Crankcase pressure affecting fuel pressure? I hadn't thought of this.

[DXB]

This is getting really interesting.  Here's a couple of reported cases of a blocked breather tube causing engine failure related to excess fuel pressure:

http://forum.canardaviation.com/archive/index.php?t-138.html

"The extra pressure in the crankcase affected the mechanical fuel pump by adding additional pressure to the top of the diaphragm. The extra pressure, caused the pump to increase fuel pressure beyond the carbs needle and seats ability to stop it from entering the float bowl. Once the float bowl became filled it would run out the float bowl breather port directly into the venturi throat. This caused the engine to become flooded with fuel and drowned out the plugs."

I can't say I know enough about how the engine driven pump works to grasp this yet but look forward to learning.  It does give me pause to know that the pressure can get strong enough to overwhelm the ability of the carb float to keep it out of the bowl!  I mainly see the a pegged high fuel pressure reading  at low power with boost pump on.  Killing the engine by flooding the carb is kinda the opposite of what I'm going for when I turn the boost pump on during the GUMPS check 

[N6758N]

4 minutes ago, DXB said:

This is getting really interesting.  Here's a couple of reported cases of a blocked breather tube causing engine failure related to excess fuel pressure:

http://forum.canardaviation.com/archive/index.php?t-138.html

"The extra pressure in the crankcase affected the mechanical fuel pump by adding additional pressure to the top of the diaphragm. The extra pressure, caused the pump to increase fuel pressure beyond the carbs needle and seats ability to stop it from entering the float bowl. Once the float bowl became filled it would run out the float bowl breather port directly into the venturi throat. This caused the engine to become flooded with fuel and drowned out the plugs."

I can't say I know enough about how the engine driven pump works to grasp this yet but look forward to learning.  It does give me pause to know that the pressure can get strong enough to overwhelm the ability of the carb float to keep it out of the bowl!  I mainly see the a pegged high fuel pressure reading with at low power with boost pump on.  Killing the engine by flooding the carb is kinda the opposite of what I'm going for when I turn the boost pump on during the GUMPS check 

Interesting find Dev. I am pretty sure my crankcase breather isn't totally blocked, but I wonder if I could have something restricting it which is causing higher than normal crankcase pressures...My boost pump doesn't seem to have a noticeable affect on fuel pressure at low power settings ( it is already high enough as is) I might have to pull the breather line and check for restrictions. 

[N201MKTurbo]

The mechanical pump works by the spring driving the diaphragm down. The fuel is below the diaphragm in a chamber with two one way valves. One to let fuel into the chamber and one to let fuel out. The pressure in the chamber is created by the spring, but seeing that the spring chamber is connected to the crankcase any pressure in the crankcase will add additional pressure to the diaphragm and hence the fuel in the lower chamber. 

The fuel is not pumped out of the pump by the engine, the only thing forcing fuel out of the pump is the spring and the crankcase pressure. What the engine does is lift the diaphragm and spring, drawing fuel into the pump and then releasing the diaphragm to let the spring push the fuel out.

If there is no fuel flow the diaphragm never descends from its lifted position, so it will just sit there waiting for some fuel to flow. If there is fuel flow, the spring pushes he diaphragm down providing pressurized fuel to the carb or FI system. Before the diaphragm bottoms out the engine will again lift the diaphragm replenishing the lower chamber and the cycle continues.

 

The diagram below shows a cutaway of the pump. It is upside down from the typical aircraft illustration. The important part is the lever pushes the rod down in this case by pushing on a shoulder on the rod. Once the diaphragm is pulled down the lever comes off of the shoulder on its up stroke. When fuel is pushed out of the chamber by the spring the lever will once again pull the diaphragm down drawing fuel into the chamber then coming off the shoulder letting the spring push it out. 

Edited January 4, 2017 by N201MKTurbo

[N201MKTurbo]

One more thing, On a turbocharged pump, the lift rod has a seal and the spring chamber is vented to the outlet of the turbo to maintain a constant pressure with altitude changes.

[captainglen]

  You are probably overthinking the problem. The fuel pump has no pressure regulator and puts out a fixed volume per stroke. As the pressure climbs it will eventually overcome spring pressure. The carburetor feeds the engine the fuel it needs for any given power setting which at idle is quite low and the float valve remains in a nearly closed position causing the pressure to increase. If the carburetor were at fault and fuel flow too low you would have high EGT.

  Your POH start checklist just asks you to verify you have fuel pressure after engine start; your runup checklist asks you to verify all instruments are in green arc. If these conditions are met the aircraft is fit to fly but if any non green arc readings are observed in flight this is to be considered a non airworthy condition. As the springs in your brand new fuel pump wear in the idle pressure will come down.

[Reflex]

Very interested to hear more about this topic.  My M20C just started doing this very thing.  What is concerning is that until a week ago, fuel pressures remained in the green at all settings.  The problem was noticed when I turned on the fuel pump prior to landing.  When switched on, the pressure reading was right at 8.5psi.

I decided to stay up and "tinker" with various throttle settings.  Here's what I found:

1) At cruise settings fuel pressure was at the high end of the green arc (roughly 5.8 psi).  Normally I see about 4.5 - 5 PSI.

2) When power is pulled back, fuel pressure increases over the red line (6.0 psi)

3) On the ground at low power settings, fuel pressure is roughly 7.5 psi.

The engine runs smoothly and transitions well.  EGT's are in the normal range and can be adjusted with the mixture and I see no change in how much I must move the mixture to achieve the desired EGT.

One other observation.  Yesterday, after the aircraft had been sitting for a week, I made sure the throttle and mixture were all the way back and then flipped on the Fuel pump without the engine running.  My thought was that with everything shut down and the throttle and mixture all the way back, I should have been able to repeat the problem.  No demand should equal high fuel pressure.  With only the pump running, I got a reading of 5 psi.  Exactly where it normally is before start-up.

321 SMOH

I haven't flown the aircraft since discovering the problem  I've contacted my mechanic and he's scratching his head.  Again, everything was fine until a week ago.  Had this been the norm for this aircraft, I wouldn't be quite as worried.  I consider this aircraft not to be airworthy at this time.

I look forward to additional thoughts about this problem.

Reflex

Edited January 9, 2017 by Reflex

[DXB]

11 minutes ago, Reflex said:

Very interested to hear more about this topic.  My M20C just started doing this very thing.  What is concerning is that until a week ago, fuel pressures remained in the green at all settings.  The problem was noticed when I turned on the fuel pump prior to landing.  When switched on, the pressure reading was right at 8.5psi.

I decided to stay up and "tinker" with various throttle settings.  Here's what I found:

1) At cruise settings fuel pressure was at the high end of the green arc (roughly 5.8 psi).  Normally I see about 4.5 - 5 PSI.

2) When power is pulled back, fuel pressure increases over the red line (6.0 psi)

3) On the ground at low power settings, fuel pressure is roughly 7.5 psi.

The engine runs smoothly and transitions well.  EGT's are in the normal range and can be adjusted with the mixture and I see no change in how much I must move the mixture to achieve the desired EGT.

One other observation.  Yesterday, after the aircraft had been sitting for a week, I made sure the throttle and mixture were all the way back and then flipped on the Fuel pump without the engine running.  My thought was that with everything shut down and the throttle and mixture all the way back, I should have been able to repeat the problem.  No demand should equal high fuel pressure.  With only the pump running, I got a reading of 5 psi.  Exactly where it normally is before start-up.

321 SMOH

I haven't flown the aircraft since discovering the problem  I've contacted my mechanic and he's scratching his head.  Again, everything was fine until a week ago.  Had this been the norm for this aircraft, I wouldn't be quite as worried.  I consider this aircraft not to be airworthy at this time.

I look forward to additional thoughts about this problem.

Reflex

This new behavior of your plane matches my C model's behavior precisely for the past 2 years I've owned.  Though I have given up on investigating for now, I may be inspired to do some more if you find an explanation.   

[N6758N]

I haven't had any time lately to troubleshoot this anymore. I plan on diving into it deeper when I do my bladder install in the next few months.

[carusoam]

Is this at all related to the very cold temperatures lately?

Best regards,

-a-