marcusku

another fuel pressure question

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I did do some searching first but haven't come up with the answer.  About a year ago I was getting some fluctuating pressure readings and generally the pressure was showing toward the bottom of the green.  The engine driven pump was replaced and the result was higher and more stable fuel pressures.  After replacing, every so often shortly after start up, fuel pressure will be slow to build.  When this happens it often also seems to take more cranking to start the engine and when hitting the boost pump prior to start up pressure builds more slowly and doesn't get as high.  .  If I hit the boost pump the pressure goes up to the middle of the green which is typical but then when I let go of the pump the pressure goes back down.  It's also happened on take off after shutting off the electric pump, pressure drops quickly but slowly comes back up.  On my last flight the pressure was reading close to the middle of the green and as an experiment I hit the boost pump, this brought up the pressure slightly as you'd expect but as soon as I turned it off the pressure quickly fell to the red line then slowly built back up to the middle.

 

Is it possible something weird is going on with the boost pump?  I tend to think it's not an indication error.  Any clues?

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Getting the expected indication on the FuelP gauge makes me feel that the instrument is at least showing the right trend...

FuelP... there are a few threads around here related to the fuel pressure exceeding the green arc... on some engine OHs...

The mechanical pump builds pressure in relation to the engine rpm... really slowly until the engine fires up...

The electric pump reacts quicker, because it isn’t related to the engine rpm...

+1 always in the green arc is generally good...

Sinusoidal quirkiness of actual pressure reading, after the electric pump is turned off... could be related to how fuel pressure is controlled...

PP thoughts only, wait for somebody with better fuel system experience...

Best regards,

-a-

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Is it possible air is getting into the fuel line somewhere?  

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10 hours ago, PT20J said:

Is it possible air is getting into the fuel line somewhere?  

I’d take a look at your gascolator orings and screen.  If those have not been replaced in a year or two- they might be the source of your problems (it’s also an easy maintenance item to perform). 

A nicked or swollen o-ring on the gascolator piston can pull a little air into the gascolator, which shows as fluctuations in the fuel pressure (or so I’ve found).

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Some good things to check.  Has anyone seen a problem with the electric pump causing some pressure fluctuations?  It's seem strange that immediately after shutting off the electric pump the pressure will quickly fall then slowly build back up with the engine pump running.

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  I have a similar problem, which is the FP looks OK at startup, drops to 20psi on takeoff and really starts to drop when I climb above 6000'. At 9000' it's about 15psi, just above redline. The engine runs smooth and all the JPI readings look good. Turning on the booster added maybe 2 psi.

  Some people suggest making sure the pump stays cool. There is a shroud you can buy at Aircraft Spruce to direct air around the pump. Other people thinks it's trash in the fuel strainers. Many swear that replacing the pump works wonders.

  My A&P doesn't see any problems. I had him replace the pump, which didn't cost too much (new Tempest $310) but I fear the labor is going to be multiples of that because the pump was hard to reach. Anyway, it didn't help.

  I honestly never looked that often at the fuel pressure as it was in the green, i.e. I don't think it was ever 25psi except at startup, otherwise it was just in the green somewhere. I'm not sure what to try next.

  To recap: I tried many different things, such as changing the throttle, prop speed, etc but the main factor seems to altitude. Could it be vapor in the lines at 10,000'? Maybe I shouldn't worry but an engine loss would ruin my whole day, especially in the mountains.

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  They checked that on the ground using a reference gauge. They checked the fuel flow by running the output into a bucket.

  When I run the boost pump at startup the pressure is good, about 25psi. It's only after takeoff roll that it drops and stays down. I'm starting to wonder if it's an air leak that only appears after the throttle is firewalled and it really starts sucking the juice. Maybe the gascolator? It was serviced at last annual.

  I'll call the A&P on Monday, but I have to wonder if it's air or vapor in the fuel lines. I'm not sure where the fuel pump is on the accessory case and whether cooling is an issue. I'll make another test flight this next week.

 

 

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In the other similar thread on FP issues...

How does the fuel injector system adjust for the change in altitude?

It sounds as if this adjustment isn’t occurring as expected...

PP thought that may not be very helpful... not a mechanic.

Best regards,

-a-

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  The main adjustment would be the mixture, which is a manual process. I'm concerned by the low fuel pressure, which I currently think might be due to an air leak. Another possibility might be that the fuel system is getting hot and causing bubbles in the fuel system, but I'm not sure that it would get that hot on first takeoff. If other people reported 15 psi as normal I'd feel better, but most report 20-25 psi.

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There are things that cause variations in fuel pressure...

Failure to build pressure is one...

Failure to maintain pressure is another...

Blockage is an interesting situation where pressure will be maintained ahead of the block, and low on the other side....

Leaks have a way of lessening pressure... but fuel would be showing up everywhere if half the normal pressure is lost.

It may be possible to have a pressure leak and have the fuel go somewhere else... Some Mooneys have fuel return lines back to the tanks...

Air leaking in, causes sensor difficulty, especially FF sensors... air bubbles and fuel vapor bubbles won’t be noticed by a pressure sensor... they will all coexist at the same pressure.

 

First, it is important to know your Fuel P sensor is working properly...

When discussing fuel pressure, it is important to mention what plane or engine you have... put some data next to your avatar...

Best regards,

-a-

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  It's an '81 M20J, IO-360-A3B6D, I'll have to look into doing the avatar stuff.

  The gauge was check against a mech gauge and found OK.

  I'm going to da plane this morning and going over stuff w/theA&P. Here's hoping!

 

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On 7/14/2019 at 8:21 AM, johnw413 said:

  The main adjustment would be the mixture, which is a manual process. I'm concerned by the low fuel pressure, which I currently think might be due to an air leak. Another possibility might be that the fuel system is getting hot and causing bubbles in the fuel system, but I'm not sure that it would get that hot on first takeoff. If other people reported 15 psi as normal I'd feel better, but most report 20-25 psi.

The mixture adjustment is only an idle setting along with the idle screw.   The way a fuel servo works is the for little tube along with the rubber bellows provide for altitude compensation by the ramming of air into the tubes.   Mixture is controlled by the red knob and the loose nut behind the yoke.

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You have a weak mechanical pump or air is getting sucked into the system somewhere.

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  OK,  just came back from latest test of the fuel pressure. Here is what I found:

  With new pump it reads about 25 psi at idle, maybe a little higher. On takeoff it drops to about 20-22psi. The airport elev is about 2,300'. As it climbs above 6000' the pressure drops until it's about 17-18 psi @8500'. Not as high as some have noted but above the red line of 14psi. Switching on the booster adds maybe 1-2psi. I'll call these results acceptable. I'm wondering if having the cowl flaps open helped ( somehow the linkage was messed up and I couldn't close them ).

  Another A&P wondered if the pump vent was clogged but he wasn't sure if that would cause the symptom, viz dropping FP with altitude.

  I've thrown over 1 amu at this problem. Replacing the fuel pump may have helped along with blowing out the fuel lines, etc. I'll need some more time to figure this out but I'm moving on to other ways to shower my baby with amus.

  Thanks ya'll!

 

 

 

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