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Fuel Pressure intermittent…..ideas?

Robert Sailor

Well Known Member
So this issue has been going on for a long time and I want to solve it. I have an IO-320 Lycoming with 500+ hours. I purchased the aircraft 4 years ago and for the first year or so everything operated perfectly except the fuel flow transducer which I replaced.

As soon as I started long cross countries my current problem showed itself. I live on the Westcoast and unless I’m flying locally I’m climbing between 10-12000 feet. During this climb at high altitudes or more commonly when I reach cruising altitude and my cylinder head temps are hot, oil is hot and I start leaning more aggressively and reducing rpm my fuel pressure starts to drop. Normal pressure is about 23-25 lbs and it starts to drop to 10-12 lbs or even less. My EFIS alerts me and I throw on the electric boost pump, leave it on for a few minutes and my problem goes away only to return later or it may run for several hours with no issues. When I put on the boost pump the pressure rises immediately to about 28 lbs.

If I wasn’t over mountains I’d be tempted to leave the boost pump off to see what happens as the engine has never stumbled or lost rpm.

I’ve seen pressure drop to 6 or 7 lbs before I got on it but even then it was running fine. The boost pump trick has so far solved the issue BUT it shouldn’t be there.

My thoughts…it’s a mechanical engine driven pump, basically a diaphragm pump.
I had one fail on my Cessna 210 and it didn’t screw around, it failed, prop went full fine and I was pulled from my belts while I went through my normal sequence to get it running again….it’s what I imagine a diaphragm pump does at end of life.

So my friends say this….it’s a mechanical pump, it either works or it doesn’t.
They think the problem is with my transducer…I don’t buy into that, I of course agree that a mechanical pump usually works or it doesn’t however I don’t think there is anything wrong with the pressure transducer because when I hit the boost pump it immediately shows 28 lbs. if it was the transducer I’m sure it would show up when I turned the boost off and on, it doesn’t.

This situation is always associated with heat and higher altitude…..I’m stumped! Help me solve this if you can, id be very appreciative. Robert
 
Mechanical pumps do fail gradually - I’ve seen and had it happen. Slow degradation over years and hours. Can’t say for sure that it is your problem, but the fact that your pressure comes right up and steady with the boost pump suggests its not instrumentation, but a real pump problem…unless, of course, it drops with the boost pump on.

Some mechanical pumps last until engine TBO, and some fail at 500 hours -0 it happens. You could have a check valve going bad, or a diaphragm crack….nothing is perfect. Check to see if there is any sign of fuel in the drain line - it is there to tell you if a diaphragm is leaking.

Paul
 
This situation is always associated with heat and higher altitude…..I’m stumped! Help me solve this if you can, id be very appreciative. Robert

These two parameters would say something with vaporization or air leaks.

I would suggest you do a fuel system pressure leak test with air, from fuse bulkhead fitting forward to the FP outlet (AN plug or cap). Blow most fuel out then pressurize from the bulkhead end, use 30 psi with a trusted gage. Use a ball valve to shut off the air and then watch the gauge. If you watch it drop it is big, find the leak. I use a progressive approach to find that. Soapy solution etc are good detectors. Since the system will not have much volume, this leak test is pretty sensitive. I would leave it overnight for the final test. The several I have performed or assisted (7, 10 &14) all lasted the night after find a leaks. One had a leak in the boost pump on the inlet side. If that passes, then consider heat on the mechanical pump or the pump itself.

I have measured fuel temps and they get hotter than one might think, so a little air is a significant issue.

My 2 cents.
 
Mechanical pumps do fail gradually - I’ve seen and had it happen. Slow degradation over years and hours. Can’t say for sure that it is your problem, but the fact that your pressure comes right up and steady with the boost pump suggests its not instrumentation, but a real pump problem…unless, of course, it drops with the boost pump on.

Some mechanical pumps last until engine TBO, and some fail at 500 hours -0 it happens. You could have a check valve going bad, or a diaphragm crack….nothing is perfect. Check to see if there is any sign of fuel in the drain line - it is there to tell you if a diaphragm is leaking.

Paul

Thank you Paul for your observations and opinion, much appreciated
 
These two parameters would say something with vaporization or air leaks.

I would suggest you do a fuel system pressure leak test with air, from fuse bulkhead fitting forward to the FP outlet (AN plug or cap). Blow most fuel out then pressurize from the bulkhead end, use 30 psi with a trusted gage. Use a ball valve to shut off the air and then watch the gauge. If you watch it drop it is big, find the leak. I use a progressive approach to find that. Soapy solution etc are good detectors. Since the system will not have much volume, this leak test is pretty sensitive. I would leave it overnight for the final test. The several I have performed or assisted (7, 10 &14) all lasted the night after find a leaks. One had a leak in the boost pump on the inlet side. If that passes, then consider heat on the mechanical pump or the pump itself.

I have measured fuel temps and they get hotter than one might think, so a little air is a significant issue.

My 2 cents.

Thank you Bill for your thoughts and opinions, much appreciated.
 
You could have a leak anywhere from the engine pump to the fuel valve, assuming it happens on both pumps.
You didn't mention which fuel valve you have. If it is Van's brass valve one, then there is an O-ring in there that I have seen leak and cause this problem.



Vic
 
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so if there is a leak then isn't there going to be raw fuel all over at the sight of the leak? i would think that's a big leak if the pressure goes down 5 psi.
 
so if there is a leak then isn't there going to be raw fuel all over at the sight of the leak? i would think that's a big leak if the pressure goes down 5 psi.

Likely sucking air, Bob, then it gets hotter(expands) as it goes forward and then reduces the suction at the mechanical pump. I could also be some kind of restriction, acts more like a suction leak.
 
Air succion

so if there is a leak then isn't there going to be raw fuel all over at the sight of the leak? i would think that's a big leak if the pressure goes down 5 psi.

I had a air leak ( aspirating air at one gascolator fitting ) No fuel leak. No blue stain. Just saw bubble on the pressure test at 30 psi. Mecanical fuel pump was creating a negative pressure in this area when boost pump was off. Low presssure indication, returning back to normal after boost pump on. Rock solid since leak was corrected. Engine was fine most of the time at low pressure but once where i felt a lost of power for fraction of a second. ( bubble passing in the mecanical pump)
 
You could have a leak anywhere from the engine pump to the fuel valve, assuming it happens on both pumps.
You didn't mention which fuel valve you have. If it is Van's brass valve one, then there is an O-ring in there that I have seen leak and cause this problem.

Thanks Vic…I have the basic vans fuel selector, I’ve never noticed it losing pressure on the electric pump. When it loses pressure it’s always on the engine driven pump and when I select the boost pump it heads up to 28 lbs and stays there until I turn it off.
 
This kind of leak seems pretty challenging to find - anyone have any hints, besides just tightening all fittings? I can't see a good, safe way to pressurize the fuel lines before the pumps to test for leaks, but perhaps there is a clever way to do this.
 
This kind of leak seems pretty challenging to find - anyone have any hints, besides just tightening all fittings? I can't see a good, safe way to pressurize the fuel lines before the pumps to test for leaks, but perhaps there is a clever way to do this.

Mickey, I have done this test a few times using a connection for pressure gage and air input at the fuse fuel bulkhead fitting and a AN plug or cap FWF. Obviously the fuel tank is eliminated from the test. Originally, I rigged a servo bypass and plugged the hose at the fuel distributor (spider). If it leaks a progressive check works best to isolate the leak to a section and reduce the number of connections to check. Both sides are tested and fuel valve cycled.

BTW - I bench tested my fuel pump assembly as it is the bundle of tubes style design. And was very happy no leaks in the Andair valve area, it is tight on my 7.

Use of a non corrosive, bubbling leak detector is recommended, just tightening the B nuts is a bad idea.

Is there some reason you see it unsafe?
 
Mickey, I have done this test a few times using a connection for pressure gage and air input at the fuse fuel bulkhead fitting and a AN plug or cap FWF. Obviously the fuel tank is eliminated from the test. Originally, I rigged a servo bypass and plugged the hose at the fuel distributor (spider). If it leaks a progressive check works best to isolate the leak to a section and reduce the number of connections to check. Both sides are tested and fuel valve cycled.

BTW - I bench tested my fuel pump assembly as it is the bundle of tubes style design. And was very happy no leaks in the Andair valve area, it is tight on my 7.

Use of a non corrosive, bubbling leak detector is recommended, just tightening the B nuts is a bad idea.

Is there some reason you see it unsafe?

Sounds like you are doing it in a safe way since you are isolating the fuel tanks to avoid accidental over-pressurization. I've read some people use a lighter! :eek:

I don't suspect a leak in mine, but I have not tested it, which is why I'm curious about the various techniques people have used. I've only tested by pressurizing using the electric fuel pump, but that won't catch leaks between that pump and the fuel tank pickup.
 
I have the exact same problem with my new IO 360 (100 HRS). Runs fine on the ground. In flight every now and then the fuel pressure with the electric fuel pump off the fuel pressure would drop to 6-7 psi, turning the electric pump on would birng the fuel pressure back to 27 psi. At high altitude (11,000 ft) engine would cut out. Can not duplicate the problem on the ground. Thought it was the engine driven fuel pump(was new). Sent it back and put a new one on, same problem. They (I think Tempest) bench tested my old one, said it was fine.
I will check O ring since I have the Vans fuel valve and do a pressure test.

Gregg
 
You could have a leak anywhere from the engine pump to the fuel valve, assuming it happens on both pumps.
You didn't mention which fuel valve you have. If it is Van's brass valve one, then there is an O-ring in there that I have seen leak and cause this problem.

Thanks Vic…I have the basic vans fuel selector, I’ve never noticed it losing pressure on the electric pump. When it loses pressure it’s always on the engine driven pump and when I select the boost pump it heads up to 28 lbs and stays there until I turn it off.

That's normal. The electric boost pump will help to hide system leaks on the suction side.

To answer someone else's question, sometimes you might see a blue fuel stain, and sometimes not. It is easier for the pump to suck air than fuel, so even a miniscule leak at a fitting will allow it to suck air.

Vic
 
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Given that this issue tends to happen at a combination of high altitudes and high oil temperatures, I would be leaning toward the fact that it's a fuel vapor pressure related issue. Given that the mechanical fuel pump is bolted to the back of the engine and is oil lubricated, it is essentially at the same temperature as the oil. As the fuel is drawn up into the engine, as it approaches the pump is will be heated, especially as you start leaning, because now your fuel flow rate has gone down, increasing the dwell time of the fuel in the line and subsequent temperature, thus increasing the likely hood of reaching the vapor pressure. The situation is worsened sometimes by full tanks. If you fill the tanks before departure, then taxi on slightly uneven ground, fuel will slosh into the vent lines and run down to the low point near the wing root. When the engine is running, this fuel becomes a restriction that air must pull past to get into your tanks to replace the fuel consumed. Due to the small diameter in the vent lines and the surface tension of the fuel, the air will struggle to get past until there is a large enough vacuum in the tanks, at which point the air will pull past the fuel in one big slug, which you will notice by the fuel pressure rapidly rising again. The situation is worsened further if the aircraft has been sitting on the ground and the fuel is already heat soaked, meaning that it is more likely to become vapor when drawn in by the mechanical fuel pump. Eventually the fuel in the vent lines will fully vaporize and pressure fluctuations will go away, even at altitude. If the fuel is hot enough, altitude high enough or you are using a high Reed Vapor Pressure fuel like mogas, this may result in surging of the engine. The immediate fix is to turn on the boost pump, as this is cold and situated low in the aircraft, so very unlikely to have a vapor pressure issue on its own. The longer term fix is quite a bit more difficult. You can try to reduce your oil temperature, or adding external blast cooling to the mechanical fuel pump. Keeping the lines and fuel as cool as possible between the tanks and the fuel pump will also help, by making sure your tanks are painted white, minimizing heat gain with heat shields and insulation and making sure there are only swept bends (not sharp bends) where possible between the tanks and the pump.
Tom.
RV-7
 
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