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Strange fuel pressure drop issue

edclee

Well Known Member
Patron
Problem Found !!!! Strange fuel pressure drop issue

I flew the RV-9A(mechanical fuel injection) to Savannah this morning and after about 30 minutes of flying noticed the RPM fluctuating somewhat. I noticed that the fuel pressure was also varying and turned on the boost pump. Pressure came back up to 30 psi. Turned the boost pump back off and the fuel pressure dropped a bit and it was variable. Fuel flow reduced whenever fuel pressure got to 20 psi or so, Boost pump back on and all is good. I left it on for 5 or 10 minutes. Turned it off, Same drop in fuel pressure. Changed tanks, and same symptoms. This went on for another maybe 10 minutes then when I turned the boost pump off, the fuel pressure dropped to 26 psi and stayed there for the rest of the flight, No varying. I left Savannah and returned home late morning and all was good until I turned off the boost pump. Pressure started varying all over again just like before. After about 45 minutes of playing with the boost on, boost off, I turned off the boost and like before, the fuel pressure went to 26 psi and stayed there. No change in throttle position or altitude or mixture strength affected these symptoms.

Any words of wisdom?
Ed :confused:
 
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More details needed. Anything unique about your fuel system config? Are you using aviation fuel? etc.

Making some assumptions:

Doubtful = fuel system leak prior to main fuel pump
More likely = Your main fuel pump is on it's way out.

Will be interesting to see other's thoughts/ideas.
 
Fuel Pressure

More details needed. Anything unique about your fuel system config? Are you using aviation fuel? etc.

Making some assumptions:

Doubtful = fuel system leak prior to main fuel pump
More likely = Your main fuel pump is on it's way out.

Will be interesting to see other's thoughts/ideas.

System is per Vans plans, nothing unusual. I run mix of Avgas and 90 octane non ethanol mogas. Have run this system and this fuel for over 100 hours with no changes and no issues.
 
sounds like a failing fuel pump. I would not test fate with another flight before either replacing it or finding an alternate reason that the mechanical pump won't work but the boost pump does, like air introduction upstream (less likely due to variable nature or excess heat causing vapor lock in the pump.

Larry
 
It's changeover season and the possible winter 90 octane is boiling. What was OAT and altitudes?

I run similar, 50/50 ratio, noted similar- however, Carb'd. Went to 9500 to check for it and found it, just barely- electric boost pump cures it and I had to climb hard to find it.

Does not happen below 8500, corrected to standard day. I used to own a 15psi pressure carb Pitts that would do it a bit at 6500' on up under same conditions/blend.

The REC gas is great to thin down unneeded lead without risking ethanol, but it does have the negative of vapor pressure in the spring changeover season if you have the winter blend in warmer weather.

I'd jiggler siphon one tank dry and try pure Avgas to confirm it's not a mechanical fuel pump issue. Save the other stuff for lawnmowing.

IF it shows up on Avgas it's the mechanical pump or the always possible air leak into the fuel stream.
 
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It's changeover season and the possible winter 90 octane is boiling. What was OAT and altitudes?

I run similar, 50/50 ratio, noted similar- however, Carb'd. Went to 9500 to check for it and found it, just barely- electric boost pump cures it and I had to climb hard to find it.

Does not happen below 8500, corrected to standard day. I used to own a 15psi pressure carb Pitts that would do it a bit at 6500' on up under same conditions/blend.

The REC gas is great to thin down unneeded lead without risking ethanol, but it does have the negative of vapor pressure in the spring changeover season if you have the winter blend in warmer weather.

I'd jiggler siphon one tank dry and try pure Avgas to confirm it's not a mechanical fuel pump issue. Save the other stuff for lawnmowing.

IF it shows up on Avgas it's the mechanical pump or the always possible air leak into the fuel stream.

+1

I missed the part about the mix. No real mogas experience here to help. Though I would expect that 50% avgas would eliminate any vapor lock issues, especially with spring temps vs summer. Have to guess that temps at 8000 were still relatively low and problem occurred well after the heat from climb was gone. The vapor lock issue seem to happen as folks level off in cruise and go away after several minutes when things cool down.

I still vote failed pump.
 
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I would run it with 100% avgas. Sure sounds like fuel vaporizing and quits when the engine has fully cooled in cruise. We’re you higher than you normally fly and how old was the fuel?
 
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I was also thinking about RVP issue. Would love to experiment with this one but not worth it.

In a different application, you'd fill one tank with 100LL and see if it goes away. Accidents tend to be a combo of things. Winter blend on a hot day, maybe a filter (or two) that is unusually dirty creating a high DP, extra head lose to the main pump, etc.

Easiest place to start would be looking for leaks in the line. Also see if it will replicate on the ground. All said, I wouldn't fly it until the root cause is determined.

Best Vegas odds still on the main fuel pump. Second and third are close with a suction line leak or VP issue.

IIRC, car gas VP is ~10 psia @100degF. Taking 3-4 pounds (4-5 including suction head elevation loss) out of that low a flow is pretty tough if temps are maintained. Of course, one hot gas leak or radiant shield loss in the wrong spot and that local VP number rises quickly.

Let us know what you find, Stay safe.
 
I have run this mix in temps of 100 degrees F and cyl hd temps at 400, while climbing and no issues in the past. It gets really hot here in SC in August. I have flown this mixture in the summer at 14000 ft with no issues. I cannot believe it is boiling fuel or RVP issues. I think the suggestions of a bad fuel pump or possibly an air leak between the boost pump and engine pump. I will investigate the air leak and if no evidence found, will just replace the pump.
Ed



I was also thinking about RVP issue. Would love to experiment with this one but not worth it.

In a different application, you'd fill one tank with 100LL and see if it goes away. Accidents tend to be a combo of things. Winter blend on a hot day, maybe a filter (or two) that is unusually dirty creating a high DP, extra head lose to the main pump, etc.

Easiest place to start would be looking for leaks in the line. Also see if it will replicate on the ground. All said, I wouldn't fly it until the root cause is determined.

Best Vegas odds still on the main fuel pump. Second and third are close with a suction line leak or VP issue.

IIRC, car gas VP is ~10 psia @100degF. Taking 3-4 pounds (4-5 including suction head elevation loss) out of that low a flow is pretty tough if temps are maintained. Of course, one hot gas leak or radiant shield loss in the wrong spot and that local VP number rises quickly.

Let us know what you find, Stay safe.
 
The summer cut of REC 90, 50/50 with Avgas, I find no issues to 10,500'- msl, hotter/higher DA in summer heat. The winter stuff, this time of year, seems to always catch folks come Sun N Fun season.

Pumps, filters, vacuum leaks can always happen- but it's vapor lock season.
 
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Problem Found, Strange Fuel Pump Issue

Well those of you who voted for the RVP being the problem win a Kewpie doll!. After a few hours yesterday and this morning looking over the entire fuel system for leaks, loose connections, pressurizing with the boost pump with the cowling off, checking vent lines on the tanks, etc. and researching fuel pumps and finding little help I decided to drain the right tank, fly over to the local airport on the left tank, and fill the right tank with Avgas, then test fly. I was able to successfully prove that the problem would not occur on Avgas and would reliably occur on the mix I have in my hangar tank, which is winter blend mogas and Avgas. The Reid Vapor Pressure of the mogas must be damned high. I have run mogas mix in much hotter weather and very high altitudes before with no problem but never with mogas purchased in the cold winter and left over in the hot weather.
Ed
 
Wow. Bullet dodged. I would have had a higher suspicion if you had straight Mogas. A fifty-fifty blend is a bit of a surprise.

I’d consider trying to find what the local temps are around the fuel line before considering anything other than 100%. It may be getting much hotter there than expected.

Can any contributors explain the relationship that mogas/avgas blends have on RVP?
 
Glad to help and keep this topic going each spring.

Thank you for checking mechanicals first and then posting your outcome.
 
Vpor Lock Solution

I have an independent fuel pressure sensor feeding a timer circuit that turns on my Aux fuel pump whenever it senses a low fuel pressure to the fuel body. Once a low fuel pressure is sensed, the pump is turned on until fuel pressure at the fuel body is normal, plus 10 seconds. Totally automatic, just like your car! I run 93 Oct E10 gas 100% of the time with NO issues!

Well those of you who voted for the RVP being the problem win a Kewpie doll!. After a few hours yesterday and this morning looking over the entire fuel system for leaks, loose connections, pressurizing with the boost pump with the cowling off, checking vent lines on the tanks, etc. and researching fuel pumps and finding little help I decided to drain the right tank, fly over to the local airport on the left tank, and fill the right tank with Avgas, then test fly. I was able to successfully prove that the problem would not occur on Avgas and would reliably occur on the mix I have in my hangar tank, which is winter blend mogas and Avgas. The Reid Vapor Pressure of the mogas must be damned high. I have run mogas mix in much hotter weather and very high altitudes before with no problem but never with mogas purchased in the cold winter and left over in the hot weather.
Ed
 
Using round numbers and assuming everything else remains the same:

Fuel line fluid velocity is ~53ft/sec at 10gpm
The head difference of ~ 2 ft causes a head differential of ~0.65 psi (SG of 0.75)
That small of a delta p causes a phase change in Mogas but not Avgas.

I get more respect for the associated boundary conditions and design margins every day. I’ll say again, bullet dodged.
 
Fuel Pump

I have an independent fuel pressure sensor feeding a timer circuit that turns on my Aux fuel pump whenever it senses a low fuel pressure to the fuel body. Once a low fuel pressure is sensed, the pump is turned on until fuel pressure at the fuel body is normal, plus 10 seconds. Totally automatic, just like your car! I run 93 Oct E10 gas 100% of the time with NO issues!

Fred, did you make this device? If so can you share the design? It is a great idea. By the way, for those who thought my mix was 50/50, it is not. My mix is 80% mogas, 20% avgas. Average Octane is 92. Still have never had a detonation issue (8.5:1 CR) but clearly the RVP is a problem, at least with this winter mogas.
Ed
 
I used my 50/50 numbers and carb/low pressure example data, plus a hotter run PS5C 15psi pressure carb'd Pitts/non- RV plane. I have not gone below 50/50 myself. I prefer diluting the lead over saving $.

80/20 I'd expect just what you experienced- on winter blended Mogas at 20-30 psi.
 
Does the Reid Vapor Pressure problem with winter blend mogas go away if you are running a fuel injection system at 45 to 55 psi, instead of carb?
 
It is a pump suction side phenomenon. Related losses are proportional to the square of the flow (all other things kept constant). Downstream pressure really doesn’t come into play.
 
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