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  #1  
Old 01-14-2016, 07:56 AM
jhk770 jhk770 is offline
 
Join Date: Aug 2015
Location: Morristown, NJ
Posts: 38
Default Intermittent Fuel pressure problem

I own a RV7A with an IO-360, fixed pitch prop. with a Dynon SkyView that I bought from the builder and have been having an annoying fuel pressure issue since I bought it. At WOT during takeoff and in a climb with the electric fuel pump on, fuel pressure seems fine (30-35psi) If I shut electric fuel pump off I sometimes get a drop in my fuel pressure and an audio "low fuel pressure" warning from the SkyView. Drops to about 12-14psi. This doesn't happen all the time. When this first happened, I thought it was the mechanical pump, so I replaced it with a new one and checked my filters. That didn?t solve the problem. Engine always runs strong and I haven't had any issues. A week ago, after preheating the engine with a red dragon heater, starting and waiting for oil temps to rise above 100 (OAT was 25F) I performed a run up and I was getting a high fuel pressure warning. I believe it was around 40-45+ PSI. without electric fuel pump on. I waited a while to observe engine performance. All seemed well. I decided to take off and stay in the pattern. After engine warmed to operating temps all seemed fine, fuel pressure came back to normal and I had an uneventful 2 hour flight.

My questions to you all, is: Since I already replaced the mechanical fuel pump, what should I check next? Does temperature effect anything? Why was I getting high fuel pressure in cold weather and low in regular temps? Is it possible my fuel lines are effected by different temperatures? Is it possible it is just a bad sensor? Also, when I turn on my electric fuel pump, according to the SkyView fuel flow goes up a bit. Is this common?

Thanks,

Jim
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  #2  
Old 01-14-2016, 10:41 AM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 6,328
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Jim, it would be best to know how the system is plumbed to get to a final solution. One thing to check: verify that the fuel pressure reading is accurate. Take off the sensor and note the type and part number. Then note if it has a local ground or the ground goes back to a central grounding point. You can either test the sensor within the plane system by attaching to air pressure with a good gage to compare to. Even mechanicals are inaccurate too, so be careful not to think it is traceable to NBS. An aux air tank with a ball valve can help in this comparison if you have to carry tools to the hangar. A good picture of the sensor would help if you can not identify it.

Once you do this report back and the more knowledgable will weigh in on the possible failure points to check. To the extent possible, it always best to diagnose a problem rather than replacing parts and flying.
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Last edited by BillL : 01-14-2016 at 09:55 PM.
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  #3  
Old 01-14-2016, 10:56 AM
alpinelakespilot2000 alpinelakespilot2000 is offline
 
Join Date: Jan 2005
Posts: 3,728
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Especially if you don't have the newer (2 or 3? wire) Kavlico fuel pressure sender, that's where I would start. The older versions without the 5V excitation wire have a reputation for being flaky. Mine included. What I found that seemed to solve erratic fuel pressure was one or more of the following:

(1) Bleed the fuel system by removing the oil pressure hose at the transducer and boost pumping fuel through it to remove any air.
(2) Take the transducer off and lightly tap it a few times on something hard.
(3) If the transducer relies on a non-wired ground, hose clamp a ground wire around the outside of the case and run it to a good airframe ground.

I've not had any problems in the last couple years since incrementally doing the above. Not sure which if any really helped, but so far so good.

Good luck.

PS, FWIW, mine is a lower pressure sensor for a carbed engine. Not sure if you might be using a different sender for IO's that might not have the problems mine was known to have.
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Last edited by alpinelakespilot2000 : 01-14-2016 at 10:58 AM.
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  #4  
Old 01-14-2016, 05:53 PM
RV7A Flyer's Avatar
RV7A Flyer RV7A Flyer is offline
 
Join Date: Jul 2013
Location: US
Posts: 2,529
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Quote:
Originally Posted by jhk770 View Post
I own a RV7A with an IO-360, fixed pitch prop. with a Dynon SkyView that I bought from the builder and have been having an annoying fuel pressure issue since I bought it. At WOT during takeoff and in a climb with the electric fuel pump on, fuel pressure seems fine (30-35psi) If I shut electric fuel pump off I sometimes get a drop in my fuel pressure and an audio "low fuel pressure" warning from the SkyView. Drops to about 12-14psi. This doesn't happen all the time. When this first happened, I thought it was the mechanical pump, so I replaced it with a new one and checked my filters. That didn’t solve the problem. Engine always runs strong and I haven't had any issues. A week ago, after preheating the engine with a red dragon heater, starting and waiting for oil temps to rise above 100 (OAT was 25F) I performed a run up and I was getting a high fuel pressure warning. I believe it was around 40-45+ PSI. without electric fuel pump on. I waited a while to observe engine performance. All seemed well. I decided to take off and stay in the pattern. After engine warmed to operating temps all seemed fine, fuel pressure came back to normal and I had an uneventful 2 hour flight.

My questions to you all, is: Since I already replaced the mechanical fuel pump, what should I check next? Does temperature effect anything? Why was I getting high fuel pressure in cold weather and low in regular temps? Is it possible my fuel lines are effected by different temperatures? Is it possible it is just a bad sensor? Also, when I turn on my electric fuel pump, according to the SkyView fuel flow goes up a bit. Is this common?

Thanks,

Jim
What kind of transducer? VDO or Kavlico? The VDOs suck. I had both oil pressure and fuel pressure transducers get flaky on me (I have a Skyview system, as you do), so replaced with Kavlicos and they've been fine. As you are, I'm FI (IO-360-M1B).

I have, on extremely rare occasions, had the fuel pressure drop as you describe...almost exactly as you describe: boost pump on, switch tanks, wait a few seconds while watching flow and pressure, boost pump on; then, maybe 5-15 seconds later, the pressure drops down to 12-14 PSI, I get the audio alert, which will really get your attention and your heart going, then it immediately goes back up to normal. I think this has happened 3 or 4 times in the last 3 years, and EACH TIME it scares the snot out of me for a second or two.

BUT...the engine continues without even a hint of a hiccup, and I suspect it's got to be some sort of "blurp" in the line for the newly-selected tank, which blasts on through and/or gets removed, no harm no foul. I don't know what else it could be. It's definitely not the engine-driven pump, and I don't think it's the transducer.
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  #5  
Old 01-14-2016, 08:46 PM
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Bruce Bruce is offline
 
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Replace fuel sensor.
End of problem.
Order 2
They fail often.
Boomer
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Last edited by Bruce : 01-14-2016 at 08:47 PM. Reason: Typo
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  #6  
Old 01-14-2016, 09:17 PM
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Jesse Jesse is offline
 
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Location: X35 - Ocala, FL
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It is not uncommon to have the fuel flow read higher with the boost pump on. In some planes I have seen as much as a 50% increase. The flow doesn't actually increase, it's just pulses or cavitation or something due to the location of e flow sensor in relation to the boost pump.
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  #7  
Old 01-15-2016, 12:49 AM
jay bell jay bell is offline
 
Join Date: Sep 2013
Location: Olds, AB
Posts: 86
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Hi Jim, I have had all those issues, and just started addressing the last one you mentioned about fuel flow increasing with boost pump. Apparently the problem is not uncommon.

I often get drop in fuel pressure without boost pump when I use Mogas. It is caused by vapour locking (aka flashing off), when you suck too hard on warm fuel. My triggers include winter blend Mogas on a warm day, high power taxiing on soft turf, slow flight on warm days, flying with less than 1/3 tank, and flying above 10,000'. Avgas has a higher Reid vapour pressure than Mogas, so it doesn't vapour lock As easily. I carry Avgas in my left tank in case the Mogas flashes. Symptoms of vapour lock are fluctuating fuel pressure followed by pressure drop to 20 psi and roughness, then to 15 psi, at which point engine runs poorly, then to 10 psi, where it is mostly quit. Switching to avgas or turning on boost pump resolves my problem. I once had mild vapour lock problems on avgas under extreme conditions (10 minute, 60 Kt slow flight on 90 degree day)

Lots of low wing planes require vapour lock solutions, especially those with turbochargers. Look at a Cherokee fuel pump and sump shroud for one example. Solutions include:

If you use Mogas, try pure avgas.
Smooth anything that makes it harder for engine pump to suck fuel including tank pickups, hard angle 90 and 45 degree fittings, fuel valve not opening properly, unions, couplers, kinked lines, sharp bends, excess fuel line length, and sections of too small fuel line. Then keep the fuel cool; light coloured paint on wings, reflective heat shield on fuel lines, exhaust heat shields to protect fuel lines, blast air shrouds on fuel pump and gascolator, extra cowl ventilation louvers. As you noticed, running lots of fuel helps because it has no time to heat. Managing CHT low, (330ish) and keeping airspeed up helps too.

Other responders had good comments on gauge accuracy. Just remember, if your engine runs rough, you have more than a gauge problem. If you must have boost pump to prevent vapour lock, if boost pump fails, you will become a glider
Jay
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