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Symptoms of a dying fuel pump?


S3flyer

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I had the new Italian fuel pump installed at my annual approximately 4 flight hours ago. Don't actually recall the pressure readings but were all steady and over 4.0 for those hours. Went up today with everything safely in the green but on climb-out the pressure dropped slowly from 4.8 to around 1.9. I leveled off and the fuel pressure went back up to a steady ~4.5. I went up to 5500rpm in level flight with steep banks and the fuel pressure remained steady. Started a climb at full throttle and the pressure slowly dropped again. Landed and went through several 4500rpm run-ups and pressure varied between 4.5 and 5.2.

 

The engine ran fine at all times.

 

I have the Dynon Kavlico fuel pressure sensor remotely mounted.

 

Checked the various grounds on my S3 and all were tight and clean. No detectable fuel leaks.

 

Any ideas?

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How full were your tanks? Was your fuel flow consistent at full power in level flight, and in a climb? If the pump isn't getting fuel quickly enough, the pressure will drop.

 

Fuel pumps also have a relief valve that is always in use. They are calibrated for an upper pressure limit before they open, and the pumps always exceed that pressure limit (ensures uninterrupted pressure to the engine). The fuel that is relieved is routed back to the inlet of the pump (or in big planes, back to the tanks).

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I have a Sting with a single belly tank that was over 3/4 full. I don't have a fuel flow sensor but fuel pressure was constant in level flight, even at 30 degree bank, and with varying RPM.

 

Do you have a boost pump? You should be using it on a climb. Coordinated banks and level flight are virtually the same thing when it comes to weight distribution of the fuel, but climbing will want the fuel to move away from your nose, so there will be less pressure to move it to the engine. It may very well be normal for your aircraft make though.

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Thanks for the ideas. The Sting has an Aux pump that is to be used 'as needed' but I've not needed it for climbs (or any other time) in the six years I've flown the plane. Is there a transient condition that can cause this behavior?

 

It really sounds to me that your pump is having to work "uphill" in a climb, thus the pressure drops. This does not happen in a bank, descent, or any other condition. I'd just monitor pressure on climbs and use the boost pump when the pressure drops. It doesn't sounds like the engine is starving even at the low pressures, but if you have a boost pump you might as well use it for the few minutes you are climbing.

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Hey Dave,

I have a similar set up to yours (Kavlico fuel pressure sensor), and I have about 90 hours on the sensor and pump (they put in a new pump with the sensor). I've had the pressure hit 1.8 - 1.9 psi on occasion and I think it was during climb but can't be certain. What I've done is just reduce throttle a little and then the pressure starts to climb, I usually set my throttle to around 5100 - 5200 rpm on climb, but I'm climbing at 500 fpm after I get to a safer altitude after takeoff. The SV auto pilot is set to climb (and descent) at 500 fpm.

 

With my earlier setup (VDO and flaky pump) it had as much to do with attitude as it did to climb. I remember the first time the fuel pressure hit the red was during slow flight where the deck angle was quite steep (mid to upper 3000's rpm). Fortunately the engine never faltered and I was able to get the aux pump on quickly which immediately got the pressure back into the normal range, but have had nothing below 1.8 to 1.9 with the new set up, and it has probably happened twice with the current set up.

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In my Sky Arrow with stock 2007 ROTAX-provided pump and FACET boost pump pushing fuel up to the engine...

 

...on turning the boost pump off I sometimes see the fuel pressure drop to about zero on the analog gauge before climbing back up into the green.

 

Never a hesitation, but I've never been able to visualize what's going on with that, and it's pretty much always done that.

 

Probably going to bite the bullet and order the new style engine driven pump to install in March when I do my annual, even though I've had no issues with my circa 2007 stock pump.

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Found this on the Vans sight which may explain the behavior:

I believe that many of these reports of low fuel pressure indications in a climb are due to the use of fuel pressure senders designed for automotive applications. The fuel pressure sender has pressurized fuel on one side of a diaphragm, and the other side of the diaphragm is vented to the atmosphere. The sender measures the difference between those two pressures.

 

I believe that many pressure senders designed for automotive use may have diaphragms that are vented to atmospheric pressure through very small orfices. This is OK in a car, which doesn't see rapid changes in altitude. The small orfice size helps keep contaminates out of the sender.

 

If you are in a climb, this may cause the pressure on the vented side of the diaphragm to respond slowly to the change in atmospheric pressure. The pressure on the vented side of the diaphram would be higher than the atmospheric pressure and the fuel pressure indication would be lower than the actual fuel pressure.

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Hi Dave,

That might be true with some senders, but not a VDO like ours. here are a couple pictures for you. These are spring operated and move over a rheostat. The spring operation verses the correct position on the rheostat can and does quite often become inaccurate. I just changed a VDO oil pressure sender the other day that read 10 psi on a cold engine. Fuel pressure senders go bad too. Most of the time they usually will read low. Some of the fuel pressure senders will read .5-1 psi on a cold engine before a start.

 

For those that have had dipping fuel pressures have you installed a new sender and checked it in flight?

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post-3-0-59336300-1388510171_thumb.jpg

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In my Sky Arrow with stock 2007 ROTAX-provided pump and FACET boost pump pushing fuel up to the engine...

 

...on turning the boost pump off I sometimes see the fuel pressure drop to about zero on the analog gauge before climbing back up into the green.

 

Never a hesitation, but I've never been able to visualize what's going on with that, and it's pretty much always done that.

 

Probably going to bite the bullet and order the new style engine driven pump to install in March when I do my annual, even though I've had no issues with my circa 2007 stock pump.

 

Does this spike happen quickly, or over a few seconds? And, do you know the design of your boost pump?

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It drops to near zero or zero over a couple seconds, stays there for a couple seconds, then starts a smooth climb back into the green over 4 seconds or so.

 

Not the kind of thing I'm prone the write off as normal, but it's always done it though not every time. If I had a low fuel pressure alarm like CTLSi has, I'm sure it would be sounding.

 

The fuel pump is whatever ROTAX was using back in 2006/2007.

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That might be true with some senders, but not a VDO like ours

Hi Roger -- I have the new Kavilco solid state sensor from Dynon for both fuel and oil pressure. I have no idea how they actually determine pressure.

 

I did go flying today and drained a little debris from the gascolator. Fuel pressure dropped to around 3.3psi on initial climb out to 3K AGL then stabilized around 4.6 in level flight. Did several climbs at different power settings and pressure stayed above 4 psi.

 

On my flight yesterday, the plane had set for a couple weeks and we've had unseasonably cold weather (for Dallas). Ground temp was at 40 and air temp was 32 at my altitude. Today, temps were 45 on the ground and in the air. Is it possible that temperature played a role in the lower-than-my-normal readings?

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Hi Roger -- I have the new Kavilco solid state sensor from Dynon for both fuel and oil pressure. I have no idea how they actually determine pressure.

 

I did go flying today and drained a little debris from the gascolator. Fuel pressure dropped to around 3.3psi on initial climb out to 3K AGL then stabilized around 4.6 in level flight. Did several climbs at different power settings and pressure stayed above 4 psi.

 

On my flight yesterday, the plane had set for a couple weeks and we've had unseasonably cold weather (for Dallas). Ground temp was at 40 and air temp was 32 at my altitude. Today, temps were 45 on the ground and in the air. Is it possible that temperature played a role in the lower-than-my-normal readings?

 

I use the new sensor that rotax just started recommending (the one that replaces the honeywell), with a D120 EMS. What is your panel?

 

It drops to near zero or zero over a couple seconds, stays there for a couple seconds, then starts a smooth climb back into the green over 4 seconds or so.

 

Not the kind of thing I'm prone the write off as normal, but it's always done it though not every time. If I had a low fuel pressure alarm like CTLSi has, I'm sure it would be sounding.

 

The fuel pump is whatever ROTAX was using back in 2006/2007.

 

Wait what?

 

We're talking about an AIRFRAME pump, right? Not the engine driven pump? The two are very separate, and airframe boost pumps are of different design (they are centrifugal, vs the constant displacement engine pump)

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Wait what?

 

We're talking about an AIRFRAME pump, right? Not the engine driven pump? The two are very separate, and airframe boost pumps are of different design (they are centrifugal, vs the constant displacement engine pump)

 

Sorry, I thought I had made it clear. This is what happens when I turn off the electric FACET boost pump. It's in series with and before my engine-driven pump in my fuel system.

 

I really need to fly the little plane soon. Next Saturday the 4th if not sooner. I'll see if I can get it to do it and capture it on video.

 

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  • 2 months later...

Well, my low pressure came back today but with different symptoms. Was a warm day in Dallas with ground temperature around 80 and in the mid-60s at 5500. I did a long climb from the ground to 5500 and pressure stayed between 3.5 and 4 through 3500. It dropped to the high 2s by the time I hit 5500. Slow trended upwards and was stable around 3.4. I did a touch-n-go maybe 20 mins later, then climbed back to 3500 for the return trip. Fuel pressure was between 3.5 and 4 for the 30 mins home.

 

After landing and on taxi back I noticed the pressure had dropped to 2.8. Turned on The boost pumps hitch got it up to 5. Turned off the boost pump and pressure slowly dropped to 1.0. Boost pump would get it back to 4.5+. Did a run up without the boost pump and the engine didn't miss a beat. Let the engine idle awhile and the pressure stayed around 1.5.

 

Any ideas?

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Plan is to check it out with a mechanic this week so we'll put a gauge on it then.

 

I'm leaning towards a bad pump since the aux pump was able to push the pressure to 4.5+ and was steady but don't know if my logic is correct.

 

I'll also check the power and ground (this is the 5V Kalvico sensor) on the sensor as well.

 

Any chance this is vapor lock due to the higher than normal temps and 'winter gas'?

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  • 1 month later...

Another fuel pressure problem. I have a 2008 CTLS 912ULS, 450TT. Never had problems until now. Fuel pump was changed about 50 hours ago at annual along with Rotax rubber hose change out. to the made in Italy 993110 pump.  Pressures were pretty steady 3.8 on the old pump and with the new pump now at 4-5.5. Now after fifty hours this happened. Flew for an hour with pressures fine. Landed and killed engine for 30 minutes. Right at start up I got low fuel pressure below 2.2 warning on my Dynon D120. It would vary between 1.5~3 or so. I taxied out and did a runup and pressure was a little erratic going from 1.5~4. Parked the plane and the next day cranked up and pressures were back to 4~5.5 I few for 30 minutes and no problems. I changed out the fuel sensor a VOD type. Same type sensor as before.  Started up and pressures were 4~5.5. I ran the rpms 1800-4000 time and time again about 20 minutes with good pressures. Thinking a a sensor problem had been fixed I started to taxi back to the hangar when pressures suddenly dropped back to 1.5~3 setting off the alarms. Did some more run ups and pressures never went and stayed normal. Is pump failing after getting hot? Engine never lost power. Will the 912 run on gravity feed only if pump goes out?

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Sounds like vapor in the line. The engine heat was causing bubbles to form, and when you shut down and let it sit, they either worked their way out or condensed back into fuel.

 

It's around this time that the refineries change from winter to summer blends because of the vapor issue.

 

Someone want to chime in if mixing a little 100LL with mogas will lower the vapor pressure any appreciable amount?

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Normal

 

Vapor. Mine does it all the time after I park for a while. The pressure will come back after a couple minutes of running. Idle around 2000+ and it will come back sooner.

 

The kicker here is it only affects the pressure sender and isn't an issue for flying. The carbs get all the fuel they need.

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