Low fuel pressure... again

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Harry Range
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Re: Low fuel pressure... again

Post by Harry Range »

Well I think Diamond's recommendation of using the boost pump continuously is a ridiculous recommendation. If they feel that the system is inherently weak then they need to formally issue an SB or an AD or something requiring us to run the boost pump.

I don't beleive the system is that weak. How many DA40's have actually had the engine quit, or think about quitting as a result of low fuel pressure ? Not many I suspect. I wonder how many mechanical fuel pumps have been replaced and of those, how many were actually defective. If the mechanical pump is working we do not automatically have low fuel pressure. We have an indication of low fuel pressure. The boost pump, while a known weak item, is not the culprit here as most of time the warnings happen when the boost pump is off. There is definitely an issue, as lots of us are getting the warning but I, for one, am not convinced it is actually low fuel pressure. But it still needs to be solved to restore our confidence.

Does anyone know the physical location of the feul pressure sensor ? I would like to pull my cowling and inspect the wiring and connections as my number one bet is still on the sensor.
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Jean
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Re: Low fuel pressure... again

Post by Jean »

Brichards wrote: Our latest thought is that the electric pump may in some way be causing a fuel flow restriction,
How could that be ?
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Brichards
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Re: Low fuel pressure... again

Post by Brichards »

It is just a guess, but it appears from the maintenance manual section 28 that all fuel flows through the pump or through a bypass in the pump. At this point I am on a mission to find out what the problem is and how to fix it. The pressure indicator is between the engine driven pump and the metering unit so the problem is between the tanks and metering unit. I will replace every piece in-between if that is what it takes.

I am still not convinced that the check valves at the fuel tank vents are not the culprit. It seems logical that with inadequate venting, the tanks are forming a vacuum until the pressure differential allows a gulp of air into the tank at which point the pressure somewhat equalizes and a vacuum starts the cycle over again. At least it makes sense to me and with my technical background, it would most certainly be fair and possibly wise to tell me I am full of it.
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Tim M
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Re: Low fuel pressure... again

Post by Tim M »

In an earlier post I said that I have succumbed to the boy who cried wolf syndrome and have learned to ignore this warning. But if the pressure drops are real, maybe ignoring it is a dangerous mistake. It has certainly effected my aeronautical decision making so I'm beginning to think that FAA Engineering involvement might be a good idea. Guys are carrying around spare pumps for heaven's sake. We should all be carrying spare batteries for our flashlights and other gizmos, but fuel pumps? if Diamond or Lycoming are unwilling to meaningfully engage us as a community then by all means let's dial 911 and get the FAA to at least join the dialogue:

http://www.faa.gov/regulations_policies ... tition.pdf
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Lance Murray
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Re: Low fuel pressure... again

Post by Lance Murray »

I have a 2002 with the same problems. It never has caused an issue but I do get the fuel pressure fluctuations. I suspect inadequate fuel venting. I wonder if it is possible to find a vented fuel cap that fits so we can test this theory.
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Harry Range
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Re: Low fuel pressure... again

Post by Harry Range »

A very interesting flight just this morning. Two major low fuel pressure warnings, one lasing about 6 minutes commencing in the takeoff roll. I have uploaded the data file for those who would like to have a look at it. It is number 1177 at CYBW I think. Obviously during the take-off roll I had the electric boost pump on as well and all systems ran and indicated normally other than fuel pressure. Since both pumps were on I think it is safe to assume it is unlikely I actually had low fuel pressure as that would mean both pumps would have to be failing or underperforming at the same time, and then both functioned just fine for the last half of the flight. As you can see from the data I tried multiple power settings once established in cruise to try to replicate the problem but no luck. Could the sudden increase in fuel flow rate have set the sensor off ? I don't know. I just still feel it is the sensor as the aircraft ran just like it always does .... perfect.
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Re: Low fuel pressure... again

Post by Graham »

Hi Harry,

I am by no means an expert but I noticed a few things in this versus some of your older flights. Did you lean for the climb since the elevation in Calgary is high? Your EGT seems high for a full rich climb and indicates a lean mixture. Also, your fuel flow is low for a full power climb and is lower than your previous flights. I usually see a flow near 16gal/hr on climb when full rich. Also, for the winter temps, your cylinder temps hit 400F which is what I would see on a hot summer day with a low speed climb at full power.

Not sure if this means anything, but it makes me suspicious about less fuel getting to the engine. Could you have had some ice in the lines?
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Harry Range
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Re: Low fuel pressure... again

Post by Harry Range »

Graham, The EGT and Cylinder head temps are definitely high for takeoff climb for my plane. I usually climb at about 1325 EGT and try to keep CHT down below 380 if possible. The winter kit helps the oil temp but hurts the CHT. I may fly without it in the winter because I don't like the elevated CHT's. I must have been lean during the climb as I never climb from takeoff at those EGT's. Not sure why that happened, possibly as I started fumbling with stuff with the audible "low fuel pressure warning" dinging away I may have bumped the mixture control. Not sure. And then I am sure I did not pay super close attention to mixture for a little bit as I was trying to diagnose this "warning". I see I got my act together after a few minutes. Ice in the lines I had not thought of but we almost never get water in our fuel here. Haven't seen any in my entire 650 hours on this aircraft and did strain the fuel inside my very warm hangar. If there was ice in the lines I would think it would have quit for sure.
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Re: Low fuel pressure... again

Post by DiamondDave »

I would be suspecting the fuel pressure sensor. I have seen sensor issues in the past with these units. It is located behind the alternator under #3 cylinder. These sensors are the ones recommended by Garmin for the G1000 installation. It sounds like the sensor could be giving fluctuating readings. If it were a wiring issue the fuel pressure indication would X out on the Garmin, So I would not suspect that.

I would also check your fuel filters. There are two screens, the main being the gascolator on the belly, the other is a small finger screen inside the fuel feed line to the servo on the LH side. Remove the feed line from the servo and the screen is right there. If those are both clear then I would try a new pressure sensor. They do use the same manufacture of sensor as the oil pressure by they are different so make sure you get the right one.
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Harry Range
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Re: Low fuel pressure... again

Post by Harry Range »

Thanks Dave, we'll go looking for those.
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