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Low Fuel Pressure Warning

I was test flying my 7a -180 Lycoming, injected, CSP- at 12.5k ft. in prep for a cross country in June (Cleveland Ohio-Eastern Oregon) and my Dynon eng computer flashed a "Low Fuel Pressure" warning banner. It dropped from the usual 25ish range down to 5ish. The performance seemed to be ok; man pressure, airspeed, rpm all seemed stable, but I'm not positive as I was being more attentive to the fuel pressure issue. I opened and closed the mixture a bit, it didn't seem to have much effect. Lastly, I turned my Booster pump on and the pressure number immediately returned to normal. I left the Boost on about a minute and then turned it off. Everything was fine/normal for a few minutes and I got another Low Fuel Pressure warning. I descended and everything again returned to normal. So, any thoughts on cause and or arguments for/against using the Booster for extended periods in higher alts?
 
Very likely you had vapor bubbles in the suction line at the inlet to the mechanical pump. It happens with a combination of higher altitudes and warmer fuel - and our mechanical fuel pumps, being bolted on to that great big heater up front, do a wonderful job of heating fuel.

You will probably find it helpful to install a cooling air shroud around the fuel pump, it's a commonly used device.
 
It can also be an indicator that your mechanical pump is getting tired and may need replacement.
How many hours are on your pump? How many years old?

I agree with Airguy in the first response as a possible cause.

Monitor closely in cooler temps and after you instal the heat shield /cool air blast tube.
 
I was test flying my 7a -180 Lycoming, injected, CSP- at 12.5k ft. in prep for a cross country in June (Cleveland Ohio-Eastern Oregon) and my Dynon eng computer flashed a "Low Fuel Pressure" warning banner. It dropped from the usual 25ish range down to 5ish. The performance seemed to be ok; man pressure, airspeed, rpm all seemed stable, but I'm not positive as I was being more attentive to the fuel pressure issue.

If you've got a Dynon monitor, you should be able to dump the data, upload to a free account at Savvy, and then see what the MP, FP, RPM were at and before the event.

I just had a fellow do this with some Dynon data. Feedback " easier than I thought it would be ". And now we can look at OAT v. CHT and see progress (or not).
 
Purge Valve as a culprit

I have a similar setup with a purge valve on the fuel spider that dumps back to the port tank service line prior to the fuel selector.

Once in a while I'll get a pressure drop operating on the port tank at altitude. I have checked that purge line many times with both pressure and vacuum and can't find a leak.

I have settled on air or fuel vapor in the purge line that expands and enters the fuel stream at altitude. At most I can see a momentary leaning/EGT reaction but it is very brief.

I would love to find the problem, but it has eluded me so far.
 
It can also be an indicator that your mechanical pump is getting tired and may need replacement.
How many hours are on your pump? How many years old?

I agree with Airguy in the first response as a possible cause.

Monitor closely in cooler temps and after you instal the heat shield /cool air blast tube.


Fuel pump is original; build was 2005, TT now about 1100 hrs.
 
I had pretty much that same scenario a couple of years ago..turned out to be a pinhole in the boost pump diaphragm. $800 later, the fuel system was working fine.
 
Shawn. Just as another reference to a similar problem, my IO-360 started doing this almost from the beginning. Drop from normal 28psi to about 12-15, usually above 9.5K, seemed to do it most at higher power settings ... boost pump always put the pressure right back up to 30psi and there was no engine monitor indications whatsoever. Ran like nothing happened.

I queried the builder (ASP) and the injection builder (Precision) about the issue. They both suspected an air leak but I was never able to find one. After about 100 hours the problem went away on it's own!

The system has been rock solid, never junk in the filters or screens and has now passed 235 hours with no issues.

Hope you find a source for your gremlin!
 
I have had numerous cases where pressure would drop to somewhere between 15-20 for a bit and then return to normal shortly thereafter. Sometimes this will happen several times in a row before it stops. However, if I saw my FP drop to 5 on two different occasions, I would replace the pump or otherwise found cause. I think the minimum pressure for the servo is around 12. I do not consider your case to be normal and advise that you address it. While I have read about MANY incidents where FP drops into the teens, but rarely read of them dropping as low as is happening to you. It could be an anomaly or it could be your pump screaming at you that it is on it's last leg. Running a boost pump is fine, but if that becomes the primary, you have lost redundancy.
 
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If it were only actually 5PSI, airspeed and power would not remain the same and the engine would have quit.

That would mean it has to be a gauge malfunction - which doesn't make sense if the boost pump brought it back up.

An injected engine will still make power when lean, sometimes very lean. You will make less power, and lose airspeed - but it takes a minute or so to see that. When you're flipping switches and doing a fast scan of the gauges and the adrenaline starts pumping, you're not going to notice that you lost 5 knots over the last 90 seconds.
 
I have had numerous cases where pressure would drop to somewhere between 15-20 for a bit and then return to normal shortly thereafter. Sometimes this will happen several times in a row before it stops. However, if I saw my FP drop to 5 on two different occasions, I would replace the pump or otherwise found cause. I think the minimum pressure for the servo is around 12. I do not consider your case to be normal and advise that you address it. While I have read about MANY incidents where FP drops into the teens, but rarely read of them dropping as low as is happening to you. It could be an anomaly or it could be your pump screaming at you that it is on it's last leg. Running a boost pump is fine, but if that becomes the primary, you have lost redundancy.

Thank you for this sobering reply. Seems strange to me that it has run perfectly
for the past couple years at or below 10k. I'll ask a couple mechanics I know about replacing the FP. I had a horrifically expensive fix last summer (new prop/$10k). UGH!
 
Assuming airport altitude 900 ft MSL, temp 1 C, 28.9" Hg, climbing to 12.500 ft -23 C, 18.8" Hg. It´s a pressure drop of 10" Hg above the fuel in the tank.
This will cause bubbles in the tank and fuel line, seeing a fuel pressure between 5-15 psi is normal under this condition. It usually settles after 10-20 min at altitude because the fuel gets cold and this prevents more bubbles.
Normal fuel pressure at the fuel spider varies between 1-7 psi.
At 12.500 ft the power would be 55-60 %, 3-4 psi would be normal.
Therefore engine runs just fine.
The mechanical fuel pump has behaved as one could expect, not a problem.
But I´m not ruling out a problem with the mechanical fuel pump.
I suggest repeating this climb to find out.

Good luck
 
Thank you for this sobering reply. Seems strange to me that it has run perfectly
for the past couple years at or below 10k. I'll ask a couple mechanics I know about replacing the FP. I had a horrifically expensive fix last summer (new prop/$10k). UGH!

I have that problem and did try a new mechanical fuel pump and it made no difference. Of course each case is different and I could have gotten a new pump with the same problem.
 
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