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  #1  
Old 11-18-2019, 03:40 PM
Starlifter Starlifter is offline
 
Join Date: Aug 2016
Location: Granbury, TX
Posts: 29
Default Fuel /Pressure Problem

Equipment: RV-7A / Superior IO-360 / Airflow Performance FI / LW 15473 Dual Diaphragm Fuel Pump

Symptoms: Often, in-flight, I’ll notice a drop in fuel pressure into the yellow range around 17 – 19 psi. I turn on the aux pump and pressure returns to the normal range, and after turning the pump back off, all will be well for an indefinite period, but normally anywhere from several minutes to well over an hour.

On a few occasions, if I have not “caught” the low reading, it has dropped as low as 8 psi and the engine has sputtered. Again, turning on the aux pump returns pressure to normal for another indefinite period.

The second issue, not as unsettling as the above, is after starting a warm or hot engine, often after topping the tanks, the engine RPM will surge cyclically, at an even rate, by around 100 – 200 RPM, at idle or very slightly above. Once out of the near-idle range, there is no issue, ever, and in-flight operation is normal.

Occasionally, I have seen a drop or two of fuel on or below the "witness tube" on the pump which indicates a loss of integrity of the diaphragm, but this is not consistent, and the pressure issue now happens most every flight.

As I did not build this airplane, I am hesitant to do any serious troubleshooting of the fuel system without a more thorough understanding of its operation.

Speaking with Tempest, it is recommended to replace the fuel pump after 10 years of operation. Mine is 13 years old so I will replace it. I've heard it can be tricky so any tips or tricks would be appreciated.

I appreciate any insights you may be able to provide.
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  #2  
Old 11-18-2019, 04:27 PM
erich weaver's Avatar
erich weaver erich weaver is offline
 
Join Date: Aug 2006
Location: santa barbara, CA
Posts: 1,635
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Quote:
Originally Posted by Starlifter View Post
Equipment:
The second issue, not as unsettling as the above, is after starting a warm or hot engine, often after topping the tanks, the engine RPM will surge cyclically, at an even rate, by around 100 – 200 RPM, at idle or very slightly above. Once out of the near-idle range, there is no issue, ever, and in-flight operation is normal.
This part is simply due to the tightly cowled nature of RVs, the fuel injection system, and the very low fuel flow at idle. The fuel is vaporizing in the small tubes leading to the injectors, causing the minor stumble. Very common, and nothing to worry about. Higher fuel flow and air flow during flight makes this go away


erich
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  #3  
Old 11-18-2019, 04:31 PM
gfb gfb is offline
 
Join Date: Jul 2014
Location: Madison, WI
Posts: 580
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I have a similar oil pressure problem with my 2 year old engine. It happens maybe once every 50 hours for me so has been tough to track down.
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  #4  
Old 11-20-2019, 03:37 PM
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RV7A Flyer RV7A Flyer is offline
 
Join Date: Jul 2013
Location: US
Posts: 2,013
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Quote:
Originally Posted by gfb View Post
I have a similar oil pressure problem with my 2 year old engine. It happens maybe once every 50 hours for me so has been tough to track down.
Same here, once maybe every 50-100 hours on average, but engine never stumbles. FP drops down to anywhere from 14 or so PSI to 17/18 PSI, triggers an alarm on the EFIS, then returns to normal, over about a 2-3 second period.

I figure it's cavitation at some point causing a bubble of air to very rarely pass through the system, but I actually have no idea. By now, I just live with it since I may never find it.

It's clearly there, though, when you look at the EMS data in Savvy. But, and believe me, I've tried, it doesn't correlate to *anything*...not MAP, not RPM, not ALT, not AS, not bank angle, not pitch, nothing nothing nothing.
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  #5  
Old 11-20-2019, 04:11 PM
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AlexPeterson AlexPeterson is offline
 
Join Date: Sep 2005
Location: Maple Grove, MN
Posts: 2,198
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I posted this on an identical thread, but have copied it to this one also:

Here is a good thread on replacing the fuel pump.

If your diaphragm is leaking fuel, you should not be flying it. You are having classic symptoms of a bad fuel pump.

Be aware (I believe it is discussed in the above thread) that there are dangers to not getting the pump installed correctly (lever inside the engine can be damaged...).

I've replaced mine again since that thread was started, and it was quite a bit easier than the first time! Biggest pain is getting/modifying an allen wrench so that it will go into the screw heads without a side bending load. I ground one down on one side where it interferes with the pump casting. A Bondhus brand ball driver would not go into the sockets on the bolts on my plane, just too snug.
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