What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Fuel Pressure Saga

Alchemist

Well Known Member
I have an RV6-A with a lyc d1a engine. I've started to notice some low fuel pressure readings on my gauge 2 PSI at 2400rpm. I also notice that when I put my boost pump on I see no change in fuel pressure. I know that my boost pump works as I tested the flow and you can notice the difference in sound when it starts developing some head pressure. I am curious on what the best practice and procedures are for identifying a bad gauge or sending unit ?

I have an electric boost pump, engine driven fuel pump, and up draft carb.

Fuel Pressure Gauge is a Modular Mitchell aircraft products
Sending unit is a PS-211-9000

Thanks in advance!
 
Pics below
 

Attachments

  • 71D6F7EA-4BF8-48E5-A19C-49840C5C5301.jpg
    71D6F7EA-4BF8-48E5-A19C-49840C5C5301.jpg
    173.5 KB · Views: 195
  • 80BF9431-5802-46F9-B884-3C8F746534D0.jpg
    80BF9431-5802-46F9-B884-3C8F746534D0.jpg
    357.1 KB · Views: 304
  • B73F0535-D583-433C-9849-D1FDC16F1CD6.jpg
    B73F0535-D583-433C-9849-D1FDC16F1CD6.jpg
    313 KB · Views: 191
  • 5F49DEE9-33FD-4A7B-B882-6566232CA4E0.jpg
    5F49DEE9-33FD-4A7B-B882-6566232CA4E0.jpg
    223.3 KB · Views: 206
Pressure

I had the same problem, after checking every hose clamp I put a mechanical pressure gauge on my line going into the carb and found normal pressure
It was my sending unit
Put another one in and my pressure was fine
Don’t know if that’s your problem, but I would check that first.
Dave
 
Wire connection

Pics below

Looking at he second photo of the sender:- it appears like the terminal on the left is bent about 90 degrees, so maybe start with checking those two crimped connectors as well as the tabs. It also seems like there may be some surface corrosion due to humidity or ? around that area, so cleaning the connections could help.
 
Justin, many threads here on pressure indications. Grab a known good mechanical gauge and plumb it in the hose before the sender. That way you can see BOTH the sender and mechanical gauge readings. If they read the same, check the pumps, both the boost and mechanical, the plumbing for a possible restriction, especially if rubber lined hoses are used.
If they read differently with the test gauge closer to 'normal" specs, then sender or possibly a wiring/terminal/ground/power thing.

Tom
 
So I checked it with a mechanical gauge and it read 3 psi with engine driven pump and with the electric boost pump it went to 5 psi. I’m going to check the wires and terminals tomorrow before ordering a new sender.
 
Justin, pressure senders are the Achilles heel on my 6A / d1a. My 2 cents worth is .5 is lowest I’ll accept, and I always suspect the sender when pressure reads too high. Each have their comfort level on this.
 
So I checked it with a mechanical gauge and it read 3 psi with engine driven pump and with the electric boost pump it went to 5 psi. I’m going to check the wires and terminals tomorrow before ordering a new sender.

Justin, let us know what you find when you run the wires down. Have a similar issue I am troubleshooting. Mechanical gauge to test against should be here next week
 
I have put new connectors on and changed grounds which resulted into no noticeable gains. Next order of business is order a new sender and try it. I have no signs of fuel starvation and the mechanical gauge read 3 psi and 5 psi with the electric boost pump on. I buttoned it back up and going to fly on until the new sender comes in.
 

Attachments

  • 429EDFC3-E180-4DE4-AD67-9D94A083BA0B.jpg
    429EDFC3-E180-4DE4-AD67-9D94A083BA0B.jpg
    316.4 KB · Views: 155
I have put new connectors on and changed grounds which resulted into no noticeable gains. Next order of business is order a new sender and try it. I have no signs of fuel starvation and the mechanical gauge read 3 psi and 5 psi with the electric boost pump on. I buttoned it back up and going to fly on until the new sender comes in.

Food for thought: Yes, you now know that as of your last test, FP is fine. However, there are no guarantee that the FP will be fine on your next flight and you now have no way of knowing it. Sure, some planes are not equipped with FP instruments, but I would think twice about flying it without instrumentation. We have these instruments to warn us of conditions that are abnormal and could potentially lead to engine failure. This early warning can often give us the chance to get on the ground before complete failure.

Larry
 
Last edited:
If the sender is substantially higher than the fuel pumps, could the head that has to be overcome give a lower reading?

Also, if one replaces the sender or does anything which exposes the gas line to the air (sender and others) , is it possible that air in the line prevents an accurate reading?
 
Generally speaking pressure transducers are not affected by air in the lines. Air will compress to a similar pressure as the surrounding fluid.

If you have air near a pump, you may cause cavitation and not generate enough or no pressure at all.

My fluid dynamics is a bit rough but I just calculated the pressure gain of moving the transducer down 1 foot on the firewall. I made the assumption that you would lose 1 foot of .305 id tubing using the same number of adapters and bends, using a 30gph fuel pump running 100ll. You save .04 psi by making that change.
 
Last edited:
That's a standard VDO sender that Mitchell re-labels. The VDO part number info is on the "NUT" sides. It should be available at any good parts house for around $35.
 
VDO

Ive had one of these fail as well at around 500h in service.
The internal wiper wears a groove and gets stuck, reading the same pressure.
I believe it is quite common.
Cheap and easy fix
 
Well after all the trouble shooting its confirmed that the sender was bad. Mechanical gauge and electric now show identical readings. Plane is back to reading 5-6 psi. Thanks guys !
 

Attachments

  • C01FBEBF-76DA-4279-8B3F-CF2D2C4D8AB5.jpg
    C01FBEBF-76DA-4279-8B3F-CF2D2C4D8AB5.jpg
    249.6 KB · Views: 153
Per: Lycoming Operator's Manual
Carburetor equipped engines- fuel pressure PSI Max 8.0, Desired 3.0, Min 0.5.
So... do you really have a problem?
 
Well after all the trouble shooting its confirmed that the sender was bad. Mechanical gauge and electric now show identical readings. Plane is back to reading 5-6 psi. Thanks guys !

Good to hear.

I found the picture of mine after it failed if anyone is interested.
I pulled it apart to take a look.
You can see the wiper has worn through the coil.

56CE952F-B006-4C6D-8BA2-9457215A64F3.jpeg

Cheers.
 
Per: Lycoming Operator's Manual
Carburetor equipped engines- fuel pressure PSI Max 8.0, Desired 3.0, Min 0.5.
So... do you really have a problem?

Considering mechanical Read 5-6psi and my electric read 1 yeah it proves the sender was junk . Now it’s readings match the mechanical.
 
Back
Top