Piper Arrow Fuel Pressure

[ArrowFlyer86]

Over the last couple months my arrow's fuel pressure gauge has been on the fritz. For a while it was jumpy -- oscillating wildly in the green back and forth like a metronome. Then it would read near 0 after start-up and during taxi, but then come up and stabilize in the green during run up. Now the gauge bounces around a little bit and sometimes reports low. FWIW, the fuel flow dial on the manifold pressure gauge doesn't jump around at all -- and engine performance is perfectly fine.

The shop I brought it to for annual said the gauge is probably fine and we need to replace the engine driven fuel pump. They suggested a check-valve in the fuel pump could be getting stuck closed/open resulting in actual sporadic fuel pressure. The fuel pumps aren't that expensive and I'm all about critical components being top-notch, but I also don't want to replace it if that's not the culprit and I'm still stuck with an unreliable gauge afterwards. Any troubleshooting suggestions or people who have experienced similar issues?

 

[Bell206]

Any troubleshooting suggestions

As mentioned in another thread a good place to start is to verify the gauge indications you are seeing. The art of troubleshooting is to determine what is and what isn't and not what it appears to be. The are various methods to do this. Perhaps check in the maintenance manual for some option or additional T/S itmes.

 

[mandm]

Feel like mechanics love changing fuel pumps, but it’s quick and easy I guess.

 

[MJR Pilot]

I have the same issue. ‘75 Arrow that I bought about 4 years ago. When I did a test flight before buying, the fuel pressure needle was going crazy - but always stayed in the green. Seller replaced the needle after pre-buy but it kept happening. Asked my AP about it during my last annual and he said it’s fairly common on older Arrows (“you should start worrying when it stops acting erratically” - direct quote ). So I’ve learned to live with it - I just make sure it’s in the green when it’s going crazy and haven’t had any issues.

 

[Dan Thomas]

I have the same issue. ‘75 Arrow that I bought about 4 years ago. When I did a test flight before buying, the fuel pressure needle was going crazy - but always stayed in the green. Seller replaced the needle after pre-buy but it kept happening. Asked my AP about it during my last annual and he said it’s fairly common on older Arrows (“you should start worrying when it stops acting erratically” - direct quote ). So I’ve learned to live with it - I just make sure it’s in the green when it’s going crazy and haven’t had any issues.

If it is a sticking check valve in the pump, and you keep flying it, someday it might stay stuck and your pump is dead. You'd better hope that your boost pump is up to keeping up with the fuel demands. Those boost pumps get old, too.

 

[Daleandee]

The shop I brought it to for annual said the gauge is probably fine and we need to replace the engine driven fuel pump.

Hopefully my doc won't approach my health matters in such a manner ...

 

[Ed Haywood]

IMO the fact that your flow gauge reads fine suggests a problem with the FP gauge. If throwing parts at it, I'd probably start there. But be ready for a scavenger hunt; standalone FP gauges are becoming scarce, as everyone goes to engine monitors.

Does the pressure indication change when you turn on the boost pump? How does it indicate when the boost pump is on without the engine running? Both of those are clues.

Do you have a wet gauge or electric sender? If a wet gauge, start by blowing out the line. Then check to see if the line is supposed to have air in it or be purged. If that doesn't help, you can get a used gauge or auto gauge for 50 bucks to test.

If electric, those senders live in a harsh environment and are known to go bad, so I'd probably swap that first.

I have the same engine on my Decathlon. The wet gauges definitely oscillate, but at a very high frequency and within a defined range. That can be fixed with a snubber fitting. Yours doesn't sound like that.

 

[William Pete Hodges]

IMO the fact that your flow gauge reads fine suggests a problem with the FP gauge. If throwing parts at it, I'd probably start there. But be ready for a scavenger hunt; standalone FP gauges are becoming scarce, as everyone goes to engine monitors.

Does the pressure indication change when you turn on the boost pump? How does it indicate when the boost pump is on without the engine running? Both of those are clues.

Do you have a wet gauge or electric sender? If a wet gauge, start by blowing out the line. Then check to see if the line is supposed to have air in it or be purged. If that doesn't help, you can get a used gauge or auto gauge for 50 bucks to test.

If electric, those senders live in a harsh environment and are known to go bad, so I'd probably swap that first.

I have the same engine on my Decathlon. The wet gauges definitely oscillate, but at a very high frequency and within a defined range. That can be fixed with a snubber fitting. Yours doesn't sound like that.

I agree with Ed on this. I will only add that if you have an electric sender and gauge I would want to replace both at the same time if possible and check the sender wire to be sure it is not intermittently open or grounded. If you have a mechanical wet gauge and feed line, I would consider replacing both of those together as well. It is easy for a line to partially collapse inside especially if there is a tight bend or some other stress involved. I hope this helps.

 

[pburger]

Piper Arrows have wet gauges (both oil and fuel pressure), at least up to the Piper Arrow II. Later year models may have senders, but I don't know for sure. I have a 75 Arrow II. I struggled with the fluctuating fuel pressure gauge for a few years. Three years ago my A&P change the engine-driven fuel pump. That didn't fix it. Oh well, at least I've got a new fuel pump... We eventually did get it fixed, and it's been rock solid ever since:

The fitting that the gauge line is connected to is supposed to be a 90°elbow with a restriction (orifice) in it. Piper part no. 69033-00. I think that orifice dampens out the pulsations from the engine-driven diaphragm pump. (I'm also assuming this will prevent a high flow of fuel coming out in the event the gauge line is severed somehow in the cockpit.) I'm not sure how/when, but apparently that restricted elbow on my plane may have been replaced with an elbow w/o the restriction at some point in the past. So, my A&P installed a restricting orifice in the gauge line and the gauge has been rock solid every since.

By the way, if you search the various forums you will find people that will say to remove the gauge line, blow it out and reinstall it so that there is air in the line to dampen the pulsations. The Piper troubleshooting chart says to do the opposite: Crack the line at the gauge and purge all air out, and then tighten it back down. I'm a firm believer that air should be purged out of liquid gauge lines. In the end, adding a restricting orifice in the gauge line did the trick.

 

[Bell206]

I'm a firm believer that air should be purged out of liquid gauge lines.

FWIW: I've never had the equal success with recip engines on the fuel side due to the make up of avgas. But have on the oil side except were cleaning the oil line of deposits worked first. However, on turbine engines adding a bit of air into the fuel and especially the oil indicating systems has fixed a number of fluctuating needles. And on certain helicopters providing an air cushion in the engine torque lines was as common as topping off the oil level.

 

[Ed Haywood]

Here is a picture of the restricting orifice for an IO-360. I have been told it is an injector nozzle, but I did not check that against part numbers.

On my Decathlon, it did not dampen out oscillations. I added a snubber fitting with microscopic mesh instead of an orifice, per current TC spec for my aircraft.

Note: the oscillations this addresses are at the same frequency as the fuel pump pulses, which match engine RPM, so say 20 Hz. Fast enough that the needle is a blur. A wandering range or slower oscillations is probably a different issue.
From a safety perspective, I would not want a wet gauge without an orifice or snubber, or both. The snubber restricts flow to a drop every 10 seconds or so, which seems to me unlikely to sustain a dangerous cockpit fire in the case that the line breaks.

 

[ArrowFlyer86]

Sorry folks -- I didn't see everyone's responses to this (PoA never alerted me to this thread being updated ).
I ended up replacing the engine driven fuel pump. I should know in the next few days if that does the trick... At a minimum I'll get some solace in the fact that I have a shiny new pump to replace the old one that was there... From what you all have written it sounds like there might be a couple culprits to check if the misbehavior continues. So thank you for the tips!

If it is a sticking check valve in the pump, and you keep flying it, someday it might stay stuck and your pump is dead. You'd better hope that your boost pump is up to keeping up with the fuel demands. Those boost pumps get old, too.

This is exactly what the A&P was concerned about, the check valve staying stuck.
My boost pump crapped out a couple years ago and I did a repair on it. Ever since the repair it has worked great. But I still don't want to be in a situation where it's the only thing keeping the plane in the air just b/c that pump is quite old

 

[Skymac]

Just take apart the line, clean it, and blow it out with a air gun. Usually fixed it right up. The blue dye from the fuel builds up in my experience and causes the flutter. Seen it many times on a arrow and decathlon/citabrias

 

[ArrowFlyer86]

Just take apart the line, clean it, and blow it out with a air gun. Usually fixed it right up. The blue dye from the fuel builds up in my experience and causes the flutter. Seen it many times on a arrow and decathlon/citabrias

I'll add this to the troubleshooting list that I request from the A&P.
I don't believe I'm allowed to do this myself...

 

[ArrowFlyer86]

As expected based on the comments here -- the A&P called and let me know that replacing the engine driven pump didn't help the gauge stabilize. On the plus side I now have a shiny new fuel pump.
Will be doing the other troubleshooting steps you all outlined here.

 

[Domenick]

Your shop is taking you for an expensive ride. I wonder what they will suggest replacing next?
Do ALL the troubleshooting BEFORE replacing expensive parts.
If you have a wet gauge, bleed the oil line to the gauge for air.

Here is a picture of the restricting orifice for an IO-360. I have been told it is an injector nozzle, but I did not check that against part numbers.

Looks to me more like a primer nozzle.

 

[ArrowFlyer86]

They admitted they did little to no troubleshooting. They saw the age of the pump and said "let's replace this!". I agreed to it before seeing the responses on this thread.

Given the $120/hr shop rate and god knows how many hours they'd bill me for troubleshooting, I decided to take the path of least resistance and just replace the pump. Didn't work. Lesson learned...

 

[hindsight2020]

yeah, mine does it too, another vote for gas dye gunk in the wet gauge line, clean and burp, and it goes back to normal. I haven't messed with it because mine is mild, but I know what it is. As has been mentioned before in post 13, it's incredibly common.