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-   -   Fuel Pressure unreliable (https://vansairforce.net/community/showthread.php?t=188431)

f14av8r 10-30-2020 09:54 AM

Fuel Pressure unreliable
 
Hello All,
Looking for some help troubleshooting a fuel pressure issue. My RV-8 has an IO-360 being monitored by my EFIS, an Advanced Flight Systems 3400s with the AF-3400 Engine Monitor Package AF-3400-EM-4.

I acquired the airplane in May of 2019. The fuel pressure readout has been a constant issue and I'm trying to focus on getting it fixed.

Here's the problem. The fuel pressure reading is accurate and reliable when I'm running just the engine driven pump, or just the boost pump (on the ground obviously) but when I have both operating, such as for takeoff and landing, the fuel pressure readout goes haywire. It runs into the triple digits, then back down to zero and, eventually, the indication gets XXXed out.

I have the standard VDO sensor installed. I've replaced the sensor. No difference. I've checked the wiring. All good. I've checked the EMS setup. All good. I've checked that the reducer is installed in the line from the pump to the sensor manifold.

I'm considering replacing the VDO with the AFS/Dynon Kavlico pressure sensor. I would have already tried this but it the Kavlico requires +5v which I'll have to run from the EMS. That's a lot of work and the opportunity for maintenance induced failures are always high anytime one starts mucking around behind the panel!

What other troubleshooting steps would you take? Any thoughts / ideas much appreciated.

Thanks,
Randy

lr172 10-30-2020 10:30 AM

Quote:

Originally Posted by f14av8r (Post 1473759)
What other troubleshooting steps would you take? Any thoughts / ideas much appreciated.

Thanks,
Randy

Be sure that the FP line to the sender is coming from a point downstream of the engine fuel pump. I wouldn't be surprised to see this behavior if plumbed between the two pumps.

Larry

Ralph Inkster 10-30-2020 10:40 AM

My first thought too. But I can't imagine getting a mechanical pressure (normal engine running without boost) reading if the sender feed is before it.

Can you describe where the press sensor is mounted & where the line is T'd into the main fuel line?

f14av8r 10-30-2020 10:41 AM

Hi guys, the sender line is coming directly off of the engine driven pump, straight to the manifold.

Ralph Inkster 10-30-2020 10:46 AM

Through a -03 line about 18" long?

Also review what sensor profile is used in the EFIS setup.

f14av8r 10-31-2020 03:59 AM

Quote:

Originally Posted by Ralph Inkster (Post 1473775)
Through a -03 line about 18" long?

Also review what sensor profile is used in the EFIS setup.

Hi Ralph,
Yes, that's the line. And, I have reviewed the EFIS setup and it is correct. I even tried other settings to see if that made a difference. Using other settings did not produce acceptable results, even when the pumps were running individually - wrong sensor type.

74-07 10-31-2020 04:42 AM

Fuel Pressure
 
Is the boost pump the AFP boost pump type setup? Are the fuel lines run as depicted in the plans? I'm asking because our recently acquired 8 (built around the same time) had a non-standard boost pump/fuel line/check valve system that produced weird pump pressure readouts and had other operational characteristics that I didn't like at all. In doing some research, I discovered that there was apparently a magazine article published in the early 2000s that described this "low cost" alternative and several were built and flown this way. Once we got ours home, it all came out and the airplane is now plumbed and pumped according to plans. All is good. I know it's a long shot but thought I'd mention it.

f14av8r 10-31-2020 05:20 AM

Quote:

Originally Posted by 74-07 (Post 1473930)
Is the boost pump the AFP boost pump type setup? Are the fuel lines run as depicted in the plans? I'm asking because our recently acquired 8 (built around the same time) had a non-standard boost pump/fuel line/check valve system that produced weird pump pressure readouts and had other operational characteristics that I didn't like at all. In doing some research, I discovered that there was apparently a magazine article published in the early 2000s that described this "low cost" alternative and several were built and flown this way. Once we got ours home, it all came out and the airplane is now plumbed and pumped according to plans. All is good. I know it's a long shot but thought I'd mention it.

Were your pressure readings weird all the time, or like my case, only when BOTH pumps were running?

74-07 10-31-2020 06:54 AM

Fuel pressure
 
Randy,

Rather than suggesting I had the exact same problem, I was really suggesting that any deviation from the standard install can introduce both indication problems and operational issues.

In our case, I noticed on the first pre-buy run that there was no pressure indication with just the boost pump on. We knew it had a non-standard boost pump install so we checked the pump pressure later and it was working so we figured it was an indication problem. Got it home and dove into it. It had essentially two firewall forward fuel delivery systems from the firewall penetration. It had one line that went to the mechanical pump and then to the throttle body AND another line that went to the Holley pump then to the throttle body. The two systems were separated by one-way check valves. The fuel pressure was taken only from the mechanical side SO you never, ever knew if the boost pump was really working. We ripped it all out, installed a new AFP pump IAW the current drawings, installed new T&S fuel lines and fittings, put everything back the way it should be and all works perfect now.

So, I was only suggesting that you get the drawings and go through everything and make certain you are starting your investigation from a known point. You may just find a surprise somewhere!

TS Flightlines 10-31-2020 10:42 AM

Randy, you are getting the variation only when the boost pump is on? And it goes haywire? Quite probably an electrical ground. A couple of things:
1) how long, has this been going on; anything changed that brought this on?
2) have you verified the fuel pressure with a mechanical gauge?

Since things seem to be fine in ground tests, I feel that there is an electrical issue, maybe ground, maybe power at the sender. MIght also be that in flight there is enough movement in the pressure manifold to distrupt any additional grounding of the sender. Try this---short piece of grouning wire, clamped to the sender, and terminated on one of the mounting bolts on the pressure manifold. Probably a long shot, and its probably a sender issue itself, but worth the 10 minutes to find out.

UH---any other electrical anomalies in flight?
Tom


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