Help Troubleshooting Fluctuating Fuel Pressure

[Woodpile]

So my fluctuation is on the low side, my cruise is around 15 psi (1 above red). I have a new fuel line, but maybe I just need more pressure. Can the mech pump be turned up, or time for a new one? Boost runs at 18

[haymak3r]

13 hours ago, Ragsf15e said:

Well that’s definitely different than what we’re discussing on this thread, at least my symptoms.  Mine consistently bounces between about 20-30psi, never settles low and the electric pump doesn’t change the behavior at all.  The electric pump works fine but it doesn’t settle the fluctuations or raise the psi of them.

Exactly. Mine bounces high. Never goes below 23-24. boost pump makes no difference. 

[Woodpile]

Thanks, I didn't see this on the analog, so I presumed it was similar to my manifold pressure drops. Looks more like a mech pump issue now. 

[Ragsf15e]

23 minutes ago, Woodpile said:

Thanks, I didn't see this on the analog, so I presumed it was similar to my manifold pressure drops. Looks more like a mech pump issue now. 

The mechanical pumps are supposed to be dead reliable.  That being said, I had mine replaced because it was leaking oil.  They have two seals in them.  One keeps oil out of the fuel, one keeps fuel out of the oil.  If one is leaking, it’s not a problem.  If both leak, it’s not good.  If they leak, it comes out the tiny drain on the pilot side under the engine, near the nosewheel.  You can trace the drain line to see exactly which drain because the other tiny drain is the sniffle valve and it’s common to get a little fuel out of that after shutdown.  When my pump was changed, psi increased to right at 30psi.

[Shadrach]

14 hours ago, Ragsf15e said:

Well that’s definitely different than what we’re discussing on this thread, at least my symptoms.  Mine consistently bounces between about 20-30psi, never settles low and the electric pump doesn’t change the behavior at all.  The electric pump works fine but it doesn’t settle the fluctuations or raise the psi of them.

Seems likely that this is an issue with the instrument and not the fuel system.  My mechanical gauge reads rock solid in flight at about 27psi (IIRC), turning electric pump raises pressure to a hair under redline (30psi).  I would be bummed to be dealing with your issue.   I rely on a timer and the gauge to run the tank to (near) empty on long XCs.  I get the alarm and sure enough see the needle bobble within a minute or minutes if my estimation was conservative.

[Ragsf15e]

53 minutes ago, Shadrach said:

Seems likely that this is an issue with the instrument and not the fuel system.  My mechanical gauge reads rock solid in flight at about 27psi (IIRC), turning electric pump raises pressure to a hair under redline (30psi).  I would be bummed to be dealing with your issue.   I rely on a timer and the gauge to run the tank to (near) empty on long XCs.  I get the alarm and sure enough see the needle bobble within a minute or minutes if my estimation was conservative.

It definitely seems like the “problem”appears when we switch to fancy eis vs the stock gages…

[haymak3r]

Hey All, Just wanted to follow up and state that after added the snubber, my fuel pressure fluctuation has continued. Not quite sure where to go from here. No other engine indications show an issue. FF is consistent, engine sound does not change when the FP changes, EGT's look good too. I am going to try and fly today, and will try and remember to pull the logs and provide them for analysis to see what the FP shows in them. 

[Shadrach]

On 8/30/2023 at 7:40 AM, Woodpile said:

So my fluctuation is on the low side, my cruise is around 15 psi (1 above red). I have a new fuel line, but maybe I just need more pressure. Can the mech pump be turned up, or time for a new one? Boost runs at 18

I just checked my pressures today. At taxi rpm my 67 F is at a solid 29psi. The boost pump pushes it up another pound right to the red line of 30psi.

[ArtVandelay]

Hey All, Just wanted to follow up and state that after added the snubber, my fuel pressure fluctuation has continued. Not quite sure where to go from here. No other engine indications show an issue. FF is consistent, engine sound does not change when the FP changes, EGT's look good too. I am going to try and fly today, and will try and remember to pull the logs and provide them for analysis to see what the FP shows in them. 

Mine is the same, I was thinking of trying the snubber, but now I think I’ll leave it alone, FP varies 23.5-28.5 at 6000’.
It seems to vary more at higher altitudes, maybe 15-25 at 12,000’.

[201Mooniac]

Snubber seems to help damp the large FP variations but not the small ones.  I had it bouncing about +/1 5 psi and with the snubber it now bounces +/- 0.3 psi.  Still a little annoying but much more manageable.

[Woodpile]

FWIW, I replaced the mech pump. Pressure is around 18 and no spikes noted on ground run. I guess I can say that the change to the Garmin Engine monitor made this issue apparent and was well worth it.

[haymak3r]

I know this is an old thread, but wanted to close this out, and say that my mechanic, and I troubleshooted the fluctuating FP yesterday while the plane is in for it's annual inspection. Turns out that there is something in the pigtail that we received from the Garmin install kit for the FP sensor. not the connector, but something a few inches up IN the wiring. While the elec pump is on, if we squeeze/pinch the wiring there, or bend it, the FP spikes all over the place. It seems like a defect in the wiring somehow? Most likely the normal engine vibration during flight has been vibrating the wiring enough that it was causing the noise. Contacting Garmin today to see if we can get a replacement pigtail for that sensor. So strange. But happy to find the root cause. 

[Woodpile]

Thanks, I am still not satisfied with my values and will have a look at this too...when I finally get a prop

[AndreiC]

I'll add myself to the long list of people who have had this kind of issue. But what is different for me is that I do have the original equipment mechanical gauge. I posted a month or so ago a different thread about fuel pressure variations. In my case they were going on about a 5-7 second cycle, with a quick drop from 25psi to about 20, then slowly recovering back to 25, steady for a couple of seconds, and repeating. With the boost pump on pressure was rock steady. At no point was engine operation affected in any way.

This behavior stopped and things were fine for the last 10 hours of flying. But a couple of days ago, before I went flying, advancing the throttle from taxiing power (1000 rpm) to run-up position (1800 rpm) made the pressure do a big drop, from 26psi down to 15psi or even lower, and it took maybe 10 seconds to come back. Pretty unsettling.

At the suggestion of the guys at Top Gun we checked the finger screen at the inlet to the servo, nothing found there.

Another IA suggested that maybe the flexible fuel line between the firewall and the mechanical fuel pump may be getting old and under suction could partially collapse internally. Has anyone heard of something like this? The line is about 14 years old, but it is the teflon type, which from what I understand is not time limited. Is there a way to test if the fuel line is in good condition? I'd hate to start replacing stuff without testing if it is good or bad.

Here is a link to a video I took of the first type of pressure drop.

 

[Woodpile]

My fluctuating problem was cured with a new mech pump. My GI275 EIS seems to be related to a (likey false) low constant pressure - bottom of green. 

[802flyer]

My fluctuating problem was cured with a new mech pump. My GI275 EIS seems to be related to a (likey false) low constant pressure - bottom of green. 

Roughly what did that day you back for installed cost?


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[Woodpile]

I put the pump in - nothing special. 500 for the high  pressure variant from spruce, and 100 refundable core. If memory serves I got a new set of very stubby wrenches, that and some wobble extension for your ratchet (a rubber arm would have been handy) plus the cost of a quick inspection