Low Fuel Pressure - Troubleshooting

[n82rb]

being a cheap b....d, i buy the cheaper ones. i'm willing to bet the parts are all made by the same company.

bob burns
N82RB RV-4

[GLPalinkas]

Hi Kyle, hope you get this issue resolved. I'll be following it for my own reasons.

I'm trying not to hijack your thread but maybe this will factor in for you also.

I get low pressure readings ONLY when I climb to altitude and ONLY with the Facet fuel pump off. Turn on the pump and pressure comes back up from 1psi to normal 5-10 psi.

When climbing to 18' to 3000', or cruising at 3000' (approx) no problem. Only happens when I go from 18' (field elevation) to above 8-10K'. Pressure drops to 1-2 psi. The engines continues to run fine at full power and cruise LOP.

Fuel sensor temperature fluctuations? (Van's electronic fuel pressure sender and gauge)

Is there any chance that a fuel viscosity/temperature issue is affecting these mechanical fuel pumps? I'm smart enough to know I'm not smart enough to know.

Kyle, what was the temp at your field when you had the problem?

[Kyle Boatright]

Originally Posted by GLPalinkas

Hi Kyle, hope you get this issue resolved. I'll be following it for my own reasons.

I'm trying not to hijack your thread but maybe this will factor in for you also.

I get low pressure readings ONLY when I climb to altitude and ONLY with the Facet fuel pump off. Turn on the pump and pressure comes back up from 1psi to normal 5-10 psi.

When climbing to 18' to 3000', or cruising at 3000' (approx) no problem. Only happens when I go from 18' (field elevation) to above 8-10K'. Pressure drops to 1-2 psi. The engines continues to run fine at full power and cruise LOP.

Fuel sensor temperature fluctuations? (Van's electronic fuel pressure sender and gauge)

Is there any chance that a fuel viscosity/temperature issue is affecting these mechanical fuel pumps? I'm smart enough to know I'm not smart enough to know.

Kyle, what was the temp at your field when you had the problem?

The temperature yesterday was 65-70F, and it was the first flight (or taxi) of the day.

The low fuel pressure while climbing issue has been hashed out several times in this space - it seems to be a common problem. I've seen it, as have many others. Several theories have been put forth. Since there have never been reports of engines losing power during that type event, I believe it is a transducer problem, and is related to poor venting for the static system in the transducer. Effectively, the transducer has a <nearly> plugged static system and it takes a while for the static (or reference) pressure inside the transducer to catch up with the altitude of the airplane...

[GLPalinkas]

Originally Posted by Kyle Boatright

The temperature yesterday was 65-70F, and it was the first flight (or taxi) of the day.

The low fuel pressure while climbing issue has been hashed out several times in this space - it seems to be a common problem. I've seen it, as have many others. Several theories have been put forth. Since there have never been reports of engines losing power during that type event, I believe it is a transducer problem, and is related to poor venting for the static system in the transducer. Effectively, the transducer has a <nearly> plugged static system and it takes a while for the static (or reference) pressure inside the transducer to catch up with the altitude of the airplane...

OK, Thank you. I must have missed that thread. Anyway, keep us posted on your solution.

[Sam Buchanan]

Originally Posted by Kyle Boatright

The obvious follow-up question is "If I do buy a replacement pump, which brand do I buy? Tempest or Lycoming?" Looks like about a $100 difference at Aircraft Spruce.

Kyle, I bought a Tempest pump and it has given good service.

Interestingly, I was seeing more pressure fluctuations than usual (per threads already discussed) as the old pump started downhill. The new pump was much more consistent. One A&P suggested the check valve may have been getting weak.

A Tempest pump is reasonably priced, you are due one with 1000 hrs...go ahead and do it.

Tip; When you replace the pump, rotate the engine so the pump pushrod arm isn't under full tension, otherwise it is a real pain to get everything tightened down.

[tturner]

Kyle

Have you checked your fuel filters? I have had a similar problem on my RV4 with an O-320. On two occasions cleaning the fuel filter fixed the problem. My filter is very fine and had a small amount of trash in it.

[Kyle Boatright]

Originally Posted by tturner

Kyle

Have you checked your fuel filters? I have had a similar problem on my RV4 with an O-320. On two occasions cleaning the fuel filter fixed the problem. My filter is very fine and had a small amount of trash in it.

Nope, but I plan to. No stone goes unturned on something important like this.

[GLPalinkas]

So, Kyle, have you figured out your pressure issues? New pump installed? Keep us posted.

[Kyle Boatright]

Originally Posted by GLPalinkas

So, Kyle, have you figured out your pressure issues? New pump installed? Keep us posted.

I have not resolved the issue. Since the last time I posted, I have done the following:

1) Plumbed a manual fuel pressure gauge into the system. It agreed with the electronic gauge when reading the pressure from the mechanical or electric pump.

2) Conducted fuel flow tests. With the electronic pump, the flow is ~35 GPH at the carb. On the mechanical pump (at ~300 rpm - starter RPM with the sparkplugs out), it showed 11 GPH. Both of those figures are satisfactory.

3) Checked the gascolator screen. Clean.

4) Checked the facet pump for inlet obstructions. Also checked the adjacent flared connections for cracks and the like. No problems identified.

5) Checked for loose fuel fittings. Some could be slightly snugged, but none were "loose".

6) Confirmed (by observing valve position) that the fuel pressure event was when I was feeding from the left tank, if I need to chase plumbing back to the fuel source.

Next Steps:

- Check to make sure the vent lines are working properly on both tanks.
- Reinstall the sparkplugs.
- Check the electrical connections on the 2 year old GRT fuel pressure sender.
- Run the engine, hoping to repeat the problem. If it repeats, do a full run-up to see if the engine actually starves for fuel at "low" pressure readings.

If I can't isolate the problem after all of that, I'm kind of stuck as to definitive troubleshooting ideas. At that point, the question is whether to throw parts at the problem (new fuel pump) or go flying under the conviction that whatever the problem is, it is solvable with the electric fuel pump. Going that route, as long as I'm at altitude when there is a problem, I can afford to wait until the engine stumbles to prove or disprove whether the low fuel pressure indication is real or not.

I'm wide open to other suggestions, whether in the forum or by PM.

[Kyle Boatright]

Update...

I was down for a weekend due to the plague or some other malady, but got to take another pass at the problem last weekend when a buddy and I checked all electrical connections, tightened all fuel connections, and eyeballed the entire fuel system. We also scratched our heads a bit. I performed a run-up and no problems were noted. On that note, I went flying and saw no problems during a 15 minute flight.

This weekend, I repeated the test flight. Upon arriving at altitude, and after turning off the electric fuel pump, the fuel pressure (as measured by the GRT EIS4000) dropped again. It fell to a negative value of between -0.3 and -0.7 PSI. The engine continued to run fine in that condition for the duration of the flight - about 30 minutes.

I suspect I have an intermittent measurement problem. Funny enough, if you look at GRT's online troubleshooting guide, it mentions that their VDO senders often fail and become erratic at 200-500 hours, particularly on carbureted engines. On the other hand, the troubleshooting guide did mention that the erratic pressure tends to happen on the high side, rather than the low indicated pressure I saw.

So, I guess I'll order a new sender from GRT and see if that resolves the problem Unless one of you has a better idea. If you do, I'd appreciate hearing it...