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Low Fuel Pressure - Troubleshooting

Kyle Boatright

Well Known Member
On the way out to the runup area today, I had a low fuel pressure warning. The digital gauge showed 0 or even slightly negative fuel pressure, but the engine continued to run. I turned on the electric fuel pump and the pressure came up to the normal operating range of 5-6 PSI. I turned the electric pump off, and the low pressure indication returned. Restarting the electric pump raised the pressure to normal again. I returned to the hangar, pulled the cowl, and looked for any obvious issues - blue fuel stains, etc. No joy.

The engine is a 160 HP, carbureted O-320 with an electric fuel pump and mechanical pump in series.

Here are the possibilities I've considered:

-Sender/gauge problem: Since the electric fuel pump raised the fuel pressure both times I turned it on, I'm going to assume the sender and gauge are not the problem.

- Intermittent failure of the low pressure mechanical fuel pump. Sam Buchanan had this problem once upon a time, but as far as I can tell, his posts didn't explain how he diagnosed it.

- Leaky fuel fitting or line, allowing air to enter the fuel system somewhere upstream of the mechanical pump (but after the electric pump?). Should I be able to test this (at least the connections between the electric pump and carb) just by turning on the electric pump with the engine shut down and looking for leaks and/or seeps?

- Collapsing fuel line. The fuel lines are stainless braided with a Teflon liner and should be good for a lifetime, but there are no guarantees in life.

This is a potentially dangerous problem and I'd rather identify and fix it with certainty before I fly again.

So I'd appreciate any thoughts people have on the subject. In the meantime, I'm going to dig through my Sacramento Sky Ranch book...
 
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Kyle, my engine would randomly lose rpm in flight, quite suddenly while climbing, and would instantly pick back up when the electric pump was switched on. Loss of rpm was inversely proportional to rise in pulse rate.....

After this happened three times (at first I thought it was carb ice) I decided I'd had enough and replaced the mechanical pump. Problem solved, the pump had ~700 hrs on it.
 
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Check pump pressure with a mechanical gauge. I had same problem- thought it was a sender problem - turned out to be a bad mechanical fuel pump. Replaced the pump and all is well. Pump had less than 500 hrs. on it.
 
Kyle, my engine would randomly lose rpm in flight, quite suddenly while climbing, and would instantly pick back up when the electric pump was switched on. Loss of rpm was inversely proportional to rise in pulse rate.....

After this happened three times (at first I thought it was carb ice) I decided I'd had enough and replaced the mechanical pump. Problem solved, the pump had ~700 hrs on it.

How'd you determine it was a pump issue and not something else?

BTW, the Fokker looks nice.
 
How'd you determine it was a pump issue and not something else?

BTW, the Fokker looks nice.

Thank you, Kyle.

Because the engine immediately regained rpm when the boost pump was engaged I assumed the mechanical pump was failing. I have had zero issues of this sort in the 500 hrs since the pump was replaced.
 
That is what I would suspect as well. If the gauge reads normal with the boost pump on, then I would assume you have a bad mechanical pump. On the ground the carb will likely draw enough fuel from the tanks to run, but not in flight probably. I would check the mechanical pump.
 
I have had intermittent fuel low fuel pressure problems also. In the process of chasing this down I have replaced all my fuel lines etc, eliminating many restrictive fittings and elevation changes etc. I thought I had it nailed when I replaced the Kavilco pressure sender and afterwards did not see the problem again, until now...

I even temporarily replaced the fuel line between the Red Cube flow meter and the servo and ran the engine while observing,,, no bubbles.

After explaining all that I have done to solve this problem to the folks at Aerosport they have offered to send me an overhauled Tempest fuel pump and it is on the way.

Talking to the fuel pump tech experts, they all tell me the mechanical pump cannot fail this way, that they either stop pumping or they are OK. I have now heard of several instances where they have failed on an intermittent basis.

We will see how things go after changing out the pump but I may have to wait quite a while to feel certain I cured the problem as it has been hours and hours of flight time between the failures I have experienced.

Mine is on an IO-375, the mechanical pump will normally maintain 25 PSI and the backup pump bumps it up to 32 PSI.

The fuel injection folks clued me in to the fact that looking for other engine parameters like fuel flow or MAP etc to drop along with the pressure does no good as the servo adjusts for pressure up to a point. I think I have always been too fast in switching on the backup pump so in looking at graphs of engine parameters from my Dynon, I can see the fuel pressure drop down to about 13 PSI in cruise. They tell me if this happened on a take off run I might see decreased power and high EGTs etc as the fuel demand is much higher, but in cruise conditions, the problem is hard to detect other than the fact that "Betty" says into my headset, "Fuel Pressure", and my hand goes very swiftly to the fuel pump switch when that happens.

I have wanted to repeat the problem at altitude and make myself hold off on turning on the back up pump but whenever I am ready for this, of course the problem will not appear.

Just passing this on in case others get the same message I have from the "experts" that say this can't happen.

Randall in Sedona
 
I know your airplane has been flying for a while, has there been any other pressure problems up till now? sounds like a flaky pump to me. lycoming says that you really only need about 1/2 psi to run at idle. people saying that they do not fail that way are wrong, I have seen them fail in a lot of different ways. my last one would pump fine at power, and idle when cold, but pull it back to idle on landing and the engine would surge then quit. pressure gauge stayed solid until it quit running. I am willing to bet your pump is headed south. how many hours is on it?

bob burns
RV-4 N82RB
 
I know your airplane has been flying for a while, has there been any other pressure problems up till now? sounds like a flaky pump to me. lycoming says that you really only need about 1/2 psi to run at idle. people saying that they do not fail that way are wrong, I have seen them fail in a lot of different ways. my last one would pump fine at power, and idle when cold, but pull it back to idle on landing and the engine would surge then quit. pressure gauge stayed solid until it quit running. I am willing to bet your pump is headed south. how many hours is on it?

bob burns
RV-4 N82RB

Never had problems until yesterday. The pump, engine, airframe, and everything else have ~1,000 hours. I'll get to put one of those fancy tags on the prop next year at Osh. ;-)
 
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.
 
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
 
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?
 
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...
 
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.
 
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.
 
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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

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.
 
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.
 
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...
 
Kyle, thanks for the update. Seems like a very complex problem. I hope your new sender cures the problem.

Hopefully, someone else will chime in with ideas. I have none.
 
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.

Kyle, I have the VDO sender for fuel injection pressures, but it did indeed get funky at around 400 hours, over east Arkansas on the way home from Petit Jean 2014. The indication would drift downward in cruise from its typical 23 or so to 18, which would trip my alarm preset. The boost pump (an old-school AFP) would push it back up above 20.

I almost always check instrumentation first. In this case I pulled the sender, which is oriented nipple-up as you see in the photo....



....and holding it nipple-down, squirted the restricted liquid passage with a quick, gentle squirt from an air nozzle. The idea is to "milk" the internals without breaking them. Sure enough, the return spring pushed out some black gunk. After repeating a few times, I reinstalled the sender, and it was fine for another 115 hours.

Recently I've had the airplane grounded for an annual and some updates. After sitting about 6 weeks, I fired it up and right away got a low fuel pressure indication, about 14 psi. Again, the boost pump would bring it right up. So, I repeated the "clean-out and full range cycling" process with the air hose, and bingo, all is normal again.

I don't know if any of this will transfer to the low pressure VDO sender. Heck, I don't even know why it works, not in detail; I plan to cut the sender open the first time it does not respond to the air hose treatment. But, it might be an interesting experiment worth trying, in particular if you plan to ditch the old sender anyway...doesn't matter if you break it.
 
I did go ahead and order a new sender today. Should arrive later in the week for a quick replacement. Pulled the cowl (again) instead of eating lunch today, retorqued the prop, and put myself in good position for a quick return to service when the sender arrives.

It would be nice to find a more robust sensor that would work work with the GRT EFIS. I know I used a Rochester fuel pressure gauge for the first 10+ years, and never had a problem. I guess I need to dig through my stuff and see if I can find that old sender to see what it is and perhaps compare its resistance curve against the VDO unit.
 
Kyle

I have replaced several of the VDO senders.

I have the GRT box also.

Last summer I relocated the transducer via flex hose to the engine mount rather than the engine itself.

I believe its a vibration problem that kills the transducers.

No problems since.
 
Oh .. another thing.

The packaging it rather funny on the sender unit when you get it.

something to the effect " Do not use on any airplane, ever"

The product is not intended for any use and should not be used for that purpose.
 
Kyle,
I have the same EIS4000.

I have just read through the posts here and on HA and I am a little confused with the mechanical fuel gauge measurement agreement with the EIS. Did the mechanical gauge read 5-6psi with the boost pump on and off during an engine run unlike the EIS? If so it certainly seems like an electrical issue.

Recall the f/p sender uses the 4.8V output from the EIS plus a series resistor. More than normal possible failure points.

May I suggest disconnecting the sender and checking the voltage on the ring terminal for 4.8V with and without the fuel boost pump on.

Also try wiring a fixed resistor in place of the sensor (not sure of the value) again check with the fuel pump on and off. This will at least zero in on the wiring.

You might also want to measure the resistance across the transducer terminals (with the sensor completely disconnected from the electrical system) using a good multimeter under similar conditions.

I can't think of how the boost pump on/off would alter the reading other than if the sensor supply wiring (4.8V) was shorted to the pump supply somehow. I do think a faulty sensor is a long shot, but I have had my own VDO sensor failure.

Good Luck,
 
The new sender didn't help things. I still had intermittent low fuel pressure readings. Only, they weren't intermittent - I finally noticed that pitch attitude (not climb or descent rate) was driving the pressure indication. Low pressure in a nose up attitude, normal pressure in other attitudes.

What could cause that? Differences in fuel "head" pressure due to the airplane's attitude! Yeah, I know, the math doesn't show much of a "head", but it certainly drove fuel pressure differences in my airplane.

So, I replaced the fuel pump a couple of weeks ago and have been flying around with my fingers crossed (and good fuel pressure) since then.

A hint from a different thread about using safety wire around the fuel pump plunger to put it in a bind so it would stay "up" while I replaced the fuel pump came in very handy. It only took an hour or so to remove some obstructions (fuel lines, breather, a piece of SCAT), pull the old pump, retract and bind the fuel pump plunger, and install the new pump. Even safetying the new one wasn't bad.

Thanks to all for the suggestions in the thread. Maybe it'll help someone in the future.
 
Fuel Pump

Kyle, please keep us posted if anything changes. I was following your thread but it really got my attention when you mentioned GRT EIS 4000. Same one I have and I too am experiencing fuel pressure fluctuations. Mine started with the right tank only, would go to normal when I would switch tanks or turn on the boost pump. I suspected blockage in right tank plumbing. Made sure lines were clear all the way to boost pump. I have since got low readings on both tanks but they always come back up with the boost pump. Had them on ground and in air but never had any engine roughness. Have not found any material in lines or in my gascolator screen. I may try sender but don't see how it would not work for the mechanical pump yet does with the boost pump on. Like you I may be forced to go the expense of changing out the mechanical fuel pump. Thanks again for posting such details.
 
Fuel pressure challenge

About 7 hours ago, I began to notice that the fuel pressure at times would oscillate between 24 and 28 psi with the engine mechanical pump. I have an IO360. The fuel flow seemed oK. I looked to see if the voltage or current was fluctuating and it did not. Today, the fuel pressure gauge was fine and then went off scale on the high side. I have a Dynon 180. The engine was running fine. I turned on the electric pump and the pressure came back to about 32 psi and no longer had a red indication on the Dynon. I turned the electric pump off and the gauge started to fluctuate again. repeated the same cycle again.
Is this a sender problem or a mechanical pump problem as the failure is a fluctuating reading and an intermittent very high pressure reading?
 
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