What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Fuel Pressure

Tsabean

Active Member
Trying to track down inconsistent fuel pressure readings.
RV4, O320, Dynon display with Kavlico sensor. Running 91octane mogas.
Electric boost pump and gascolator in tunnel in cockpit as opposed to on firewall.

Pressure normally reads 3-5 psi when running on engine driven pump. On occasion, it will drop down to 0-.05psi. I believe these readings are accurate as the engine will eventually stumble when pressure reads 0. Selecting the electric pump on brings the pressure back up to 5-6psi. After a few moments , selecting the electric pump off results in 3-5 psi from the engine pump but not consistently. Pressure will occasionally drop back to lower readings.
I have read multiple threads about pressure issues but still unsure if I have an engine driven pump problem or it is something else.
Another data point from test flight today, pressure will drop when doing a loop or when pulling 3-4G during a tight 360 when electric pump not on. Tanks have flop tubes.
I have Pressure tested the system and did find a small air leak on the electric boost pump but repaired it. Going to pressure test system again while I await comments.

Thanks, Tom
 
I was having fluctuating fuel pressure readings on my Advanced Flight (Dynon) SV-EMS too (without boost pump on). My engine-driven pump has about 100 hours on it. I insulated the fuel lines and added some more exhaust shielding. Then, while doing an oil change I had my A&P change out the fuel and oil pressure sensors to replace the older Kavlico sensors that were the subject of that AFS, Garmin, and Dynon service bulletin. Problem is now solved (so far). The new Kavlico sensors recommended are both 0-150 psi sensors.


https://dynonavionics.com/bulletins/support_bulletin_050620E.php

https://dynonavionics.com/includes/guides/Addendum_Kavlico_Sensors-Rev_E.pdf


(ETA: not saying that the service bulletin from 4/20 had anything to do with fluctuating readings. Only that replacing that old Kavlico sensor with the new one let to more consistent readings.)
 
Last edited:
I was having fluctuating fuel pressure readings on my Advanced Flight (Dynon) SV-EMS too (without boost pump on). My engine-driven pump has about 100 hours on it. I insulated the fuel lines and added some more exhaust shielding. Then, while doing an oil change I had my A&P change out the fuel and oil pressure sensors to replace the older Kavlico sensors that were the subject of that AFS, Garmin, and Dynon service bulletin. Problem is now solved (so far). The new Kavlico sensors recommended are both 0-150 psi sensors.


https://dynonavionics.com/bulletins/support_bulletin_050620E.php

https://dynonavionics.com/includes/guides/Addendum_Kavlico_Sensors-Rev_E.pdf


(ETA: not saying that the service bulletin from 4/20 had anything to do with fluctuating readings. Only that replacing that old Kavlico sensor with the new one let to more consistent readings.)

To be clear, it appears the OP has a carb'd engine so Dynon says for the SkyView/AFS, EMS-D10, EMS-D120 and FlightDEK-D180 systems to use the new 15 PSI Kavlico sensor (P/N 103755-000/P255-15G-EA4) for most carbureted fuel systems with 0.6–15 PSI requirements, not the 6-150 PSI sensor for injected engines.
 
Last edited:
Clearly you are having intermittent fuel delivery issues, as you notice the engine react to the low pressure. I would first analyze WHEN the problems occur. If it happens at times when the temps around the pump are high (like aggressive climbs) it could be a heat/vapor issue. If it also happens at cooler times, like in cruise after the cool down from climb, it is not likely vapor issues. It could also be introduction of air into the lines upstream of the pump, but not as likely. A carb needs no real pressure, just delivery, and air bubbles create less issues with carbs than FI, due to the lower pressure requirements, especially if the pump doesn't have to lift the fuel to get to the carb. Also odd for a failure like this to just pop up. It is usually related to installation or doing something to the lines.

Fuel venting blockage can also create symptoms like this, but typically is more prevelant. If this only happens when on one tank and not the other, it is worth looking for a dauber in your vent line. If it happens on both tanks, venting issues are unlikley. The fact that it struggles under G load further points to the pump being weak.

I would replace the pump. They are known to have issues like this just appear out of the blue and often don't make TBO. If it is the pump, it will likely just get worse and at some point leave you stranded. Behavior like this warrants quick action. Also clean the inlet screen after pump replacement, as it may have been shedding rubber from the diaphragm.

Larry
 
Last edited by a moderator:
Thanks for the reply Larry. I flew today and the fuel pressure remained normal for the entire flight including during high G loading.
The only issue today was low pressure reading while taxiing with the electric pump off. Up until recently the electric pump was always on while on the ground. Decided to turn it off as part of the post landing checklist and noticed the issue.
Is it normal for the engine driven pump to produce less/insufficient pressure at low rpm settings on the ground?

Thanks, Tom
 
Rpm

Tom, what is RPM @ idle when seeing 0 lbs / stumbles? My idle, 850.

Me same engine, wonky fuel pressure readings I change the sender. Ive never had below .5 or a stumble. If I had a stumble, Id likely change engine pump.
 
12 yrs with low fuel pressure

In the first 12 years of flying my RV6 with carb my fuel pressure would always go below 1 psi when climbing and would go to 3 to 5 psi once the electric fuel pump was turned on. After trying several remedies I finally found the problem, a slit in the fuel pump inlet O-ring which caused air suction. Once replaced I no longer had an issue. One potential cause to investigate.
 
Thanks for the reply Larry. I flew today and the fuel pressure remained normal for the entire flight including during high G loading.
The only issue today was low pressure reading while taxiing with the electric pump off. Up until recently the electric pump was always on while on the ground. Decided to turn it off as part of the post landing checklist and noticed the issue.
Is it normal for the engine driven pump to produce less/insufficient pressure at low rpm settings on the ground?

Thanks, Tom

I don't think so. was it a large variance or small. The pump is RPM dependant, so low RPMs = low pump cycles/output. That said, fuel required is also RPM dependant, so low RPMs = low fuel required. Pumps typically deliver around 3 times the volume your engine needs (4 cyl anyways), so I would say that No, a pump should not drop pressure when at low RPMs. The pumps are volume based. Therefore, they deliver max pressure (just a spring, so can vary a bit engine to engine) up to a specific volume and then the pressure starts falling towards 0 as volume increases up to it's max volume capacity (capacity is RPM dependant of course). Generally speaking, a low pressure reading is an indication that the pump is struggling to meet the volume required and is at least 2/3 of the way to max volume capacity. I mentioned several things that can cause a pump to be inefficient and therefore a reduced volume capacity. In a healthy 4 cylinder engine, the pump should easily deliver the necessary fuel volume at idle and low RPM. Our typical pump should deliver arounnd 45 GPH at 3000 RPM. Don't know how linear the pump is, but expecting 15 GPH at 1000 RPMs is probably not far from reality. A 4 cyl at high idle should need around 2 GPH.

My recommendation to change the pump hasn't changed unless you find a flaw that is reducing volume output, such at air introduction upstream from the pump.

Larry
 
Last edited:
Tom, does your fuel pump has an overflow fitting with transparent tube installed? could tell you if a diaphragm is about to fail.
 
I ve had similiar lower fuel pressure in climb on my 0320 RV9 with just the mechanical pump pumping. I ve also found that either a new pressure transducer or some time just cleaning the electrical connection to the sender will correct the indication. This is also cleaning the ground wire on a two wire sender
. I also have a fuel filter in each line from the tank to the selector valve. It's an automotive strainer which does accumulate FBO Lint from their fuel filters. I have found that a lower pressure maybe indication of fuel strainer debrie. We depend on gravity to get fuel thru the selector valve and onto the mech fuel pump. Any pressure drop or restriction in that suction side of the mechanical pump will really have effect on the outlet pressure.
So even through you may not have fuel strainer set up like mine , you may need to look at the tank strainer if nothing shows up elsewhere. Some can be seen.from the fuel cap or maybe a fiber optic scope could help inspect those tank screens
I have also seen "filter Lint" blockage in the electric pump on another RV.
 
I would definitely check for vent blockage as Larry suggested.

Fluid pressure is the restriction of flow. If you have variation to flow without a change in system restriction this will manifest to a variation in pressure.

IF you have a partially blocked vent, the pump may draw fluid at a variable rate as changes to tank vacuum overcome the restriction.

In your case the blockage may be too much for the engine drive pump, but not for the electric pump.
 
My RV-7 acquired similar symptoms.

You have a fuel problem if you actually have engine stumble. Any amount of indicator replacement won't fix it.

I noticed a mis fire on climb out first. Boost pump would remedy that.

Was not long until the mechanical pump would not pump enough for cruise throttle.

Replace pump before it gets any more exciting
 
Just a follow up to original post. Replaced the engine driven pump over the weekend. Test flight had solid fuel pressure throughout. Disassembled the original pump, hoped to find some obvious issue but everything looked good. The only thing that stood out were the bolts on the bottom of the pump housing were not torqued to spec. A couple were just beyond finger tight. Although there was no noticeable fuel residue, I suspect this might have allowed air into the system.
I wish I had checked that before changing out the pump because, trust me, that was not a lot of fun on an Rv4.
 
Back
Top