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Fuel Pressure Problem

H

HDDAHL

Active Member
I have an RV-9, 0320 D2A. Fuel pressure on start up is normal (4.5 psi) but goes to zero after 10-15 min of operation. Fuel flow indications are at normally expected levels. Fuel system is an inline boost pump, gascalator, fuel flow sensor, then to engine driven pump to carb. Pressure sensor is at engine driven pump discharge. I have changed fuel sensors to no avail. Do I have a small air leak or is vapor lock the problem due to excessive suction at the boost pump? Engine runs normally at all power settings including full. Any ideas welcome as this is very frustrating!
Doug Dahl N959RV
 
Gascalator

I remember a thread about this within the last year. The analysis centered on the gascalator heating up and causing low pressure. You might want to do a search or two regarding fuel pressure, etc. There was some discussion about shielding and or cooling the gascalator.
 
Venting?

It could possibly be a vent stopped up. With such a short engine run, the tanks would have to be almost full to drop that much. To check the vent lines, open the gas caps and blow through the vents under the fuse. I had that problem; a mud dabber had built in the vent line between the time I built the wings and installed them. It showed up as fuel pressure dropping. While i would doubt it, you could have something blocking the flow, perhaps in the gascolator. Check the pressure with a standard gage (I have electronic, too!) from the same point. That should rule out any possible electrical problem.

Bob Kelly
 
What kind of gauging system have you got? mechanical, electrical, EFIS? Mine is an electrical sender hooked to my EFIS, nd about my third flight, I was looking down and saw Zero fuel pressure! Scared the daylights out of me as I headed back to the field. I then realized that the engine was running fine at full power, so it had to be a guaging problem....I finally decided that the head pressure between the carb and the high-on-the-firewall pressure sensor was probably losing half the pressure. And then the inexpensive sending unit probably wasn't much more accurate than a psi or so. Couple that with a possible "venturi" effect as the pressure sensor is reading off the side of the fuel line, and....well, as long as the engine keeps running fine, I know I have adequate pressure! I stopped worrying about it after a couple hours, and only pay attention to fuel pressure at start up. If the engine stumbles in normal flight (it never has), my first step would be "Boost Pump - ON" anyways....

If the engine is still running, you've GOT fuel pressure! ;)
 
fuel preasure

I had a low f/p problem on a 9-A ended up with blast tube to the gascolator, and a thermal blanket betwen the firewall and the electric fuel pump.Maybe a problem that shows up on the 9A because of the gearleg mess in the airfow area on that model. No problems now,125TK 450 hrs. Of note, I recently finished a RV9 and have no problems.however I always use the firewall blanket on all my planes as a precaution
 
more info needed

like does the pressure come back up when you put on the boost pump, does fuel flow change at all? Altitude?
 
Fuel pressure problem

I'm assuming you have an electric fuel pressure gauge. if so, then maybe this will help.

I recently had a fuel pressure indication problem. Instead of reading zero it would go off the upper end of the scale. The engine ran fine. I traced the problem to a poor electrical ground. It was one of those sporadic reading type problems that drive you nuts, so it took me a while to figure it out. The gauge would read fine for a while and then go high. I accidentally noticed that when I placed my hand on the aluminum cover holding the gauge that it would return to a normal readout. That's when I realized it must be a bad ground.

If your engine continues to run okay with the zero pressure reading and you're at your wit's end, you might try changing where the isntrument is grounded.

Chris
 
Thanks to everybody for response. Pressure sensor is electronic, well grounded. No altitude or attitude effects. When I turn on boost in flight, I get some positive, but erratic readings 0.0 to 0.9 psi
 
My first thought would be indication..

My first thought would be an indication problem but until this is ruled out I would be VERY VERY careful, just because it's running and assuming everything is ok could be a deadly mistake. Carb's can run with basically no pressure as long as there is fuel flowing to keep the bowl full, but you may have a bad mechanical pump or a blockage somewhere. If it were me I would not be venturing any further than gliding distance from the airport until you resolve this. I would start with pulling the fuel line from the carb and doing a fuel flow test using the electric pump. Keep in mind that most experimental crashes result from loss of power usually as a result of fuel system problems. :eek:
 
Walt has a VERY good point, and I hope that by my quick answer I didn't mislead....I took my indicated loss of pressure pretty seriously and did an additional flow test and checked pressure with a mechanical gauge as well....don't just ASSUME that you have a gauge problem because others do....

Paul
 
GREAT advice Walt and Ironflight. I am going to get to the bottom of this before I venture out of gliding distance. Thanks.
 
Gliding distance? How about a power loss on takeoff? I can't think of any sensible reason not to ground the plane until it's sorted out. If you can build an entire airplane then you can solve this little mystery. I'm not sure it will help here, but I recently learned that the mech fuel pump on my xp360 has two drain holes. One will pass oil if one diaphragm ruptures and the other will pass gas if the other diaphragm breaks. Just one more thing to examine.
 
I agree Steve

The only reason I gave him the "gliding distance blessing" was because he said he had good presure for 10-15min but otherwise I totally agree, ground it until you get it fixed!
 
The first plane that I built I saw the fuel pressure go to zero on the second flight, scare me too. I turned on the electric boost pump and saw maybe .5 pounds pressure. But the engine ran fine. I could find no problems with it. I spoke to Bart La Londe about this and he said it was quite normal to see this on some engine and carburators. He said this occurs on some production aircraft as well. Since then I have seen it on many of the RV's that I have built and flown. I have seen the pressure drop in cruise and then come back. I have never seen it completely diappear on climb out. As long as the engine is running and making power don't worry about it.
 
Norman CYYJ said:
The first plane that I built I saw the fuel pressure go to zero on the second flight, scare me too. I turned on the electric boost pump and saw maybe .5 pounds pressure. But the engine ran fine. I could find no problems with it. I spoke to Bart La Londe about this and he said it was quite normal to see this on some engine and carburators. He said this occurs on some production aircraft as well. Since then I have seen it on many of the RV's that I have built and flown. I have seen the pressure drop in cruise and then come back. I have never seen it completely diappear on climb out. As long as the engine is running and making power don't worry about it.
Click to expand...

I guess we can feel better that it seems to be a common, benign condition, but did La Londe happen to explain what causes it?
 
Thanks again. The pressure goes to ZERO, climb, cruise, whatever. When I turn on the boost pump it sometimes registers minimal pressure for a few moments, but typically goes back to zero even with the boost on. I remain baffled. Normally enough pressure stays for a take off and minimal climb, say 1000 feet, then to zero. Once I forgot to turn the boost off for an entire flight and Pressure remained at 3 psi (when the boost pump was turned off on landing the pressure went to zero again). I am very reluctant to fly until I resolve this issue.
 
I normally run 4-6 psi

On just the mechanical I normally run 4-5 psi cold and 3-4 when hot. The facet pump when turned on always brings it to about 6 psi. At altitude when hot I've seen 2-3 psi on just the mechanical and again the facet will bring it to 5-6 psi. I don't normally monitor this during the TO climb so not sure what it reads at full power but I can look at this if you like today when I go fly. I run an EI fuel computer with the EI fuel pressure transducer. I have found the EI stuff to be VERY accurate so I have no reason to doubt these numbers and they are look like what I expect to see.
 
I put some heat sheild stuff on the gascolator and removed the bottom half of the cowling (upper was off already) to put the sheild mat'l on. While both cowlings were off, I ran the engine (watched temps) for 20 min and fuel pressure held ( 4.5). Fuel pressure went to zero in 10 min the day before with the upper cowling off, lower cowling on. It is obvious to me that I have A HEAT (fuel vapor) problem. Fuel lines are sheilded so either the mechanical pump or the gascolator is the culprit, and I am reasonably sure it is the gascolator getting hot and causing the vapor problem. Thanks again to everybody; I will add blast tubes, heat sheilds, and see what happens. Doug Dahl
 
I too have an RV-9A with the same fuel pressure problem. After 10-15 minutes flying, pressure drops slowly to zero. If I flip on the electric pump, feul pressure always jumps right up to 5 psi. I know this is not a problem with the instruments. I suspect heat may be causing the problem, and put a blast tube on the fuel pump. I will also try insulating the gascolator. I am interested to see what your solution is

I have researched cooling and find that exit area must be at least 2 times the inlet area. I calculate that the stock 9 cowl has an inlet area of 45 square inches. The exit area is about 53 square inches. This may be why things ge too hot near the bottom of the cowl. Anyone tried to solve this problem?
 
Reading through this thread made me shake my head several times. Here is an aircraft in flight which indicates 0 fuel pressure. And there are pilots out there which recommend to fly on, don't worry, other planes have the same problem. Having a zero pressure indication means that you are most probably an inch away from fuel starvation. Continue to fly is like russian roulette. And if you have passengers on board you potentially become a criminal.

Why not go the analytical way and measure the temperature of your various fuel components on the ground during a test run. Hook a termocouple to the gascolator or to any part in the engine compartment in contact with low pressure fuel.

The symptoms indicate a classic vapor lock. Fuel in your aircraft gets too hot for the prevailing pressure. Get rid of the problem before you fly again, or else your statistical life expectancy will drop to a few weeks or months. Some day all factors play against you. NTSB: Post impact fire consumed the aircraft. Probable cause: unknown. Contributing factor: rough terrain in the landing area.
 
What about the location of the floscan sensor?

Have you considered temporarily removing or relocating the floscan sensor? I'm getting ready to install mine, and I'm a little concerned about the size of the orifice. According to the floscan website (http://www.floscan.com/html/blue/aviation.php), the pressure drop across the sensor can be about 1-3 psi depending on fuel flow and sensor type. I don't know how much 1-3 psi would have on the boiling point of 100LL, but I'd guess if vapor lock was going to occur anywhere, it would be right after that restriction. I'm thinking about locating my sensor after the engine driven fuel pump.

FWIW,

Paige
RV-8A
 
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