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Two Fuel System Problems

N

N223JH

Well Known Member
ONE: In reviewing a data download, I noticed that my fuel pressure seems quite low in flight. I normally only look at FP at pre-takeoff and see it north of the minimum. In flight I had 2.5 to a little less than 1 PSI. It got my attention when I got a low pressure alarm during climbout. Fuel flow was fine. Could this be vapor lock? OAT was only 60F at altitude.

After reading applicable forum threads, I have drilled two more holes in the fuel cap, checked connections to FP sensor and attached an additional ground to the sensor case.

Subsequent startup and ground run showed 2.6-2.7 PSI anywhere from idle to 4,000 RPM. Pulling the boost fuse dropped it to 2.3. Did not fly yet as I want to connect up an external fuel pressure gauge tomorrow. Will hook this up in place of the sensor for ground test purposes. With engine off and boost on, flow out of gascolator is strong. I'm thinking this is some kind of sensor or calibration problem...

TWO: Fuel shutoff valve will not close completely. Yes, I have a steady dribble at the gascolator. Tried cleaning out by cycling valve with boost on. No joy. Valve handle clears floor with ease; no obstructions. Ideas?

Jim in South Texas
#264
 
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Its a Mystery!

Try bleeding the fuel line where it connects to the fuel pressure sending unit on the fire wall. Just loosen the fitting til fuel starts coming out. Next clean the contacts where the wires connect to the sender. As to the fuel shut off go back to the plans and make sure the assembly is correct as well as to how you installed the handle. It maybe a bad valve. If you have to you may have to remove it and then close it to see if you have the same problem. Also with the fuel valve closed try being more patient when draining the gascolator. Maybe it take a few seconds to get all the fuel out down stream from the Valve. Let us know what you find. Even though I noted that you said the fuel streaming out of the gascolator seems good maybe they are connected, Bad valve that only doesn't close maybe it doesn't open all the way to.:)
 
Read this post by one of the most knowledgeable contributors to this forum:
http://www.vansairforce.com/community/showpost.php?p=458447&postcount=3
Could this be vapor lock?
Click to expand...
No. Per Scott's post, "Air in the line, not an issue." I agree with that. The sensor should work the same with air pressure or fuel pressure. Vapor lock usually occurs on the low pressure side of the fuel pump, not the high pressure side.
I have drilled two more holes in the fuel cap
Click to expand...
More holes do not help, per post by Scott above.
attached an additional ground to the sensor case
Click to expand...
Will not hurt, but not needed if sensor has two terminals.
I want to connect up an external fuel pressure gauge tomorrow.
Click to expand...
Good idea. The sensor is probably bad. Several builders have had to replace the fuel pressure sensor.
Good luck, Joe
 
Good suggestions, John. The shutoff valve worked fine 10 hours ago; can't figure how it would go bad. BTW, I drained about a gallon from that slow leak at the gascolator so it wasn't just draining from the engine unless it was backing up from the fuel return line somehow. Tank is full. What kind of FP are you and Colin seeing?

Jim
 
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Return line back flow

unless it was backing up from the fuel return line somehow.
Click to expand...
That is possible if the return line fitting inside of the fuel tank is lose, or if the return line does not go up into the tank filler neck.
Joe
 
N223JH said:
Good suggestions, John. The shutoff valve worked fine 10 hours ago; can't figure how it would go bad. BTW, I drained about a gallon from that slow leak at the gascolator so it wasn't just draining from the engine unless it was backing up from the fuel return line somehow. Tank is full. What kind of FP are you and Colin seeing?

Jim
Click to expand...
HI Jim,
We get a change in pressure as the flight progresses in that at the begining of the flight it will be up in the 4 plus range and if we cruise for an hour it starts going lower, maybe to 2psi. I haven't worked it out yet on why it does this. When I change power in the pattern for landing after cruising for a while the pressure goes right back up again. My thinking is like John's. It may be a little bit of vapor at the sender. It is not enough for me to worry about. Was hoping to get to San Antonio this weekend for a Tennis tournament my daughter is in but we have the plane apart to get some painting done.:(
Colin
 
Poor fuel pressure...

N223JH said:
ONE: In reviewing a data download, I noticed that my fuel pressure seems quite low in flight. I normally only look at FP at pre-takeoff and see it north of the minimum. In flight I had 2.5 to a little less than 1 PSI. It got my attention when I got a low pressure alarm during climbout. Fuel flow was fine. Could this be vapor lock? OAT was only 60F at altitude.

After reading applicable forum threads, I have drilled two more holes in the fuel cap, checked connections to FP sensor and attached an additional ground to the sensor case.

Jim in South Texas
#264
Click to expand...

Hey Jim,
You didn't mention the obvious. Have you checked the gascolator screen for thread sealant and other construction detritus on your newly completed system.
You have about the same hours that a local 12 builder had with a similar problem. He found the screen fouled with fuel lube.
Tony
 
Screen in Gascolator

Y'know, Tony, that was what I was going after when I found out the darn shutoff valve was leaking. Then I started wondering if I would need to order a new gasket for the upper/lower housing mating. Do you know if it can be reused after 20 hours of use? Got a recommendation for a seal-it-up replacement?

Jim
 
"O" ring

I believe that gascolator has a big "O" ring that seals the bottom. It should be reusable unless it has swollen.
Joe
 
Screen

Joe, I'll git after if if the pressure gauge is low when put on the sensor line. What are y'all seeing for FP prior to engine start with the boost on?

Jim
 
Yes, the gascolator has an O ring seal that should be reusable. I have had my bowl off a couple of times with no problem. You can buy a replacement from Van's and also replacement screens. They are inexpensive and would be good to have on hand.

Tony
 
Fuel Pressure

What are y'all seeing for FP prior to engine start with the boost on?
Click to expand...
Those sensors are not very accurate, so everyone's fuel pressure is going to be different. Mine is about 3psi with only the electric pump on and about 4psi with both pumps working.
Joe
 
Some Solutions

Hooked up an external fuel pressure gauge and got what appeared to be "normal" range readings in different ground run configurations. They were all about 2X what Dynon was showing with stock sender. Re-attached original sender...same old story. Vans says contact Dynon so I posted on their EMS board. We'll see, I guess.

Fuel shutoff valve is OK. Even though the actuator arm seemed to have plenty of fore-aft movement in the floor slot and didn't appear to be binding, I pulled the floor. Don'tcha know, the valve arm was binding with a *side load* which prevented it from coming the final 1/16" up to the "off" position. Figured I assembled that goofy arm mechanism wrong. Nope, plans showed it correct. All I can figure is that the whole valve rotated maybe just a couple of degrees on its axis, binding the actuator arm on the side of the slot. Widened slot now permits full cut-off.

Will post when and if Dynon provides an answer on the pressure readings. Thanks to all for your interest.

Jim
 
Defective FP Sensor

Contacted Mike Huff at Dynon and he emailed trouble-shooting instructions. Results indicated defective sender. They are UPS-ing replacement at no charge. Excellent service from these folks. Hope to be flying with accurate fuel pressure indication next week.

Jim in South Texas
#120264 flying 22 hours
 
It Ain't Over Yet

While waiting for the replacement FP sensor from Dynon--and the knowledge that my independent FP gauge showed normal (4-5 lbs) FP, I re-plumbed the Dynon unit and launched into clear skies on Tuesday. I was prepared for low FP alarms but that wasn't all I got...

While nose-up in a chandelle, I heard the horn sound, felt the engine wind down as the prop visibly slowed and a low fuel pressure warning lit up the Dynon. Lowering the nose and slightly reducing power returned things to normal. Time scoot back to the airport. Landed, did ground run, checked mags, etc. 2.5 lbs indicated FP from defective, low-reading sensor, took off. About 200 feet off the deck in 70 knot climb, engine lost estimated 30-40% of power. Lowered nose to level, reduced power very slightly and began crosswind turn--not enough runway left to land before trees. Peeked at Dynon and saw low FP warning flag.

Power held for uneventful landing. Checked mags on roll-out. OK, I had 16 gals of fuel aboard, OAT about 75F. About 20 hours on this engine. Downloaded the data onto Excel. Indicated FP during flight varied from 2.5-1 lbs except during incidents when it dipped below 1/2 lb. Fuel flow was completely normal at all times. No glitches observed in other data. Was able to pinpoint exact moment of incidents with FP, pitch RPM and altitude columns.

After talking to LEAF tech and others, (couldn't get thru to Vans) ran tests yesterday, using good gauge. Verified normal FP from boost with engine off: 2 1/2 lbs, still pulling 4-5 lbs on engine-driven pump with engine running. Ran engine for 1/2 hour with nosewheel blocked up, RPMS to 4,500. Nothing. Leveled bird (afraid it would jump sideways off block and ran for two minutes at 5,000 RPM. Steady FP.

Inspected innards of carbs with mirror--overrich seemed a possibility. Air filters very clean. Exhaust stack kind of sooty but this may be normal with Rotax on car gas. Will pull and inspect plugs today. My fueling is fastidious (Mr. Funnel) and I have never seen a speck of dust or droplet of water in samples in this always-hangared airplane. When I built tank, carefully blew out and vacuumed up debris inside. Sealed finger strainer with Proseal so inspection of tank will be last resort. Fuel cap has 3 small vent holes and is confirmed venting.

Detached FP hose from sender and inserted in gallon jug. Ran boost and got exactly the same, acceptable, rate of flow I got at gascolator during PAP testing. What I have not done (waiting on new O-ring) is inspect the gascolator screen. While it is possible there is some large obstruction in there which intermittently stops fuel flow, I would like to solicit helpful advice from this group on possible solutions exclusive of gascolator screen.

Because the fuel flow has been so consistently normal, I am suspicious that in certain temperature and/or pitch attitudes fuel bypasses the carbs in favor of the fuel return line. If you have installed your engine, you remember the little jet that the plans had you remove from the fuel return of the original Rotax configuration. LEAF tells me the fuel return system is to prevent vapor lock with a constant push on the fuel lines. IIRC, a restriction of some sort was incorporated in the Vans fuel return hose.

I will be getting into the fuel lines downstream of the gascolator to blow out any obstructions there and will run the engine more today.

I apologize for the length of this post but I am mystified how FP can drop off without any fluctuation in fuel flow. An ultralight site mentioned loose carb jets but LEAF says very unlikely. As far as I know, this is first RV12 loss of power due to fuel pressure problem (as indicated by Dynon, anyway).

I should add that the power loss was not a rough, loss-of-a-cylinder miss, but rather smooth, as I recall. It was much more than one mag going off-line. In each case, the low FP flag lit up and the download showed <1/2 lb at time of incident. (Remember, FP sensor was determined to be defective.)

Jim in South Texas
#120264
 
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Fuel pressure problem!!!!

Hello Jim
I believe I know what your FP problem is. I will PM you my phone if you would like some help. Regards, Allan
 
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