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High Fuel Pressure and Backfiring

Mockingbird

I'm New Here
I have an RV-6a with a Lycoming O-360 A1A
When I'm accelerating for takeoff it backfires and runs rough.
Also it fouls the plugs if I don't lean out aggressively on the previous flight. These things are telling me its running rich. The mechanic put on a new carburetor and replaced the intake gasket.
The fuel pressure is 13psi with the engine running. The fuel pressure is 7psi with the engine off and the electric fuel pump on. The engine manual says fuel pressure should be 3psi with minimum pressure of 0.5 and Max 8psi.
Would you guys say the fuel pump should be replaced? I'm pretty sure its a diaphragm pump and I don't think there's a fuel pressure regulator to adjust. Am I right?
 
13 PSI is WAY too high for a carb. Not surprising that you are running too rich. need a new pump! Shame on your mechanic for replacing the carb when your fuel pressure is 13. The pump is pretty much one of the first things to check when running filthy rich, after the needle/seat; Not that you would need to check the needle/seat with pressures that high.

Larry
 
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Hmmm

How are you measuring fuel pressure? Could it be you have a bad pressure sender? Float bowl generally cannot be over run with pressure/fuel, when it’s full, the needle shuts off flow. Engine driven fuel pumps come in two varieties... carburetor or fuel injection. If correct version on engine , pressure is derived by the spring tension per manufacturer. Pretty sure it can’t make more pressure than the spring allows. I am sure it can make less due to rupture in the diaphragm, evidenced by leakage out witness hole/ tube. Ignition or carb is where I would be focused. I just went through chasing high fuel pressure on carb engine.....turn high alarm off and looks elsewhere would be my recommendation.
 
Fuel Pressure from engine monitor

The fuel pressure is being sent from my GRT engine monitor and displayed on the EFIS. I might suspect the sensor if I didn't have other evidence that it's running rich. Also the electric fuel pump shows 7psi like it supposed to when the engine isn't running
 
I understand

Just saying , I doubt your issue is due to high pressure from engine driven pump. I have a GRT 4000 EIS and EFIS .... replaced fuel pressure sender a couple of times. Good luck.
 
h

How are you measuring fuel pressure? Could it be you have a bad pressure sender? Float bowl generally cannot be over run with pressure/fuel, when it’s full, the needle shuts off flow. Engine driven fuel pumps come in two varieties... carburetor or fuel injection. If correct version on engine , pressure is derived by the spring tension per manufacturer. Pretty sure it can’t make more pressure than the spring allows. I am sure it can make less due to rupture in the diaphragm, evidenced by leakage out witness hole/ tube. Ignition or carb is where I would be focused. I just went through chasing high fuel pressure on carb engine.....turn high alarm off and looks elsewhere would be my recommendation.

The problem is that if the fuel supply pressure is too high, the the float can't put enough pressure against the needle to shut off the fuel flow completely thus the fuel level in the bowl runs too high.

Skylor
 
Mockingbird,
I have read your posts and read the older thread you pointed to.
I have the same engine in my -6 and have chased low pressure reading and dealt with too lean issues that were corrected by drilling the main jet.

You are seeing evidence of too high fuel pressure by the way the engine runs. Your fuel pressure transducer is telling you the pressure is high and your engine is teling you your pressure is too high. You can try leaning for takeoff instead of full rich with boost pump like your owners manual says. But that would only be to prove to yourself it was the pumps not the pressure transducer. I would not fly the plane until you get normal reading. Per your logs, the fuel pump is past TBO. My opinion would be to replace the engine driven fuel pump.

Yes most likely, the high pressure is overpowering the needle valve set in the carb bowl and pushing fuel out the main jet into the airstream of the intake system. Normally the intake suction pulls the fuel out of the bowl. The mixture function of the carb has a small bowl inside the main bowl (which is just a resevoir of fuel). The mixture cable closes a door on the small bowl, thus limiting the amount of fuel the main jet can pull from. In an over pressure situation, the pump is overpowering the main needle set and pushing fuel through the small fuel bowl for the main jet and pressurizing the main jet that is normally a suction function.
Other signs of this would be excess fuel in the carb throat while the engine is running, so very rich on the ground and most likely so much that fuel may be dropping into the airbox and going out the small hole in the bottom of the airbox which is NOT a good thing since that is right above your exhaust pipes. Another would be not being able to actually or accurately lean the engine. You have seen evidence of this by fouled plugs.
Replacing the carb, if it too was past TBO was not a bad thing but carbs normally fail lean as they age and the pivot or hinge fittings leak air amongst other issues.
Takeoff is THE most vunerable part of a flight. If your plane isn't running right get it fixed, please don't temp fate. Coming here to VAF after the new carb didn't fix it was the right thing to do.
Find another mechanic or have a discussion with your current one about this topic. Changing the engine driven fuel pump is easier and cheaper than changing the carb was.
Please let us here at VAF know how it turns out, you can add to the data here for others.
 
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Fuel pump failing high

I wouldn't have believed it possible for the engine driven fuel pump to fail high, but I've seen it. A friend was reporting high pressure on his O-320. I temporarily plumbed a gauge onto it which confirmed the high pressure. A replacement pump fixed it right up.

Ed Holyoke
 
Thanks

Thank you for all the responses. I am going to have him replace the fuel pump.
I think it was an honest mistake to replace the carb first. I must have watched this video about fuel pumps 50 times yesterday to see how it could fail high. Still a mystery but that pressure has to be coming from somewhere.

https://www.youtube.com/watch?v=PYKoLUsTJU4

I'll let ya know how it turns out.
 
Thank you for all the responses. I am going to have him replace the fuel pump.
I think it was an honest mistake to replace the carb first. I must have watched this video about fuel pumps 50 times yesterday to see how it could fail high. Still a mystery but that pressure has to be coming from somewhere.

https://www.youtube.com/watch?v=PYKoLUsTJU4

I'll let ya know how it turns out.

I have had it happen to me on a boat engine (same AC designed pump). You could see the fuel pouring out of the top vents on the quadrajet carb. Failing with high pressure is really not that uncommon.

Larry
 
I have had it happen to me on a boat engine (same AC designed pump). You could see the fuel pouring out of the top vents on the quadrajet carb. Failing with high pressure is really not that uncommon.

Larry


That's very interesting. What is the failure mode that leads to high pressure?
 
That's very interesting. What is the failure mode that leads to high pressure?

Never did an autopsy. However, I know there is a small diaphragm and spring inside the champer who's purpose is to bleed off excess pressure above std rated. I speculate this gets stuck closed due to corrossion, deposits, etc.

Larry
 
Easy check... I agree with 1001001

What type ignition is in use and has timing been checked since your issue began ?

My A&P/IA , a RV multiple offender builder, and I can’t see how a mechanical fuel pump can create high pressure with a low pressure pump used on carburetor lycoming engine. Difference between pumps used on IO vs O engines is spring tension per lycoming. The back firing sounds like timing to me. Looking forward to hearing the fix. Wish the guy in the you tube video spoke more of pump interaction with carb vs FI .
 
After reading the other post discussing blow-by gasses causing excessive crankcase pressure

https://vansairforce.com/community/s...d.php?t=140100

I decided to check to see what my fuel pressure is with the dipstick unscrewed and out completely. Without with dipstick was 9psi and with it was 12psi.

I can see how positive crankcase pressure can cause elevated fuel pressure. That is something that needs immediate attention. Possibly a plugged crankcase vent. Crankcase pressure should be near 0 in most cases at least without severe blowby. If the vent is not plugged, a compression test is in order. Fuel pressure is not your only concern. At some point, you are likely to blow out the front oil seal.

Larry
 
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Crankcase vent

I was able to blow through the breather tube without restriction. I’m having them do a compression check before we order the fuel pump and will check the crankcase pressure with an airspeed indicator contraption. We’re not flying it until it’s fixed.
It has Slick Mags with new points and the timing has been checked twice recently.
The only issues with the ignition is fouled plugs from running rich.
 
I was able to blow through the breather tube without restriction. I’m having them do a compression check before we order the fuel pump and will check the crankcase pressure with an airspeed indicator contraption. We’re not flying it until it’s fixed.
It has Slick Mags with new points and the timing has been checked twice recently.
The only issues with the ignition is fouled plugs from running rich.

I would also stick a wire/coat hanger into the vent to be sure there is not an obstruction. Clearly this is an issue with surface area. Adding surface area (un-capped oil fill tube) reduced pressure. You want to be sure that the full area of the breather outlet is open. This includes the hose and piping attached to that outlet.

It's not that you can blow through it, you want to know how MUCH air can be blown through it.

Larry
 
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Wondering if the original problem was solved and how?

Me too! Backfiring is usually caused by too lean a mixture so I'm interested in seeing what happened. The OP is maybe talking about after-fire "popping" not backfiring.

Starting radials using the primer will teach you what a real backfire (intake) is like.
 
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