Low fuel pressure problem

groucho · #1 09-25-2009, 11:01 AM

VAF-

It's been a while everyone. Sadly, I haven't been flying my -6 much. It only got 50 hours in it last year. I'm now trying to fly it more, but I'm running into a fuel pressure problem.

On a recent flight, my fuel pressure slowly dropped to about 1.2psi using just the mechanical fuel pump. Thankfully I was close to home & my electric pump worked fine. I figured the mechanical fuel pump was bad, so I installed a new one. I ground tested the fuel pressure with the new pump, but the fuel pressure looks about the same. I haven't checked gravity fuel flow rates of the lines lately & I haven't done anything in the carb.

My -6 has an O-360 with a Facet A-4-5 carb. Before I tear into it, I'm curious how the filter is positioned in relation to the fuel pressure hookup. Would a clogged filter in the carb show me low fuel pressure (i.e. is the filter before the fuel pressure line)?

Here's a photo of back left of the carb. The left-most line in the photo is the main fuel line. The top two are fuel pressure & a return line. I pulled the fuel line off, but can't get to the filter without removing my exhaust. Before I did that, I figured I'd ask if that filter is a likely suspect.

Thanks everybody!

Ironflight · #2 09-25-2009, 11:21 AM

Tell us about your Fuel Pressure gauge Bryan - mechanical? electric? EFIS? Where is the transducer mounted? It is very common to see a low pressure reading on many of the EFIS gauges at high power, even though the pressure (at the carb) is fine. Hooking up a direct-reading mechanical gauge can help build a little confidence.

Paul

vic syracuse · #3 09-25-2009, 11:31 AM

Trya new sender before anything else. If you dissassembled one of these senders, you would see how crazy it is that we are actually trying to discern 1-3 lbs of pressure with a resistive wiper arm.
Also, why the return fuel line on the carb?

Vic

groucho · #4 09-25-2009, 11:33 AM

Fuel pressure gets measured by a transducer mounted towards the top of the firewall (on the engine side). That transducer feeds into my Advanced Flight Systems engine monitor. Doing a mechanical check sounds like a good idea.

There's definitely a difference in fuel pressure reading between having the electric boost pump on & off. With it off, it reads about 1.2 - 1.5. With the boost pump on, the pressure showed about 3.0. Hmmm...this really makes me wonder if it's an indication problem. (Boy, all that work for nothing...)

groucho · #5 09-25-2009, 11:34 AM

Quote:

Originally Posted by vic syracuse

Also, why the return fuel line on the carb?

Because the builder put it there.

I actually haven't asked the builder why he did that.

Sam Buchanan · #6 09-25-2009, 11:36 AM

Quote:

Originally Posted by groucho

VAF-

It's been a while everyone. Sadly, I haven't been flying my -6 much. It only got 50 hours in it last year. I'm now trying to fly it more, but I'm running into a fuel pressure problem.

On a recent flight, my fuel pressure slowly dropped to about 1.2psi using just the mechanical fuel pump. Thankfully I was close to home & my electric pump worked fine. I figured the mechanical fuel pump was bad, so I installed a new one. I ground tested the fuel pressure with the new pump, but the fuel pressure looks about the same. I haven't checked gravity fuel flow rates of the lines lately & I haven't done anything in the carb.

My -6 has an O-360 with a Facet A-4-5 carb. Before I tear into it, I'm curious how the filter is positioned in relation to the fuel pressure hookup. Would a clogged filter in the carb show me low fuel pressure (i.e. is the filter before the fuel pressure line)?

Here's a photo of back left of the carb. The left-most line in the photo is the main fuel line. The top two are fuel pressure & a return line. I pulled the fuel line off, but can't get to the filter without removing my exhaust. Before I did that, I figured I'd ask if that filter is a likely suspect.

Thanks everybody!

A lot of info in the archives about low indicated fuel pressure with carbed engines. Here are some of the search engine hits:

http://www.vansairforce.com/communit...+fuel+pressure

http://www.vansairforce.com/communit...+fuel+pressure

http://www.vansairforce.com/communit...+fuel+pressure

Check out these threads before tearing into your fuel system.

groucho · #7 09-25-2009, 11:52 AM

Quote:

Originally Posted by Sam Buchanan

Check out these threads before tearing into your fuel system.

I promised I searched...seems like others are better at searching than I am. Actually now that I think about it, I might have only searched for "fuel pump" and "fuel pump replacement."

The last thread sounds almost exactly like what I'm seeing. I'll see if I can dig up a mechanical fuel pressure gauge & compare. For now, I'm driving to my weekend destination.

Pat Hatch · #8 09-25-2009, 11:58 AM

Bryan, I would point out that the Lycoming manual for the O-360 lists the normal range of fuel pressure is between .5 to 8 psi with 3 psi desired. I have always considered that anything above .5 psi is OK. On all the RV's I have flown, the norm seems to be fuel pressure on the low side of the range and have never had even a hiccup at the lower fuel pressures. I have noticed that fuel pressure can vary widely. It seems to depend on many things including altitude, airspeed, and other factors. For example, I have always noted that fuel pressure increases in the descent for some reason. Anyway, unless you are noting something outside of your normal pattern, I wouldn't worry about what you are seeing.

Sam Buchanan · #9 09-25-2009, 12:33 PM

Quote:

Originally Posted by groucho

I promised I searched...seems like others are better at searching than I am. Actually now that I think about it, I might have only searched for "fuel pump" and "fuel pump replacement."

The last thread sounds almost exactly like what I'm seeing. I'll see if I can dig up a mechanical fuel pressure gauge & compare. For now, I'm driving to my weekend destination.

I certainly don't want to put you at risk of ignoring an actual problem, but the odds are very good that once you have investigated the situation, you will find no pressure problems in your system, just erroneous indications of low fuel flow.

groucho · #10 09-25-2009, 12:57 PM

Quote:

Originally Posted by Sam Buchanan

I certainly don't want to put you at risk of ignoring an actual problem, but the odds are very good that once you have investigated the situation, you will find no pressure problems in your system, just erroneous indications of low fuel flow.

Yup, absolutely. For the past 5 years my fuel pressure has read very consistently at 5psi. It's recently dropped to the 1.x range. I fly over lots of very unforgiving terrain & don't like taking any extra risk. The new fuel pump should be sending 4-6psi down the line, so I'm not happy with seeing 1 psi. When the AF2500's Betty starts screaming at me (and my wife) that our fuel pressure is too low, that's not cool - even if it is false.

I'll keep hunting. Hopefully a mechanical fuel pressure gauge will show that the pressure's fine. If so, I'll order a new sender. Hopefully that's all it'll take!