Paul Eastham

Paul Eastham

Well Known Member
My carburated O-320 has been showing high fuel pressures since birth, and now at 25 hours, pressures are occasionally leaping up as high as 20 PSI (max 8 per Lycoming) with the engine pump only. Gauge has been calibrated and tests ok.

Aerosport has been kind enough to rush out a replacement pump -- are there any tricks to replacing the pump (I've never really opened up an engine in any way before)? Is this something I should have the FBO lend a hand with?

Thanks!
Paul
 
fuel pump

Imho with your experience you should have an aircraft mech do it. It is a simple job and should not cost much.
RV-9A flying with Phase one just compleated. :)
 
Thanks guys,
I had the local shop lend a hand swapping the pump and it was a good learning experience (though I bet I could have done it myself ... would have taken much longer).

Anyway, after all that, the fuel pressure got really high again after 10-20 minutes of flight. Later, I put compressed air on the sender and it showed the correct values through the whole range. Perhaps when it gets hot it reads wrong? I put the heat gun on it for a while and could not reproduce the problem.

I have just collected some other ideas from searching the forums: check breather tube open, clean/redo faston connectors, purge air from the line. I may also try repointing my mag's blast tube at the sender (temporarily.) Gotta love those intermittent problems...
 
An intermittent problem could be a poor ground somewhere. Or perhaps the gage or sender is bad.

Roberta
 
Carb?

If you are measuring pressure after the pump, it could be a restriction or sticking float at the carb...

gil A
 
az_gila said:
If you are measuring pressure after the pump, it could be a restriction or sticking float at the carb...

gil A
Click to expand...

Thanks Gil,
That's a theory I haven't heard yet. (I have checked grounding on the sensor, etc) Is there any easy way to diagnose such things? Fuel flow and engine operation seem normal to me.
 
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Checks...

Paul Eastham said:
Thanks Gil,
That's a theory I haven't heard yet. (I have checked grounding on the sensor, etc) Is there any easy way to diagnose such things? Fuel flow and engine operation seem normal to me.
Click to expand...
The first easy thing to do is to empty the carb bowl and clean the inlet screen of the carb and look for **** there... check that the hose from the pump to carb is clear, no little internal "flaps" if you assembled it yourself.... :)

Then I guess it would be time to remove the carb and have an old-time A&P look at it....

gil A
 
Will do, thanks.
I thought of one other possible carb anomaly: I don't see any rpm rise when leaning. I never saw that in spam cans either; they always had tiny analog meters. Now I've got digital and it sure doesn't rise at all.
 
Maybe I'm missing something, but the carb should not be able to increase the engine pump's pressure. The fuel pump has a relief valve integral to it. You should be able to block all pump output, and the pressure should still be only a few psi.

Do you have a restriction in the hose going from the engine to your sender? If that restriction is too small, you could be seeing pressures due to boiling in this hose. Heat causes bubbles to form in the hose, but the restriction is so small that the pressure can't quickly be relieved back into the fuel system.

Just a thought.
 
AlexPeterson said:
Do you have a restriction in the hose going from the engine to your sender? If that restriction is too small, you could be seeing pressures due to boiling in this hose. Heat causes bubbles to form in the hose, but the restriction is so small that the pressure can't quickly be relieved back into the fuel system.

Just a thought.
Click to expand...

Yes I do, that's an interesting theory! The line to the sender is not firesleeved either, which might make it boil faster.

I was going to try "bleeding" the line to the sender (i.e. clear all the air out of the line), maybe this will have an effect since there will be a larger volume of fuel to heat.

I'm not sure I want to do away with the restrictor fitting entirely, but I guess I could enlarge it if this seems to be the problem. It is the standard 45-degree restrictor that Van's sells in the FWF kit.
 
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Paul,

I don't think bleeding the line will help, as there is no flow, and it becomes "ambient" quite quickly probably...

Can you remove the restrictor (put a different fitting in) temporarily? Just to see if it fixes the problem, then deal with enlarging it if it does.

Keep us posted!
 
Too many variables!!!

First off I would want to be sure the pressures you are seeing are real.

The best way to do this is to hook up an analogue pressure guage.

I would get a gauge and a lentgh of flex fuel hose (it should be rated up to 20 psi) with a barbed fitting that screws into the spare port on the Van's manifold (1/8th NPT)

The flex rubber hose not concern me for a 2 hour test flight but I know I'm going to get a good flaming for suggesting such a thing...so instread use a length of alu brake tubing with the approprite fittings.

run the hose (or tube) out of the oil filler door and place the analogue gauge on the windshield and secure with the time honoured method...Namely ductape!...Tape down the oil door on the hose


So now you have a side by side guage that you can see in flight.


Then go flying.....This is a cast iron for sure way of illiminating the instrumentation or showing you have a real problem...I.e your lectronic instrument can be relied upon.

Start your investigation after you have done this step

Good luck

Frank
 
Fuel Pressure

I'm going to guess you have a Grand Rapids EIS. If not than a comparable unit that uses a Stewart Warner Style Pressure sender. These units may on occasion go bad due to vibration of your engine. I am on my third in 300 hours. You and I may need to pay up for the Commercial sender which is more robust and will "Live" longer than the lesser quality senders. I was getting eratic pressures, than all values would be fine for a while.
Just a thought, it seems you have exhausted everything else.
Tad "Stripes" Sargent
7A Carb
 
Replace the pump

Paul,

If you say you are getting high pressure at the pump, I believe you! I had that and wasted I can't count how many hours checking gauges, cleaning screens and Yes, even rebuilding the carb. Nothing downstream of the pump causes high pressure. It takes only an hour or so to replace and is SO much less expensive than overflowing the carb and having a fire. Double check your gauge, and if it checks out right, change the FP.

Bob Kelly
 
Well folks, I think I have the problem solved. I made a bunch of changes all at once (tired of decowling and recowling), so I can't say exactly what the problem was, sadly. What I did today:

  • Removed restrictor and drilled out to 1/16" (I didn't have a non-restricted elbow, but I had two with restrictors so I just drilled it). It was a pretty small orifice to start with, perhaps 1/32".
  • Replaced fastons (old and new ones were good brand-name amphenols)
  • Noticed when pulling the faston off that it came off easier than I am used to. Usually, I find fastons to be rather difficult to remove -- these came off with little drama. They weren't loose, but they weren't tight either. Even the brand-new fastons still felt a little loose, so I soldered them on. (I had checked resistance across the old fastons, it was zero...but maybe they loosened when they got hot?) I guess the tabs on this (Dynon-provided) sender are a little thinner than standard.
  • "bled" the fuel sender line by running the electric pump while disconnected and filling it with fuel.

I have never been so happy to see "3 PSI" in my life! This is the first time I've ever seen it below 5 PSI in flight, and it sure didn't hit 20PSI like last flight. It stayed right around 3-4PSI until I landed, when it went up to 5 (electric pump off). The flight was only .8hrs, but usually I have been seeing >10PSI by .5 hr.

Thanks, everyone, for your help and ideas. I don't know if anyone has noticed, but I seem to have had more than my fair share of phase-1 gremlins, and I think this is the last of them. I couldn't have done it without you all!
 
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