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Question of the day

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Desert Rat

Well Known Member
Hey guys- I went up to help my buddy finish changing the engine driven fuel pump on his glass star today. Engine is an O-320.

His purpose in changing the pump was that he'd developed a problem with a high fuel pressure reading. I didn't see how a diaphragm fuel pump going bad could lead to high fuel pressure, but am willing to admit this isn't my area of expertise, so whatever.

During the post maintenance engine run I was standing next to the plane. Everything was fine at startup/idle, but when he did a runup, it started dripping fuel out of the weep hole in the bottom of the airbox. Maybe a drip every 20 seconds or so.

We swapped places and I fired it back up so he could see what I was talking about and it did the same thing again. No drips at startup, run up to about 1700 and it starts dripping, power back to idle and it stops dripping.

What the heck?

Fuel pressure is back down in the green now, otherwise I could see how you could make a case that a new pump was shoving fuel past the needle valve at high power settings, or if it did it all the time I would think that maybe the carb float sunk in the 3 months its been sitting.

I'd like to think that I'm a handy guy, but the last time I actually put a wrench on a Lycoming was back in A&P school about 30 years ago, so I'm open to suggestions.
 
What weep is saying

....there is fuel making its ways through the diaphragm....usually a tear/perforation of some kind. The weep hole is between the diaphragms oil engine side vs fuel side. Sorry to hear this, as a mounted engine change out is a trying job so I’ve been told.

I and several A&Ps agree, can’t see how a engine driven pump can fail high. Very often, the solution is simple, change the fuel pressure transducer, they fail often in my personal experience. VDO brand is my nemesis in this regard


Edit: Oops...never mind....weep hole in air box not pump.
 
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BH1166 said:
....there is fuel making its ways through the diaphragm....usually a tear/perforation of some kind. The weep hole is between the diaphragms oil engine side vs fuel side. Sorry to hear this, as a mounted engine change out is a trying job so I’ve been told.

I and several A&Ps agree, can’t see how a engine driven pump can fail high. Very often, the solution is simple, change the fuel pressure transducer, they fail often in my personal experience. VDO brand is my nemesis in this regard
Click to expand...

OP said it is dripping out of the airbox, not the weep hole of the pump.
 
BH1166 said:
....there is fuel making its ways through the diaphragm....usually a tear/perforation of some kind. The weep hole is between the diaphragms oil engine side vs fuel side. Sorry to hear this, as a mounted engine change out is a trying job so I’ve been told.

I and several A&Ps agree, can’t see how a engine driven pump can fail high. Very often, the solution is simple, change the fuel pressure transducer, they fail often in my personal experience. VDO brand is my nemesis in this regard
Click to expand...

I agree with everything you said. When my buddy (also an A&P) told me that he was pretty sure it was a pump going bad and not a transducer, I told him the same thing, i.e. I didn't see how a diaphragm pump could fail high.

You are also correct in that it was a pain in the azz to change out that pump, but the mission was successful, and I can't deny that the fuel pressure is undeniably back where it should be, per the gauge. I can't explain it, but thats the reality.

Confirming the other statement as well; The drip isn't coming out of the new pump. it's coming out of the weep hole in the bottom of the air box at high power settings.

My suggestion to him was to put a different pressure gauge on it to confirm where his fuel pressure is actually residing, but here's where we are as of today.
 
Desert Rat said:
Hey guys- I went up to help my buddy finish changing the engine driven fuel pump on his glass star today. Engine is an O-320.

His purpose in changing the pump was that he'd developed a problem with a high fuel pressure reading. I didn't see how a diaphragm fuel pump going bad could lead to high fuel pressure, but am willing to admit this isn't my area of expertise, so whatever.

During the post maintenance engine run I was standing next to the plane. Everything was fine at startup/idle, but when he did a runup, it started dripping fuel out of the weep hole in the bottom of the airbox. Maybe a drip every 20 seconds or so.

We swapped places and I fired it back up so he could see what I was talking about and it did the same thing again. No drips at startup, run up to about 1700 and it starts dripping, power back to idle and it stops dripping.

What the heck?

Fuel pressure is back down in the green now, otherwise I could see how you could make a case that a new pump was shoving fuel past the needle valve at high power settings, or if it did it all the time I would think that maybe the carb float sunk in the 3 months its been sitting.

I'd like to think that I'm a handy guy, but the last time I actually put a wrench on a Lycoming was back in A&P school about 30 years ago, so I'm open to suggestions.
Click to expand...

This comes up very frequently in the forum it seems, and even though I haven't yet seen a good explanation on HOW it happens, I am another person who has had it happen. In fact, my symptoms were similar. I actually made a cautionary landing with a rough running engine and elevated FP, and the first thing noticed on landing was fuel running out of the airbox hole. Turns out I needed both a carb rebuild (it was flooding) and a new pump.

Not sure if one caused the other, and it briefly flew okay with a rebuilt carb and original fuel pump, but pressures were still way too high (confirmed with an external gauge) so I replaced it. Yes, it is a tricky job, but the safety wire trick I learned here made it an hour long job as opposed to 4 hours.

Chris
 
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They can fail high and it has happened to me. The dripping fuel from the airbox is likely due to a midadjusted float. Depending upon how high the pressure got, that could cause the tab to bend, though likely just a coincidence. TOo high of a float level should cause excessively rich symptoms, including blackish / grayish smoke from the tailpipe at runup RPMs.

Larry
 
Also check that your priming valve is not leaking into the intake runner. Rough running at lower RPM, & fuel running back through intake down through carb throat to air cleaner...
 
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Thanks guys-

I'm headed out of town for a week, but have forwarded your replies to my friend. Will report back with the resolution if anyone is interested.
 
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