Fuel Pressure to zero in flight

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JohnNielsen
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Joined: Tue Sep 14, 2010 10:21 pm

Fuel Pressure to zero in flight

Post by JohnNielsen »

On the way back home from OSH, I stopped about 40 minutes out to top off the tanks and take a leak. About 30 minutes into the next leg, my fuel pressure warning began to flash and I saw pressure (as measured at the carb inlet) slowly drop from 6 to 5 to 4 to 3...to 0. Electric fuel pump (between the gascolator and engine fuel pump made no difference. I picked a 1/4 section freshly cut hayfield and began circling above exploring various fixes. Then the engine began to miss and soon it began to cut out, then restart and run for a moment as I circled down and made a perfect landing and taxied to the corner of the field next to the road. Now pressure was back up to 6 w/ electric pump and engine was running fine. I shut down, removed each fuel cap and verified good venting by listening for air coming out of the tank as I blew into the vent on top. There was a considerable amount of debris in the tank sumps and in the gascolator sump. She started right up, I took off, circle climbed until 5000' above and resumed the trip to home with no fall in pressure along the way.

The electric fuel pump is a Facet required by the DelAir O-360 conversion. The engine, carb, fuel pump are all brand new. Flow testing today showed great flow at each connection until you get to the out side of the engine fuel pump, were it fell off just a bit.

I burn 100ll. Today I pulled both tank sumps, drained a high volume of fuel from each into a paint filter. Found bug parts. Removed and cleaned gascolator bowl, found very fine sand looking mush in the bottom. Screen above was clean.Tell me about vapor lock. Can it happen with 100ll? Where does it typically occur in the fuel delivery system? Any ideas??

John
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Bruce Fenstermacher
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Re: Fuel Pressure to zero in flight

Post by Bruce Fenstermacher »

I would also pull the finger strainer from the carb inlet and drain the carb bowl and see what comes out.

Vapor lock can happen with any fuel generally speaking. It takes heat generally concentrated on the fuel supply line. Any thing change in your setup that would cause hotter air to hit the fuel lines than in the past. Maybe this was the first time your modified system was subjected to these running conditions.

Little different vehicle but shows the point. I have a friend who is on the pit crew for a champion father & son race team. Son wins championship so father builds him a new car identical to the last. Father then runs old car. Son goes out and wins 3 races with new car. On the fourth race son seems to run out of fuel at end of race and loses. Plenty of fuel. No explanation until father puts new car on a lift and sees exactly what the problem was. The first car had the fuel line insulated where it paralleled the exhaust, new car did not. Why did the new car win 3 races till he got vapor lock? First 3 races had no crashes or restarts. Son went to front and ran non stop. 4th race had many accidents and restarts. Less cooling more heat for a longer time. BTW they run 100LL aviation fuel.
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JohnNielsen
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Re: Fuel Pressure to zero in flight

Post by JohnNielsen »

I am most suspect of vapor lock due to the fact it ran fine following landing shut down and restart after 20 minutes. What can anyone tell me about hose routing that is more prone to it? What exactly is vapor lock and how does it work? The lines are all -8 flex hose with firesleeve except the last section from engine fuel pump to carb inlet. Routing is gascolator to facet fuel pump to red cube flow meter to engine fuel pump to carb inlet. All hoses run upwards from one devise to the next except the one from Facet pump to Red Cube that runs down 8" before radius turn up 10" to Red Cube. And the last one from engine fuel pump to Carb is down.

Thanks. John
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c170b53
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Re: Fuel Pressure to zero in flight

Post by c170b53 »

I know my electric pump has a screen in it as well, so you might check that as the fuel has to flow through it. Just curious is the electric fuel pump mounted vertically?
Jim McIntosh..
1953 C170B S/N 25656
02 K1200RS
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jlwild
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Re: Fuel Pressure to zero in flight

Post by jlwild »

Since you found debris in gas collator, check your parts manual to see if your plane also has screens in fuel tank outlet. My 170 B tanks does. When I R & R both tanks many years ago, partial plugging was found.
Jim Wildharber, Kennesaw, GA
Past President TIC170A (2010-12) and Georgia Area Representative
'55 170B, N3415D, SN:26958, O-300D; People's Choice '06 Kelowna, B.C., Best Modified '07 Galveston, TX, Best Modified '08 Branson, MO.
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JohnNielsen
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Re: Fuel Pressure to zero in flight

Post by JohnNielsen »

After many discussions and experiments, we think the problem was a restricted flow from debris in the gascolator, which caused a low pressure in the line to the input side of the electric pump (which was running when the pressure on the guage began to drop). This 9" -8 Aeroquip line with firesleeve was subjected to the heat of a just completed hot day full power climb following a 20 minute hot soak during fueling. The combination of a 'suction' and heat in that line might have developed an air pocket (vapor lock) that prevented the pump from operating as designed.

All tanks and hoses, filters and pumps have been flushed and cleaned. 1.5 of test flight with 4 landings, each allowing hot soak for 15 minutes, then full power climbs resulted in normal fuel pressure measurements and operation.

FWI: The screen on the Facet electric fuel pump has the finest size of all the filters in the fuel delivery system...about twice as fine as the gascolator screen. We also modified the Facet pump cap to be safety wired for security.

John
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GAHorn
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Re: Fuel Pressure to zero in flight

Post by GAHorn »

I would be unwilling to believe that you had a vapor-lock problem in flight due to cooling air flow.
The debris in the system is more likely the culprit. Exactly WHERE in the line is the fuel pressure sampled? Is it a standard installation other than the elect. pump?
'53 B-model N146YS SN:25713
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JohnNielsen
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Re: Fuel Pressure to zero in flight

Post by JohnNielsen »

The fuel pressure tap is the threaded boss just above the fuel inlet at the carb body. Have flown another 2 hrs since cleaning w no issues and samples from tanks and gascolator are now clean. John
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