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Sticky fuel pump

Old 12-12-2005, 11:20 AM
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Default Sticky fuel pump

I am currently flying in temps of zero to 5 degrees centigrade. The model lives in a warm house or warm car when not on the airfield. On Saturday I had a lot of trouble getting the engine to start the fuel pump but assumed it was due to low gas pressure not hitting the required temperature. Neverthless I had 8 flights. The next day the engine would not start the fuel pump at all. I connected the GDT and saw the gas temp was above the minimum and revs were good as well. I disconnected the fuel line and ran the primer function from the GDT but although I could hear a ringing noise from either the pump or ECU (they are next to one another) there was no movement of fuel. I connected the main turbine battery directly to the pump for a moment to break it free and it pumped fuel. After that the primer function on the GDT pumped fuel, and after that the engine started okay.

Engine has had around 100 starts so the pump is not old and should not be worn out. I wonder if oil congeals in the pump and makes it stick in cold temparatures? Any ideas?
Old 12-12-2005, 01:07 PM
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Default RE: Sticky fuel pump

Hi Harry,

Your method for dealing with the problem is exactly what I'd have recommended. Close the ball valve and give it a shot with the pack, open the ball valve and try another start, easy... Is this a flightworks pump or another brand? Regardless of brand, sometimes a chunk of debris can bind on one of the gear faces and produce this problem. I most often see it in cooler temps and after period where the engine hasn't run for a few months.

As and FYI... In a previous thread I misquoted the minimum start temp as 100C. Its actually 110C, and I'd forgotten to mention I tuned mine lower to help with the colder climate. If you'd like that adjustment made for you, you need to make contact with a PST rep (with a "factory" data terminal) Users do not have the ability to tune this parameter.

Kelly
Old 12-12-2005, 05:28 PM
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Default RE: Sticky fuel pump

It's the flightworks pump. There ought not to be any foreign matter in the pump, the BVM UAT bag should have filtered out anything that remains in the fuel. The system from UAT through the pump to the engine is never drained so it ought to have fresh kerosene in the pump, not just oil to gum up or go thick in the cold temps. I wonder now if a lot of the failed starts I have are due to the pump sticking rather than my assumption of low gas pressure due to our low temperatures. I normally don't use the GDT but I will from now on to monitor the temp and see if it fails to start when the temp is okay for the pump to be running.

H
Old 12-18-2005, 05:41 PM
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Default RE: Sticky fuel pump

Today I had to give the fuel pump a blip of 7.2 volts to unstick it every second flight. Temp was 0 degrees C. Do they tend to stick in cold weather or is it beginning to fail?
Old 12-18-2005, 06:01 PM
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Default RE: Sticky fuel pump

Its difficult to say. The comment I made earlier about debris in the pump may not be from poorly filtered fuel. It may be a result of the gear faces wearing inside the pump head, which would tend to point towards needing a new pump. I've had this happen on a few different pump brands over the years. From your description of its accumulated run time, this pump should still be working fine though.

That being said, 0C is pretty cold and there's a chance the shaft seal is getting sticky. What is the pump voltage you're seeing once the engine has started and calibrated the idle speed? A healthy pump should be around 0.8V. If its approaching 1 Volt, you've got a restriction in the system or the pumps' not healthy...

Regardless, I'd recommend contacting Flightworks ([link=http://shopping.netsuite.com/s.nl/c.ACCT103531/sc.5/.f]Contact Link[/link]) and ask them about having it checked out. If the engine is still under warrantee, there shouldn't be any cost aside from shipping it to them.

Kelly
Old 12-19-2005, 05:07 AM
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Default RE: Sticky fuel pump

Thanks for the info Kelly, next time the weather permits I will watch the pump voltage at idle. It's out of warranty now and probably just as cheap to buy new as send it all the way back and fore from UK to USA for flightworks to look at it. I might open it up and take a look in case there is anything obvious causing it to stick.

H.

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