My mechanic installed a new fuel pump during the hose change out in April 2019 Flight Design CTLS 2008 with Rotax 912ULS. These failures pictured, both fuel pressure and fuel flow, happened this morning to my renter. Do I have a defective fuel pump? It started happening when the engine is warm after an hour's flight and has been sitting on the ramp about 30 minutes, then happening during takeoff about 200'AGL. In the past it wasn't this bad, but he's noticed drops in pressure and excess pressure on the Dynon since the fuel pump installation. It's not just a sensor failure on the fuel flow today. He actually had to bring the throttle back from full power to increase the fuel flow, at 200ft AGL taking off. Your opinion please. Do I have a defective fuel pump?
IMG_20190712_101558_2019-07-12-2.jpg (You do not have access to download this file.)
  • Re: Is this fuel pump failure?

    by » 5 months ago


    Mechanical Fuel pumps do not typically become intermittent.
    Once they go bad, they stay bad.
    There are only two things that usually fail.

    The Diagram Cracks/Tears/Ruptures.
    This will be evident by the fuel leaking from the pump.

    The One-way valves fail.
    This will be evident by a loss of fuel pressure, usually totally and permanent.

    Your symptoms are typical of a fuel line Vapor issue. (Vapor Lock)
    This will make its appearance when the engine is hot soaked and/or when it is using a lot of fuel.
    The High temperatures can cause the fuel to boil.
    Unlike water, Gasoline boils in the 110-130°F range depending on the pressure.
    Low pressure can be induced by high altitudes or high flow rates.

    When the power was reduced, the flow was reduced and the suction produced by the fuel pump was reduced causing the pressure in the fuel line to increase, eliminating the vapor caused by the fuel boiling.

    This is a classic problem that can be eliminated by the use of an Electric Auxillary Fuel pump.
    Placing the Aux pump low on the firewall keeps the pressure up in the Area where the fuel tends to warm up within the Engine compartment reducing the possibility of fuel Boiling and causing the vapor lock.

    It is recommended to operate the Aux pump whenever you are at or below pattern altitude and during descents after a long cruise.
    - - -

    Also, notice that the photo shows the crazy combination of 5.4gph at 0.5psi.
    Is would be indicative of a vapor bubble rapidly passing through the flow meter causing a false Hi-Flow indication.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    whertzel1@yahoo.com
    Clicking the "Thank You" is appreciated by all.

    Thank you said by: Clayton Vincent

  • Re: Is this fuel pump failure?

    by » 5 months ago


    I am having the same issues with my 912uls with loss of pressure at high power settings. Activating the Aux pump returns the pressure to normal and switching it off after 5min the pressure remains normal. Bill,would mixing fuel ie 100LL avgas with 91RON or 98Ron PULP create a vapour lock issue? My aircraft is a high wing and fuel gravity feeds to mechanical pump. Thanks in advance.

  • Re: Is this fuel pump failure?

    by » 5 months ago


    100LL is slightly LESS prone to vapor issues but you would have to add a lot (≈50%) to make much of a difference.
    But then again, Rotax only recommends Leaded Fuel (100LL) if that is your only choice.
    Once you use leaded Gasoline your maintenance intervals need to be shortened significantly to mitigate the damages caused by the lead.

    Use of 100LL is NOT Recommended. Judicious use of the Aux pump, however, is always Recommended.
    - - -
    ??? PULP = PUMP ???

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    whertzel1@yahoo.com
    Clicking the "Thank You" is appreciated by all.

    Thank you said by: Clayton Vincent

  • Re: Is this fuel pump failure?

    by » 5 months ago


    Thanks Bill, I have just returned from a trip out to central Australia and for a couple fuel ups 100LL was the only fuel available. I generally use 98ron PULP and only started having the low pressure issue whilst on this trip when mixing the fuels . I have done a 200 hour service since the trip so it will be interesting to see if everything returns to normal now I’m back using consistent fuel.

  • Re: Is this fuel pump failure?

    by » 5 months ago


    The 100LL addition should not cause any problems with Vapor Lock.
    Is the Central Australia area much warmer or higher altitude than where you normally fly?
    - - -

    I think I figured it out.
    PULP = Premium Un Leaded Petrol. (An Australian Term)

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    whertzel1@yahoo.com
    Clicking the "Thank You" is appreciated by all.

    Thank you said by: Clayton Vincent

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