Articles, Operations, Maintenance

Leaking Fuel Pump

Posted on Jan 25, 2021 by Paul Jensen

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I was getting my airplane ready for a Thanksgiving trip with the family. On the prior flight, I noticed some fuel dripping from the bottom of the cowl. Suspecting a fuel leak, I pulled off the top cowl and looked around.

Sure enough, there was a big blue stain on the engine fuel pump. The fuel pump sits on the back of the engine, and the blue stain was dried fuel, easily seen on the bottom of the pump.

The next step was to run a test to see if the fuel pump was the source of the leak. I turned on the fuel boost switch and saw a drip coming from the fuel pump mixture shaft every two seconds. The throttle was pulled all the way back; the mixture knob was off as well, and the engine was not running. Sometimes it might be necessary to move the mixture knob back and forth to see a drip.

I canceled the trip and called my mechanic. The fuel pump had to be overhauled (or repaired). He told me the pump was easy to remove and explained how. Since I have done several owner-assisted annuals, I did not mind doing it. Two hours later I had it out – it should have taken an hour, but there was a cotter pin that was difficult to remove.

The location of the mixture shaft. The mixture control line attaches to the screw on the lower right. The drip was coming out at the base of the shaft.

On my last two fill-ups I noticed the MFD fuel amount did not match the fuel pump receipt. The receipt showed one to two gallons more each time, which is a big red flag. Where is that fuel going? In my case fuel was dripping down near the exhaust pipes – not good at all as it could cause an engine fire while taxiing. In flight the airflow would blow the fuel out of the cowl.

As a longtime aircraft owner, I have been surprised how often I could spot a problem just by looking. A new noise, a shake, or if something doesn’t add up, it’s good to check it out.

The fuel pump removed.

When the fuel pump is pulled out of the engine, be sure to pull out the breakaway shaft, shown here.

The fuel pump mechanic will check the shaft for any endto- end rotation, which would indicate a possible imminent failure. I hand carried the fuel pump into a nearby aviation fuel injection shop. They totally rebuilt it and ran bench tests for several hours. The cost was $626 (in 2012).

My mechanic installed the overhauled pump in an hour, and it has performed perfectly for the last six years.

My advice to you: Always be alert for any changes in your airplane – new sounds, leaks or shakes. It might be a simple problem or a serious issue. Don’t wait for the annual to investigate it.

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The overhauled pump installed.


This article was initially published in the January / February 2019 issue of COPA Pilot.

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