There's been a number of posts here recently concerning high idle problems with both the Wren 44 and SuperSport. I have one of each, and have been beset with high idle problems on the 44 since late last year. Discussions with Wren resulted in replacing the fuel pump but with no improvement. The motor then spent over a month back with the Wren team trying to solve the problem. Nothing specific was found, but it was a good time to get an overhaul and bearing change.

After re-installing it in my L39, the first 2 flights were normal, and then the high idle problem resurfaced. With it set at 55k prior to take off, and the throttle never below about 40% throughout the flight until the landing flare, it would taxy back in with the idle anywhere between 65-80+k rpm.

I replummed the whole fuel system with large diameter tubing to ensure a easy fuel feed and I've just ground run it. On the first run, after a smooth calibration run up and down followed by some throttle transients at part power, the idle was high at 67k. Three runs later I was pulling on the fuel pump leads and prodding the fuel pump, as mentioned in a previous post here and the engine suddenly shutdown. Interesting!!!

I pulled the protective cover off the back of the pump motor, exposing the circuit board. After another start, I started pushing and pulling on the board. Any slight pressure on the circuit board would cause the motor rpm to drop. Holding pressure for a few seconds and then releasing would cause the motor to have a high idle, replicating the in-flight symptoms. Pushing down on the center plastic motor end cap surrounding the rotating motor shaft (without touching the shaft) would also cause an rpm drop, including complete shutdowns. This would also explain two or three motor shutdowns that I have experienced on some slightly heavy landings, with both the old and new fuel pumps.


Following up on the other thread regarding Wren problems with the Hausl fuel pump, I believe that I have identified the cause of at least my high idle problem. I'll contact Sara tomorrow for a new pump to try.

This may be worth checking on the other motors that have experienced idle problems.


Paul