Charlie:

The engine is about 3 years old and has about 225 hours. The strange thing is I now do not need the boost pump for any operation, the problem has cured itself. I have taken the plane to 8500’ without the boost – in the past I have taken the plane to 18000’ without the boost. I have tried many combinations to get the same occurrence occur but everything is normal concerning fuel pressure and flow without the boost pump. I can blow through the vent lines into the tank with no problem and when I replaced the tygon tubing I found no kinks, only the water trap in each wing.

Ron Galbraith suggested that perhaps I was very lean of peak with the boost pump on and the flow went too lean when the boost was turned off. I was flying today to test that theory at 7500’. I leaned to 12.5 GPH with the boost on. I turned the boost off and the flow went to 12.4 GPH and the engine did not hiccup. I then tried 11.5 GPH with the boost on, turned the boost off and the flow went to 11.4 with the same pressure. I don’t think that was it.

Bill Hannahan was at the hanger today and we looked at the fuel pump diagrams in the manual. We concluded it could be the check valve in the mechanical fuel pump that was stuck open by something that was eventually washed out or the same thing could happen with a check valve stuck shut in the electrical fuel pump. If that is what it was I do not know how to troubleshoot. I also am having a hard time believing it could be a vent line problem in both tanks but it is the only solution I have!