Fuel Filter Mystery - The Answer!

What was the cause of the periodic RPM drops we've seen on 2 occasions this southbound trip? Seemed like it could be due to a clogging fuel filter, especially since changing the Racor primary filter element apparently cleaned up the first occurrence near Charleston SC.

 

There were some good guesses, both via the blog comments and via emails. Some suspected the secondary fuel filter and others noted that both occurrences had similar weather conditions, with strong winds that had us piloting from the lower helm in the salon. Those who thought that the weather was a factor were on the right track.

 

Did you notice that the symptoms were similar to what I had seen 4 years earlier with a clogging Racor, but not identical? With the clogging Racor the RPM would dip for a bit, and then resume back to normal for a while. The occurrences this year had dipping RPM but I had to manually bump up the throttle to restore normal RPM.

 

Here's a picture of Spray's flying bridge helm, where we weren't driving from when these RPM drops occurred:

You can see the engine controls at left, with the black knob for FNR gear shift and the red knob for throttle. Of course on the windy/rainy days in question, the upper helm actually looked more like this :

When we are piloting from down below the upper helm is protected from weather by the green canvas cover shown above.

 

So there we were last week, anchored in Lake Boca Raton, listening to the wind howl as we pondered the issue with the fuel filter. In strong winds like that Spray makes a variety of sounds from creaking anchor lines, and whistling mast stays, and flapping canvas. Flapping canvas?

That's when it hit me. Maybe the problem wasn't fuel filter related at all but instead the flapping helm cover was bumping the throttle lever (you can't tell in the above photo but the throttle lever is a bit taller than the gearshift). I checked the blog from the run into Charleston to confirm that we had strong following winds that day too.

 

We decided not to change the Racor element the next morning but instead to experiment with the flying bridge canvas as we traveled. Sure enough we confirmed that simple flapping of the canvas against the throttle lever was the source of the RPM dips, which I would then correct by bumping up the throttle from below. Doh!

 

Changing the Racor element in Charleston seemed to fix the problem only beacause our next travels didn't re-create the canvas flapping condition, until last week.

 

So now I'm hunting for just the right cover for the upper controls that will prevent this. Sorry for dragging you needlessly through the finer points of fuel filters (no I'm not). The good news is that I'll be ordering one of those nifty Racor vacuum sensors that Jeff Siegel suggested, so next time I'll know if the Racor is at fault.

 

Posted with Blogsy