Finally! We have a clear identificaiton of the cause and have fixed the issue. Following my earlier post in this topic, I sent along the writeup and video to the Nordhavn Owners Group, which has some 750 members including owners and top marine experts familiar with Norrdhavn systems. This group is an incredible resource representing a wealth of experience, and sure enough, I got some very good suggestions for potential causes and methods to troubleshoot the issue. I shared these with the Philbrooks staff and they started in on it Monday morning, Sept 30th.
The key was the “bucket test” suggested by Bob Senter of Northern Lights/Lugger (the engine manufacturer). The idea was to get a bucket filled with fuel and run short supply and return lines direct, bypassing the entire fuel delivery system. If everything is fine, you know that the issue is with fuel delivery. If not, you suspect the engine (fuel pump, inejector pump, etc). The philbrooks guys did a variation of this using clear hose so they could see what was going on.
Long story short, there were obvious, large, frequent air bubbles in the clear hose when connected to the boat’s fuel delivery system. No such bubbles (obviously) when straight to the bucket. Now the issue was to identify the source.
By the end of the day Monday we were able to go out on a Sea trial on which we bypassed the primary fuel filter assembly (which contains two replaceable fuel filters and allows you to select which one to draw fuel through). The engine ran perfectly, not skipping a beat. The conclusion was that there was a leak somewhere in that manifold, so a new one was ordered to arrive mid-day Tuesday. Unfortunately, testing at the dock after installing the new manifold still revealed air bubbles, to be chased down on Wednesday.
On Wednesday we identified a problem with the fuel selector valve in the new manifold that allowed air into the lines. Tightening that valve elminated the problem, but there were still air bubbles getting through. The source was determined to be the supply lines from manifold to the engine. These were replaced, and the Algae-X filter removed just for good measure. A final sea trial proved that all of the air bubbles had been eliminated, and the Maretron fuel flow sensors, now reconnected, showed a very steady rate of fuel consumption. Here is a clip showing the display at our normal cruising RPM and another at WOT.
We are going to depart directly from Philbrooks to Neah Bay today (Thursday, 10/3/19) and will be making the passage down the coast on our own, with the help of our weather router, Bob Jones of Ocean Marine Navigation Inc.
2 thoughts on “Engine RPM Variation – Fixed!”
Very complicated but thankfully done. Have a wonderful trip. LYPTS&B