Also, check your Intake Air Bypass (IAB) mechanism. Here is the procedure...
To check your IAB for proper operation, the next time you change out the high pressure fuel filter, check the IAB first.
• Open up both sides of the upper cowling by first removing the two bolts under the cover at the rear of the engine – those two below the exhaust outlet.
• Remove the six or so bolts on each half of the top of the cowling.
• Peel back each side of the cowling so it clears the exhaust outlets.
• (This is the same as you would do to get access to the HP fuel filter to change it out.)
There sitting in front of the HP fuel filter you will see the IAB control diaphragm. Disconnect the vacuum hose to the diaphragm. The diaphragm is held in place by two Phillips head screws that you typically remove to get to the HP fuel filter. Remove those two screws, swinging the diaphragm aside.
Grab the lever coming out of the diaphragm and pull on it as if the diaphragm was being activated. It should move until the spring arm engages (or nearly engages) the “stop” adjustment screw. Then let go and it should pop right back in place. If so, the IAB is operating as intended.
Before you re-install the diaphragm, hold it in your hand, gently push the arm back into the diaphragm, and hold your finger over the opening in the tube that the vacuum hose attaches too. When you let go of the arm, it should come out a little bit and stay there until you remove your finger. If so, then the diaphragm is operating correctly. If not, the diaphragm is leaking air and must be replaced. Unfortunately, it looks like a replacement is not available, since that diaphragm is part of the entire IAB assembly. However, I suspect that if you search around a bit, you should be able to find a replacement for this fairly standard part.