Perkins 4-108 Idle

[seabreeze384]

I have a 20 year old 4-108 with 1800 hrs in a sailboat application. It has run great for the 1000 hours I have operated it. Recently it bagan an unusual behavior. After start up and a run of parhaps 30 min to an hour it's rpm can be run up/down as normal and will idle normally at 800 rpm when trottled to minimum rpm. Just like it has done for years. The last two times I've started the engine after a few hours sailing time (preceeded by the normal operation mentioned), the engine wants to idle at 1000 rpm at minimum throttle setting. I've checked to see if the throttle min/max screws are loose - nope. Travel of the throttle cable and throttle lever seem normal - no sticking. Any ideas as to what would cause the change in rpm as described?

[AnchorageGuy]

Could be a number of issues. When was the last time your filters were changed. As the filter gets clogged the engine will idle at different RPMs trying to get more fuel. You might also have a problem with the injector pump. WG

[Stillraining]

Is it mechanically governed or Hydraulically...Perkins used both.

[seabreeze384]

To answer your questions - I don't know how the engine is governed. How do I tell? The fuel filters were changed 10 hours ago and the high pressure fuel pump was rebuilt 800 hours ago. I can change fuel filters again just as a precaution - not a big deal. The high pressure pump is another can of fish. Very expensive to rebuild. When it failed 800 hours ago, the symptom was failure to start occasionally. Currently the engine starts and runs great. What symptoms other than change in idle rpm would I look for to determine if the pump is the problem? By the way - the idle rpm is either 800 or 1000. It does not 'hunt' or fluxuate. It's either one speed or the other, no in between.

[AnchorageGuy]

One other consideration and it might be a long shot. But do you hear and feel the difference in RPMs or are you just looking at the tach? You see where I am going? WG