Darn it, my engine studder/stumble is back

anchorclanker · Jun 9, 2013

Definitely sounds like fuel starvation. Check your tank vent. That it runs 45 minutes and then starts slowing down makes me think the tank may not be venting properly. At low speed it may get enough air that you cant duplicate it. But at full power on the water it uses more fuel, possibly enough to put a vacuum on the tank. Test it by opening the filler cap when it slows down, or install a gauge so you can watch it. You could tee in a gauge to the tank feed line to the engine that reads both pressure and vacuum. Whatever the static feed pressure is, it should not drop much when running.

Also, check your fuel pump pressure and watch it as the engine loses power. Other than a clogged or kinked fuel line, fitting, faulty filter or housing, I really cant think of anything mechanical in the injection pump that could cause your problem.

FastOlson · Jun 9, 2013

About 4 years ago I replaced all the fuel lines (supply and return) in our '88 model boat with the same model engine, M25XP.

While trouble-shooting a subtile vacuum leak that was finally traced to the old Racor, we found that it was really really hard to blow air back through the OEM fuel supply line. We started with the hose run from the tank, section at a time, to be sure that our "starving" engine was not suffering from a clogged hose or tank connection.

I found that the original copper supply line had narrowed over the years due to a buildup of deposits inside (like my arteries, maybe?!) That old line had copious dings and tweeks in it, as well.

A mechanic friend of mine advised that after a lot of years it is good preventative maintenance practice to replace the old fuel lines with new current-USCG-approved reinforced diesel hose.

Sorry to have no direct help to offer you otherwise. Good luck on trouble shooting.
Loren

Witzend · Jun 9, 2013

These boats will gravity feed to about 3/4- 5/8 of a tank,after that they will start to die if the fuel pump is not running.
Does your alarm sound when you turn the key on and before the engine is started? It should, if it doesn't, check the shovel connector for the +12 on the fuel pump, it gets corroded and won't pass current. You can also hold the key in the spring loaded position, (engine not running) and have some one listen to the fuel pump below, it should be making a clicking sound.

DanM · Jun 9, 2013

iSailor,

When you say all fuel filters have been changed have you also cleaned the internal screen on the Facet electic fuel pump (if you boat has it, I have an older c30)? Easy to miss this one and mine plugged up when I first used a biocide in my fuel after I bought the boat.

DanM.

Albanach · Jun 10, 2013

Had a similar problem last year, after the 'expert' that polished my hull had clogged up the screen of the external vent.

Stu Jackson · Jun 10, 2013

DanM said:
iSailor,

When you say all fuel filters have been changed have you also cleaned the internal screen on the Facet electic fuel pump (if you boat has it, I have an older c30)? Easy to miss this one and mine plugged up when I first used a biocide in my fuel after I bought the boat.

DanM.

Also check the plumbing: should be tank, primary, fuel pump, engine. Many builders did it wrong. You want the primary filter to catch the crud, not the small filter in the pump.

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