How to Bleed Fuel Line on a Perkins 4-108 Diesel Engine

Okay, in scouring the internet I couldn’t find a good reference on how to prime (bleed) the fuel line on a Perkins 4.108 diesel, which is the 50 HP diesel engine on Zia (our Morgan 38 sailboat).

In fact, I found in some forums that owners admitted to being so afraid of bleeding the system that they have never changed their fuel filters (!).

I didn’t want to live in fear, nor did I want a dirty or plugged fuel filter, so here is how I bled the system.  The Perkins manual is pretty good, but I had problems finding the gizmos they mentioned because the pictures didn’t look like my engine and some of the gizmos were hidden on the back (port) side of the engine which I couldn’t see.  This was also the first diesel engine I’ve become intimate with, so my ignorance cannot be overstated.

If you are going to bleed the system anyway, you might as well change all of the fuel filters.  In my case, I have a primary Racor filter under the galley sink.  I changed that, then I also changed the secondary fuel filter, which is mounted on the starboard side of my engine.

The photo below is of the starboard side of our engine, via the access panel from the starboard quarter berth.  The secondary fuel filter is the white cylinder under the bolt marked with (A).

I used a 5/8″ box wrench to loosen the bolt on top of the fuel filter (A), then use the manual lift pump lever (B) to pump until clean fuel with no bubbles comes out from under the bolt.  You just raise the little lever up and down until something happens.

Then tighten bolt (A).

Next, using the forward access for the engine (removing the stairs), reach around to the port side of the engine, where the injector pump is located.  The photo below shows a red box with the location of the detailed photo that follows.


Detailed area on engine port side. This is the area shown by the red box in the previous photo.Referring to the photo above, loosen small bolt (5/16″ box wrench) on side of injector pump (A).  Facing the engine, with left hand, reach around to the starboard side of the engine and pump the manual lever on the fuel lift pump (that you pumped in previous step) until fuel with no bubbles comes out from around the bolt, then tighten bolt (A).

Next, loosen bleed bolt on top of the governor (B) with 5/16″ box wrench and again manually pump the lift pump until fuel with no bubbles issues from bolt.  I think it was draining down the back of the bolt, so I applied a little pressure to the bolt from the front to tilt it a little so I could see the fuel coming out the front.  Then tighten bolt (B).

I then cranked the engine a few times to see if it would start.  It wouldn’t start.  I’m cautious about cranking the engine for more than 10 seconds because it can pull water into the cylinders (don’t ask why I know that, it’s a little embarrassing).

Then I loosened one of the injector compression fittings (C) and cranked the engine. After a few seconds it started up and ran fine after that.  Be sure to tighten the injector compression fitting back up.

I didn’t have to loosen the other injector compression fittings, although some of the instructions I’ve seen say you might need to do that.

Note also that our engine seems to be sensitive to the cold. If it’s below 45 degrees F air temperature, I need to spray a small amount of starter fluid into the air intake.


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