Fuel Bleed Basics
Bleeding the air out of the fuel lines is something every marine diesel engine owner eventually has to deal with. Changing fuel filters is a typical trigger for obtaining air bubbles in the fuel lines. But, one might also get lucky and have an operational issue while under way. Why do this? Diesel engines don't like to run when air is interspersed with diesel fuel into the cylinders. The engine will usually stop running and be very difficult to start again.The basic process is to open the fuel lines somewhere before the engine block and then pressurize the fuel to push bubbles out of the line. This pressure can be obtained from either a manual or electric lift pump. On most of our Ericsons, there is an electric lift pump which runs when the ignition is turned circuit is energized.
The fuel lines are "opened" by unscrewing a bleed nut or valve.
M-25 Fuel Bleed Assemblies
The base Universal M25 diesel engine (Model 5421) has two fuel bleed assemblies. One is mounted on the spin-on fuel filter assembly, often referred to as the secondary fuel filter. The other connects the fuel line from that secondary filter assembly to the mechanical injection pump in the engine block. Most of us bleed the fuel lines by loosening the hex-nut on the injection pump fuel bleed assembly -- it's easier to access and is the last point in the fuel lines to let air bubbles escape before entering the fuel rails of the engine. After cracking open the nut, the lift pump is run until all the bubbles run out of the fuel line and just diesel is escaping.Bleeding the fuel lines in this way has a few downsides:
- Requires a tool or wrench to open the hex-nut
- Usually requires taking the engine bay cover off (at least on the E32-3), in order to get the correct leverage with the wrench
- Diesel fuel will sputter about the engine bay and block, leaving some mess to cleanup
An Easier Way
Enter the fuel bleed assembly from the M-25XP engine -- also, what I call a "recirculating fuel bleed assemble". It consists of a similar apparatus with a thumb-wheel valve and an extra barb where another hose will attach. Instead of requiring a tool, one can open the valve by hand. The fuel escaping from the assembly is redirected to the extra barb, which can be plumbed back to the fuel tank. Viola, no mess. And the valve is so easier to open -- one can reach it just by opening the cabinet door under the galley sink on the E32-3.Parts
- Universal #300050 - https://discountmarinesource.com/st...roduct/product&product_id=34855&search=300050 (bleed valve assembly)
- Universal #301936 - https://discountmarinesource.com/st...roduct/product&product_id=34425&search=301936 (bleed screw)
- 5/16" three-way brass hose barb - https://www.amazon.com/gp/product/B08DFFTVP4
- 3/16" ID fuel hose - https://www.amazon.com/gp/product/B08R5TKXQ1
- 5/16" ID fuel hose - https://www.amazon.com/gp/product/B08R626XBN
- Scandvik 316 SS hose clamps (I think #4's?)
Installation
The fuel bleed assembly has just one bolt to tighten it down into the engine block where the injection pump is located. The barb which accepts fuel from the secondary spin-on filter is basically just a banjo fitting. I reused the crush washers -- I probably should have bought new, but I got lazy. I've been checking regularly for leaks and have found none so far.After reassembling the... assembly
it's time to splice into the return fuel line with a three-way hose barb. The fuel hoses which need to be spliced into are all different sizes. And, then of course, the barb on the new bleed assembly is super tiny. I know the hose sizes and barbs above aren't perfect, but with a little diesel and elbow grease everything clamped together pretty well.
