2004 Fleetwood Discovery 39J
Cat C7 Blow-by Extension
As most of you already know, a diesel engine operates under high internal pressure, and
generates a fair amount of blow-by, which are combustion gases that make it past the piston
rings into the c
rankcase. In a gasoline engine, these gases can simply be routed back into the
intake and get burnt away. But because diesel intakes are pressurized instead of vacuums, you
can't simply route these gases into the intake, and manufacturers up until now have simply
vented them to the environment through a downward-pointing rubber hose. These combustion
byproducts are very oily and if allowed to touch anything, can build up quite a thick layer of
grime rapidly.

Some diesel engines have radiators on the
side of the coach, so blow-by exiting mid-engine  
simply works toward the back, soiling the chassis and toad but not affecting the radiator
much. If your coach has a
rear-facing radiator however, these oily fumes can quickly build up
in the radiator fins as they're sucked up and pushed through it by the fan, and reduce the
radiator's efficiency greatly. Many expensive engine failures have been directly attributable to
this effect.

Thus, in this circumstance you MUST route the fumes away from the radiator
at least. Neither
Caterpillar nor Freightliner nor Fleetwood take responsibility for this situation, so you're on
your own to come up with a solution. Here's what a good solution should accomplish as a
minimum:


Solving the last constraint is trickier than simply extending the tube to the back. What
we've
come up with is the following:

1. A 1-1/2" I.D. thick-walled reinforced clear PVC hose slips over the end of the existing blow-
by down tube (held on by a hose clamp) and gently extends toward the c
urb side of the open
trailer hitch crosspiece.  
PVC Hose   

2. At the lowest point in the hose (approx
imately midway), a 1/8" hole is drilled to form a
funnel-shaped path for liquids to drip out of the tube. A plastic quart container with flat sides
was drilled through with a 1-1/2" hole saw (undersized compared to the O.D. of the PVC hose)
midway up the sides, and then slid over the PVC hose upside-down to cover the hole, blocking
the release of gases but sequestering liquids.  
Sludge Container   

3. A couple of 1-1/2" steel muffler sections take a 90 degree bend into the center of the hitch,
continue through the hitch and then bend 45 degrees aft and slightly down, exiting just in front
of and below the main exhaust. The pipes are joined with J-B Weld for a leakproof seal. Short
pieces of foam pipe insulation a
round the pipe just inside both ends of the hitch crosspiece stop
the pipe from possible rattling.  
Elbow  Tailpiece  

Thus, the flow is not restricted in diameter and is self-draining. The extension cannot collapse
and is protected from the exhaust heat
. The gases are deflected down and away from the
vehicles by the powerful main engine exhaust, keeping the coach undercarriage and the toad
free from grime. Liquids remaining in the extension collect in a separate container, and these
liquids can be monitored and drained at your leisure.

You'd be surprised how clean your toad can remain when that thin film of oil is not always
being deposited on it, collecting dirt, in addition to not having those black smudges.



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