Re: [a/s] FW: [VAC] Diesel Engines

[Jim Dunmyer <jdunmyer@xxxxxxxxxxx>]

Hunter,
 That cool-down is only important if you've been pulling it very hard,
especially in hot weather. IOW:, if you pull off the Interstate where
you've been towing your A/S uphill and/or into a headwind, park it and
hold the throttle at a fast idle for a few minutes. This allows the
engine to cool down and temperatures to even out before shutdown. Don't
forget to drop the throttle to a slow idle for a few seconds before
shutting it off so the turbo can spin down while still being lubricated.

I used to have a trenching machine with a 3208 Caterpillar engine, and
when I got to the end of a long run (maybe 30+ minutes of hard pulling),
the oil pressure would drop from 60+ PSI at high idle to under 20 PSI at
idle. If I held the speed up for 5 minutes or so, the pressure would
rise to the point where idle oil pressure would be 30+ as the oil cooled
down. You could see the oil pressure rising as the engine cooled.

It's possible to boil the coolant if you've been pulling the engine hard
in hot weather and just shut it off without cooling it a bit. The engine
will 'heat soak' the coolant will boil, the oil in the turbo will
overheat and cook, and other Bad Things will happen.

The manual for my VW recommends a cooldown if you've been hammering it.

None of this is important if you've not been pulling hard, especially if
the weather's cool.

Perhaps more important is how you treat your turbocharger. The little
turbine spins at many tens of thousands of RPM, and lives only because
of constant pressure lubrication to its bearing. It will begin to spin
immediately when the engine starts, yet there's only residual oil on the
bearing. You don't want to rev the engine at all when it first starts,
let it build oil pressure, then wait a few more seconds to make sure the
oil is lubing the turbo. On the other end, don't shut the engine down
while it's revved up. When the engine stops, so does the oil pump, yet
the turbo will continue to spin for a few seconds. If the engine is
revved up, the turbo will be spinning faster and will coast longer, all
the while w/o lubrication. To my knowlege, all turbos use a plain sleeve
bearing, NOT ball or roller bearings. Sleeve bearings will last a long
time as long as they have constant lubrication, but they'll die an early
death if run w/o oil. If you think this is unimportant, ask your Ford
service manager about replacement costs next time you see him.

Of course, like most mechanical mistreatment, the effects are usually
long-term. It's the difference between getting 100K miles out of your
turbo and 200K or more.

                                          <<Jim>>