I copied this somewhere but thanks to the Author and whoever posted it.
Your question ranks right up there with the most commonly asked questions we
receive. It is also, at least in my opinion, the most commonly misunderstood
aspect of the operating parameters of a diesel engine. The short answer is we
have always recommended that if the engine is going to be idling, low idle,
longer than 5 minutes that it be shut off. The reasoning is a little more
complicated but without getting into engineering 101 basically the following is
the layman's reasoning.
Diesel engines were designed to operate at peak efficiency running wide open. As
there are no spark plugs cylinder temp., obtained by compressing air in the
compression cycle, is necessary to ignite the fuel. At low idle the cylinder
temp. drops leading to incomplete combustion. This incomplete combustion leads
to carbon build up on the valves, in the oil (crankcase), on the fire deck of
the head and the dome of the piston along with fire rings on the piston. Engine
"slobbering", what appears to be oil, out of the air box drains and the exhaust
are other side effects. As worn oil control rings will also give you both of the
last two items this is often cited as the cause of an engine "slobbering". The
irony is that the carbon build up in the oil will lead to worn oil control
Now the reasons folks give for letting the engine idle. "I need to keep up cab
heat / AC in the winter / summer". If you going to be away for less than 10
minutes the engine will keep more heat being shut off than idling for that
amount of time. The AC unit will not maintain temp at low idle. "Truckers all
leave their rigs running while at a truck stop". They aren't paying for the fuel
or the repairs. "When you've been running all day you need to let the engine
"cool" down for at least ten minutes before shutting it down". Simply not true!!
I hope this has been of some help to you. If I can be of any further assistance
please contact me.
Senior reliabilt Sales Manager