Taking your foot off the pedal does not close off the air supply on a
diesel since there is no throttle plate. It closes off the fuel supply.
The air intake is just a pipe direct to the intake valve. The older
governer style pumps this test may work on since they have a throttle
plate, but not anything past about 1976.
[EMAIL PROTECTED] wrote:
Let me take a shot at explaining this one. When you take your foot off
the throttle you close the air intake into the engine. With very little
air/oxygen getting into the intake manifold, then the fuel that is there
(if any) doesn't burn as efficiently. With the air intake closed off it
also makes it harder for the cylinder to draw air. Think of the sound
your shop vacuum makes when you plug the hose. The cylinder then has
more vacuum than normal because of the closed throttle and it tries to
draw air anywhere it can, especially thru the valve guides. If there is
extra space in the valve guides from wear, or seals with cracks or just
hardened up and not wiping well, then the engine draws in more oil
through the valves. After going down the long hill and suddenly opening
up the air supply so burning can take place the oil will burn "dirty"
giving the smoke.
You can prove the vacuum being higher by watching the vacuum guage go
off the scale to the high side when you suddenly take your foot off the
throttle. It also drops to nothing when the throttle is flat on the
floor. This happens with both diesel and gas engines. So the test will
work with both.
Does that make it easier to understand?
Ken
Don M.,
I question whether this method will work in a diesel: "To check your
valve guides and/or seals find a
very long hill and drive/coast down under compression, creating a large
vacuum draw, at the bottom of the hill apply fuel watch your rear view
mirror and if you get a noticeable puff of blue smoke it is seals/guide
issue."
Is this procedure recommended for gas engines or specifically for
diesel? Someone correct me if I'm wrong - does the mechanical fuel
injection pump on an OM 617 motor stop all delivery of fuel when
'compression braking'? If it doesn't, and some fuel is still being
burned, then I expect that the oil leaking through bad valve stem seals
would also be burned and there would be no puff of oil smoke when
throttle is applied.
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