John Bange wrote:

>>For some reason,
>>COMPRESSION BRAKING seems to throw something out of whack.
>>
>>Is this a clue? Or a red herring? If it's a real clue, where would you
>>look?
>>
>>
>>
>
>FWIW, compression braking is generally only found on diesel trucks. Theres a
>valve flap in the exhaust manifold they close and the pistons have to
>compress air against that on the exhaust stroke. On gasoline engines it's
>actually VACUUM braking, i.e. the pistons are pulling against a vacuum in
>the intake tract. The big air valve in the throttle body provides the seal
>when pressure is taken off the gas pedal. Dunno what might be the problem
>after pulling a big vacuum for a while, but it might be worth looking for
>collapsed or rotten vacuum lines.
>
>--
>John Bange
>'90 Vanagon - "Geldsauger"
>
>
>
John, would you please expand on that process a bit.  As I have always
understood it, in gasoline engines,  "compression braking"  relied on
the compression stroke to provided resistance to slow down the vehicle.
Without fuel and ignition the engine just basically becomes a glorified
air compressor, and the compression resistence will act to slow down the
vehicle. Please enlighten us with more detail   as to how "vacuum" plays
a part on "braking" with a gasoline engine.

Thanks,

Regards,

John Rodgers
88 GL Driver