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Our Lincoln Town Car -- which rides like a charm...

Dear Tom and Ray:

Our 1985 Lincoln Town Car -- which rides like a charm and
is in excellent condition -- is now giving us problems with
the air conditioner. It cools very well if I don't exceed
50 miles an hour. But on the higher-speed interstates, it
intermittently switches to "vent" and stops cooling. When I
slow down again, the cooling goes back to normal. Since we
travel the interstates often, and live in the Southwest, we
need constant cooling. What's wrong? -- John

TOM: Sounds to us, John, like you have a vacuum deficiency.
The car uses vacuum power to operate things like the power
brake booster, the cruise control and the ventilation
system. That vacuum power is produced by the engine: Every
time a piston sucks in air and gasoline, vacuum is produced
in the intake manifold and routed to these other devices.

RAY: When you turn on the air conditioner in this car,
vacuum-operated "motors" open the cooling vents or "blend
doors" and send cold air in to mix with fresh air (the
fresh air's there so you don't choke to death on your
wife's cigar smoke, John).

TOM : But if there's not enough vacuum for some reason,
those "blend doors" can't stay open, and what you get is
mostly un-air-conditioned air.

RAY: This problem would be the worst when traveling at high
speeds, when the throttle is wide open and the least amount
of vacuum is being produced.

TOM: So you've got to ask your mechanic to find out why you
have low vacuum. The worst-case scenario is that your
engine is wearing out, and you have low compression. And
that's a possibility in a car that's going on 12 years old.

RAY: On the other hand, it could be something as simple as
a hole in a vacuum hose, which would cost you 10 cents to
replace. So it's either 10 cents or $2,000. How's that for
an estimate, John?

TOM: Or maybe you have bad vacuum because it's time to
change the bag!
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