Dear Mike: When I accelerate, the air conditioner stops blowing through the vents. When I brake or simply coast, the air starts to blow again.

What’s going on here?

A: Many cars use engine vacuum to operate the blend-air doors in the underdash plenum. These doors control the air that passes through the heater and A/C evaporator.

Manifold vacuum varies widely between near-vacuum and normal atmospheric pressure. It’s very high at high rpm/closed throttle, because the engine is sucking all the air out of the intake manifold, and that air can’t get in past the closed throttle blades. At wide-open throttle, though, air rushes in as fast as it can and there’s virtually no vacuum.

There’s a check valve, and sometimes a small reservoir, that isolates the vacuum supply to the HVAC (heat/vent/air conditioning) system. If the check valve leaks, or if there’s a leak in the vacuum lines or fittings underdash, the blend-air doors won’t operate properly until the vacuum is restored when you let up on the gas.

Find and fix that leak. Dear Mike: I have a 2002 Jeep Liberty, 3.7L, automatic transmission. The engine stalls when in warm idle at rest.

Two dealers have “diagnosed” the problem as a throttle-body problem. One dealer wants to use “induction cleaning,” while the other wants to replace the throttle-body assembly.

I was hoping they could simply tell the engine computer to increase the RPMs of the warm idle, but I was told that it is not possible to adjust the warm-idle RPMs.

Can the RPMs be adjusted and, if not, which is the least expensive, induction cleaning or throttle-body replacement?

A: The idle revs are set in the computer firmware. You’ve probably got a dirty idle-air adjuster, and maybe a dirty throttle body too.

I’d try cleaning it first. Douse the throttle blade with carb cleaner. Remove the idle-air controller and douse it too. Spray a bunch of carb cleaner into the hole the idle-air controller came out of, but don’t let any of the cleaner or any resulting sludge dribble down into the engine — so it might be a good idea to remove the throttle body and do this on a bench.

Warning to other readers who may wish to attempt this operation on their own vehicles: Beware! This engine does not use a manifold-airflow sensor, but many do. This sensor is very fragile, and will probably be damaged if overspray and/or sludge come anywhere near to it, particularly to the heated-wire sensor itself. Protect this device by removing it or removing the throttle body before attempting any cleaning operations.

Dear Mike: I have a 1990 Chevy half-ton pickup truck. The blower motor runs on low only. I changed the resistor and still have the same problem.

What else could be wrong?

A: When the blower runs only on high, it’s usually because the resistor has burned out. The resistor is not in the circuit on high speed.

If the blower runs only when the switch is set to low speed, it’s got to be in the switch or the wiring. On the other hand, if the blower runs slowly regardless of what the switch is set to, it’s a bad blower or a poor ground.

Get a voltmeter and start chasing the voltage drops.