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Dear Jim,

I'm having a weird problem with my 2001 GMC Jimmy (4.3L, V6). Whenever I'm going down a steep hill and using the engine to slow down, the engine RPMs tend to go up and down without my touching the gas pedal. It only happens when the air conditioning is on, so I figure maybe the computer is telling the engine to rev up and supply extra power to the A-C system. But that should only happen when I'm idling, right?

Why would the A-C need extra power at 2,000 to 3,000 RPM going downhill? Any idea how I can fix this?

-- Bill Lyons

This is one of the trickier questions I've gotten in a while, bro, but I'm pretty sure I know what's going on here.

Everything you're describing sounds like a fuel injection problem, and one that shouldn't be that difficult to address. I haven't seen a problem exactly like yours before, but I have seen the RPMs fluctuate in many cars out on the open road.

Usually, this kind of revving issue is the fault of a clogged fuel injector -- it won't close all the way, so it sprays excess fuel into the engine's intake manifold. When that fuel finds its way into the combustion chamber, it causes the RPMs to vary unpredictably.

Fortunately, your Jimmy has a throttle body injection system, which is a lot easier to access than later systems, so if you want to take a shot at this repair yourself, here's how: Pull off the air cleaner and you should be able to see the fuel injector. Just spray a little carburetor cleaner on it, down near the plenum, and see if you can clear out the blockage by scrubbing with an old toothbrush -- not hard enough to damage the injector, just enough to get it clean. Also, go ahead and add a bottle of fuel injector cleaner to the gas tank next time you fill up.

Your other option, of course, is to take your GMC in and have a professional mechanic rebuild the throttle body. Whichever way you do it, do it soon, or that revving trouble will start showing up even without the steep hills and air conditioning.

Tip of the Week:

Dead batteries are a frustrating reality, as anyone who's gone searching for strangers with jumper cables knows. Consider buying one of those rechargeable jump starter units -- it'll cost you $100 or so, but you'll never be left stranded without the zap you need to get going.

Buffalo Jim, owner of Allstate Auto & Marine Repair in Las Vegas, writes a weekly column. Send your automotive questions to him at buffalojim@lasvegasniftynickel.com. If Jim picks your question to answer in Nuts & Bolts, you'll also be the proud recipient of a deck of official Buffalo Jim playing cards, presented in a classy, tin box bearing his hairy image.