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Dear Jim,

A couple of weeks ago, my 1986 Chevrolet half-ton pickup (5.7L, V8) developed an obvious engine miss. My mechanic replaced the plugs and wires, which I expected, but he also said I need a valve job. The thing is, I already had an $800 valve job done less than 20,000 miles ago. The truck is worth the money to me, but I hate to think I'm spending it on parts I just replaced that recently. Do you think another valve really makes sense?

Well, bro, it's hard to say anything definite without getting a look at that engine, but I'm guessing your mechanic is right about a problem with the valves. And I know that needing another valve job so soon sounds doubtful, but I can tell you a story that will make it clearer.

Back in the '70s, a car came into my shop with an engine miss, and I did all the things you're supposed to do in this situation: new plugs, wires, distributor cap and rotor. That didn't help, so I ran a compression check on all cylinders, looking for a leak somewhere. One cylinder did come up low, so I assumed the car needed a valve job.

Easy enough.

Nope -- still didn't fix the engine miss. Finally, I peeked inside at the cam shaft, and found that one of the cams -- the lobed, rotating unit that opens and closes the valves -- had worn down to almost nothing.

I replaced it, and the engine suddenly ran like new.

In other words, it sounds like there's a problem with the valves, but it's not the valves' fault. It's a cam. The moral is, don't have a valve job done until you've checked the cams first and know they're in good working order.

Actually, it's not so common a problem, but I'll bet you have it, because I've seen it crop up on several Chevy 350 engines over the years. To check, pull off the valve covers and have someone turn the engine over.

Make sure all the valves' rocker arms are moving the same way. If one of them isn't, brother, proudly tell your mechanic you need a new cam.

Tip of the Week:

Remember, drivers, if you want better service at the shop, it often helps to know a bit about the problem you're having. Research online. You won't be an instant expert, but most mechanics will respect that you're trying -- politely, of course -- and that's good.

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.