Dear Mike: I have an intermittent problem starting my Corvette. Most of the time it starts up fine, but once in a while it won’t crank over. Twice I’ve had it towed to the shop down the street, and it starts fine for them. In fact, if I let it sit for a half-hour, it starts most of the time.

I’ve even had the starter motor replaced.

— E. McE., Sharing Cross, Va.

A: Nope, it’s probably not the starter motor, and I’m surprised that the shop didn’t figure this out while your ‘Vette was still being offloaded from the flatbed in their driveway. Maybe they happened to have a starter lying around.

Like many GM cars of the late 1980s and 1990s, your car is equipped with VATS, GM’s “Vehicle Anti-Theft System,” first introduced in 1985 on the Corvette. I’m very familiar with it, as it happens, because back in 1984 I wrote Chevrolet’s training manual on VATS for service technicians. Good system. It went through a couple of variations, and was relabeled Passkey and Passkey II later. It’s no longer used, however, in part because of the problem you’re having.

Your ignition key has a small pellet pressed into the key blade. Inside this pellet is a resistor, which connects to two electrical contacts inside the lock. When the key is inserted into the lock, the resistance of the pellet is read by the VATS computer. If the resistance is correct, the car will start. If the resistance is not correct, the car cannot be started for several minutes — even if the correct resistance is substituted right away.

There are 15 different resistance values. This way, even if a car thief punches out the ignition lock and jumpers a resistance into the VATS computer, he’ll have only a small chance of starting the car the first time — and, if he fails, he’ll have to wait several minutes to guess again. The theory is that few thieves will be willing or able to wait around for the hour or more it could take to try all the values in sequence.

GM dealers had a VATS repair box that was little more than all 15 resistors wired to a switch to allow the technician to try them all, one at a time, while waiting around for that hour or more. At shop-time rates, of course.

Back to your car. I’d guess that there is dirt, corrosion, pocket lint, leftover ketchup or something on the contacts on your key’s resistant pellet that’s making the contact intermittent. Clean the contacts with a typewriter eraser — remember typewriters? You might also try to clean the contacts inside the ignition lock with a Q-tip and some electrical-contact cleaner.

And don’t ever mail your ignition key to someone by taping the key onto a piece of cardboard and putting it into an envelope. The sticky adhesive will cause problems. Don’t ask how I know this — remember, I wrote the book.

Dear Mike: I filled my minivan’s gas tank last night. My wife took it to the store today, and had a hard time starting it on the way home. There’s a strong odor of diesel fuel around the car.

I suspect that the station may have mixed in some diesel fuel, whether by mistake or deliberately. What’s the best way to clean this up, and will it cause any damage?

— G.N., Mountain Home, Ark.

A: To find out if that’s the case, assuming that you have a really accurate scale, you can weigh some of the fuel from your car and compare its weight to that of a similar volume of fresh gasoline. Diesel is somewhat denser and weighs a few percent more than gas.

More practically, though, if the fuel smells of diesel, that’s a dead giveaway that it’s contaminated.

Drain the tank of as much of the adulterated fuel as possible. Jumpering the fuel pump so that it stays on continuously and running a hose from the fuel rail to a can will get most of it. Unfortunately you’ve recently filled the tank, so now you’ll have 15 gallons or so of adulterated gasoline to dispose of. It’s no longer suitable for use in any vehicle or even in your lawnmower.

The remnant diesel will lower the octane of your fuel, so refill your tank with ultrapremium to prevent any knocking. There should be no damage, unless you’ve driven the car far or fast with this poor-quality fuel.

Dear Mike: The owner’s manual for my 1997 Mercury Sable says to use two separate types of spark plugs when doing a minor overhaul. Every mechanic that I have ever spoken to says that’s nonsense — one type of plug is all that is necessary.

Who is right?

— W.H., Staunton, Va.

A: Your engine, like many others, uses a distributorless ignition system that fires two spark plugs from a single coil. On one plug the spark jumps from the ground electrode to the center. The second plug sharing that coil fires from the center electrode over to the ground.Why should we care? The second plug uses a precious-metal electrode, usually platinum or iridium, to reduce electrode erosion. The spark jumps from negative to positive — proving Ben Franklin wrong, incidentally, when he originally designated positive and negative. Small amounts of vaporized metal leave along with the spark, causing erosion of the negative electrode.

Ford puts the precious metal on only one electrode, the one that erodes fastest. Why? Have you priced iridium lately? If you replace the originals with a precious-metal plug that isn’t made by Motorcraft, Ford’s brand, or isn’t Ford’s designated replacement plug, it will use precious metal for both the ground and center electrodes. Otherwise there’s no difference in the plugs.

That’s also why, if you have occasion to remove them, you need to reinstall each original plug into the same cylinder. If you reverse them, they’ll wear out rapidly.

Your mechanics need to do a little homework.