Q.

Our car recently started backfiring, and just last week the problem became so severe that it blew out a muffler.

We’ve taken the car to a number of mechanics, and a new carburetor was installed, but the car still sounds like a rifle going off at times. The noise is scary enough, and we can’t afford to replace mufflers regularly. What should we do? – L.R., Oakland, Calif.

A.

What you are experiencing is a muffler explosion and not a backfire. A backfire is caused by fuel igniting in the intake manifold, and the resultant explosion is ejected through the carburetor throat. A muffler explosion, not surprisingly, takes place in the exhaust system.

Have the mechanic look for a malfunctioning air valve, which is located inside the air cleaner snout, or for a bad air pump diverter valve on your air-injection reaction system.

Other possibilities include an engine vacuum leak, a bad distributor, inaccurate engine timing or worn valve-train parts. A malfunctioning carburetor can also be at fault, but in your case it obviously wasn’t.

Q.

I recently bought a ’90 Ford Ranger. The owner’s manual calls for 5W-30 oil. Why can’t I use 10W-30? Why is 5W-30 recommended? – D.S., Billings, Mont.

A.

Look at the owner’s manual again. 5W-30 is the “preferred” oil. Nowhere does it state that you can’t use 10W-30. If you look at the temperature/viscosity chart in the manual, you’ll see that 10W-30 can be used in almost any outside temperature.

Ford – and all other car manufacturers – recommend 5W-30 because it helps meet the government’s Corporate Average Fuel Economy (CAFE) standards.

Go ahead and use the 10W-30; it’s perfectly fine to do so and it’s well within the limits of what Ford recommends.

Q.

I have a ’79 Buick in my driveway which hasn’t been started or driven for more than two years. Before I try to start it, should I add anything to the gas tank so the old gas won’t harm the engine? – H.B., Quincy, Mass.

A.

I wouldn’t add anything. I would have the entire fuel system drained and cleaned, including the gas tank. I would also have a new fuel filter installed.

After that, add fresh gasoline to the tank and add a container of dry-gas additive, which will help remove any moisture left in the system.

If you ever have to leave a car for a long time and don’t drain the fuel, be certain to add a gasoline stabilizer. I’ve used a product of this type called Sta-Bil. It keeps the gas fresh and free of gunk for a year or more. I use it in my chain saw and power mower instead of draining them between uses.

Q.

My daughter is away at college, and her ’81 Olds is parked outside and is leaking oil. I’m having a problem finding someone to steam-clean the engine so the leaks can be found and repaired. I added a can of stop-leak, and it helped a bit. – L.G., East Northport, N.Y.

A.

You should be able to find someone to steam the engine by looking in the yellow pages under “Automobile Detailing.”I hope it helps you find the leak, which I imagine will be in either one or both of the engine seals.

Q.

I have always wondered what the difference is between the more popular major brands of gasoline versus the lesser known or cheaper brands. I’d like to know the quality difference – if any – and how it affects the performance of a car. – F.B., Plainedge, N.Y.

A.

Probably the main difference, besides the refining methods, between major and minor brand gasolines is in the additive package incorporated in the fuels.

Most brand name gasolines contain a high level of detergents, dispersants and other additives that ensure the engine will stay clean and run properly. Because additive packages are expensive, the price of these major brand fuels is usually higher than non-major brands.

Cheaper, offbeat brands of gasoline, on the other hand, usually don’t contain an additive chemistry comparable to the major brands, although all do incorporate some type of additive package.

That difference can be like night and day when it comes to your car’s engine. It’s important that you use a high-quality, major brand gasoline that advertises its advanced additive chemistry. In addition, major brand gasolines are sometimes customor climate-blended for the area.

Bob Sikorsky has written several automotive books. Write to him c/o Lifestyle, Sun-Sentinel, 200 E. Las Olas Blvd., Fort Lauderdale, Fla. 33301-2293.