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Q I have a question about fuels. I understand the difference between premium gas with ethanol and premium gas without ethanol. I have been told that this is also called "racing fuel." Since non-ethanol premium fuel has become more readily available in recent years, I have used it in my small engines -- snow blowers, trimmers, lawn mowers, leaf blowers.

I recently heard that the premium non-ethanol gas can damage or shorten the life of small engines because of the high octane and higher burn temperatures. Is this true? I thought it would be a better fuel as it eliminates the alcohol. I know my truck runs better on it, as does my motorcycle. What is the best fuel for smaller engines?

A It's a trade-off. The primary benefit of premium non-oxygenated fuel in any engine not operated on a regular basis is the reduced potential for moisture contamination in the fuel and the resultant phase separation and corrosion issues. The disadvantage is the higher octane rating, which is not necessary in most small engines. Higher-octane fuels are "built" with longer hydrocarbon molecules that burn more slowly. In low-compression engines -- typical of most small engines -- the slower burn rate can lead to more carbon deposits from unburned fuel.

But as I said, it's a trade-off. I'm willing to de-carbonize my small engines periodically as the trade-off to reduce the potential for moisture and/or phase separation with oxygenated fuels that sit in the tank for any significant length of time.

Adding SeaFoam to the fuel can help prevent these problems, but in my opinion non-oxygenated fuels are a better choice for any engine operated on a seasonal or intermittent basis.

The list of stations that offer non-oxygenated fuel for small engines, recreational vehicles and collector cars is listed on the Minnesota Street Rod Association Web page,

Is premium 92-octane non-oxy fuel "racing fuel?" Not really. Racing fuel is typically 100 octane or more and can be dispensed only into suitable fuel containers, not directly into the fuel tank of a licensed automobile. This fuel, as its name implies, is designed for use in motorsports events.

Q I have a '97 Ford F-150 4x4 with 150,000 miles on the 4.6-liter engine. Since I've have had it, the engine has had a knock that sounds like diesel. I have talked to many people about it, and I have heard injectors, lifters and piston slap before. But I know it's not any of these problems; piston slap goes away with heat, and the hotter it is the more noticeable this knock is.

I added SeaFoam and ran it though the engine for the last 500 miles before my oil change, and I added fuel conditioner and injector cleaner to a couple of tanks of gas -- none of which has worked. Someone else has had the valve cover off on the passenger side before I had the truck, but they must not have been able to fix it.

I read on some Internet forums that a couple of people have had the same problem, but they described the noise as a tick, and it stopped after they changed the PCV valve. Please help me before this turns into a more serious problem.

A Start by trying to find the precise source of the knock. A formal automotive stethoscope -- or a homemade version (a long wooden dowel or screwdriver) -- can help you pinpoint the noise. Hold the "tool" to your ear, then touch the tip along the top of the valve cover, then along the side of the cylinder head, then along the side of the block. If the knock is coming from the valve train, you'll hear it under the valve cover. By working your stethoscope from front to back on the valve cover, you might be able to identify the particular valve or rocker arm that's generating the noise.

If the knock is most noticeable from the side of the cylinder head, it might be a combustion knock -- ping or detonation -- or a carbon buildup that's causing combustion chamber deposit interference between the top of the piston and the combustion chamber. Such noise typically fades as the engine warms up.

If the knock originates from the side of the block, it might be a wrist-pin knock from the top of the connecting rod. If it's a lighter sound, it might be piston slap from a bit of excess clearance on the piston skirt. Piston slap might or might not go away as the engine warms up. Sometimes you can confirm piston slap or wrist-pin knock by disabling that cylinder with the engine running. Without the combustion force on the top of the piston, the noise might fade.

My Alldata automotive database identified service bulletin 00-26-1 from December 2000, which suggests checking the valve train for a "dislodged" roller finger follower that keeps the pushrod in contact with the rocker arm. Perhaps that's what the previous owner was looking for with the valve cover removed.