Carbon buildup in the good-old days

By Doug McAllister

More from a reader on "gas direct injection" engines.

Q. "Back in the day" (1970s), when I was in the auto repair business, the industry had issues with carbon buildup in some of the larger engines of the time, especially when the vehicles were not regularly driven at highway speeds. This would sometimes manifest itself by a piece of carbon breaking off at the top of the piston and creating a knocking that sounded as if the engine was about to come apart.

Prior to discovering the cause and assuming the knocking was, in fact, because of a major engine problem, engines were torn down for overhaul. Once we learned the true cause, the carbon was removed from the combustion chamber by spraying a heavy mist of water into the intake while keeping the engine rpms up. This, in effect, steam-cleaned the inside of the engine.

We further found that this procedure was an effective means to "decarbon the valves" of the British cars that recommended this procedure every so many miles as an alternative to removing the heads and breaking down the valve train to do so.

I don't know if this would effective on today's engines or not - or what consequences it might have on the catalytic converter.

A. I remember those days and cleared up a lot of carbon issues using that method.

The GDI engine today is a totally different beast with a very different problem. You need to perform the fuel-system service explained in the previous column to remove the hardened material from the back sides of the valves. A mist of water and traditional induction cleaning will have no affect on the buildup in a GDI engine.

• Douglas Automotive is at 417 W. Main St., Barrington; 123 Virginia Road, Crystal Lake; and 416 Northwest Hwy., Fox River Grove. For information, visit douglasautomotive.com. Send questions to underthehood@dailyherald.com.