Have you ever chased an occasional cylinder misfire?

Here is my story about my hunt for a perplexing problem. My 740 GLE had a sooty black tailpipe along with what I thought was a mysterious, occasional, almost imperceptible misfire.

As I would drive my 4-cylinder Wagon along the highway, it ran smoothly except for a momentary, slight shudder occurring about once or twice a minute. But, often there was no shudder...the car ran great! I chased this illusive problem for more than a year.

Looking for a source for this reoccurring, random engine misfire, I began checking all engine electrical connections, fuel systems components and engine components. In fact, I expected to check everything in the engine compartment. However, because the distributor cap, ignition coil and spark plug wires were quite old, I replaced them. Performance improved slightly, and for the rest of that year and into the Spring the misfire was gone. However, the misfire returned.

After several hours of examination, no fault was detected. I thought, "Maybe the engine was running rich or lean for just an instant." I took it to a tune up shop, but even they could not detect the problem or offer any suggestions to resolve the problem. Thus, the source of the problem remained hidden.

Additional careful observation of the problem yielded no revelation until one recent summer night when I was tuning my AM radio. I noticed while "off station" there was a click in the speakers when the car shuddered. By testing this hypothesis, a correlation was confirmed between the shuddering and the speaker clicking. Further consideration of the various systems operating revealed the air condition compressor clutch was cycling on and off in sync with the clicking and the shuddering!!!! Apparently, applying the compressor load to the small engine when turning about 2500 rpm caused a slight but noticeable, very short drop in speed which was incorrectly thought to be a misfire. I theorize that when I tightened the V-belts about a year ago, I eliminated slight belt slippage and increased the shock loading which caused this "misfire" to become noticeable. (Probably with a larger engine the compressor engagement would not be noticed.)

I was relieved that the source of the "misfile" problem was finally discovered and that a long, time-consuming and frustrating search was curtailed. If you feel a similar engine shudder, turn OFF the A/C and see if the problem goes away. If so, no need for further concern.