The Case of the Autonomous Auto

A short time ago, I responded to a reader whose letter referenced one of my previous Calamities columns. This reader described an uncontrolled acceleration incident he had with his automobile. I was unable to shed any light on the reader's experience, but thinking about his problem let me recall my own first-hand experience with unintended vehicular motion.

The Scene of the Crime

During the 1970s, I bought an old 1960 or 1961 Ford Econoline van to tow a trailer that carried my H-Modified Jabro Mk I sports car. This little car used a souped-up Crosley 750cc engine and running gear in a frame constructed of oval aircraft strut tubing and covered with a fiberglass body. The little Econoline wagon pulled my trailer and equipment faithfully, but not tirelessly, to race tracks here in the Great Lakes' area. The van's little 144 inches3, in-line six-cylinder grew more and more anemic as the miles accumulated. Finally, I found I had to draft really close behind tractor-trailer rigs just to make 55 mph.

I soon realized, if one of those rigs ever made a quick stop, I would have resembled a squashed bug on the back of the trailer. A friend at work said he had a freshly rebuilt 170 inches3 six-cylinder in-line that would be a drop-in replacement, and his labor and the engine cost would be less than just the parts needed to rebuild the 144. He took my van home and returned it a week later. My van was transformed after, as they once said in the 1930s, receiving its "monkey gland" injection. I had no trouble pulling my loaded trailer, equipment and crew. Occasionally, when not pulling my trailer, I abandoned good sense and engaged in stop-light grands prix with the fresh, torquey little mill helping to keep the little van ahead of competition up to the posted speed limit.

Of course, there always comes a time to pay for one's amusements. Payback occurred this time as I was heading home after a day's work. Accelerating smartly from a traffic signal, the accelerator pedal dropped away from under my foot and the van slowed abruptly. Recovering from the surprise, I scooted forward in my seat and I pushed my foot down to re-engage the accelerator pedal. The pedal would come back up and there would be a surge of unexpected acceleration until I lifted my foot. This action continued until I got home. The next day, a Saturday, gave me an opportunity to find out the cause of this eerie behavior.

The Investigation

I tore into the van's innards to check the operation of the accelerator pedal, the throttle linkage and carburetor. There was no observable interaction with the accelerator with the engine switched off or at idle speed, so I had to look further and try to simulate the dynamics of driving. Since the engine cover was located between the front seats, removing it allowed me to view the engine while driving. Starting off conservatively, I did not feel or see any interaction with the accelerator pedal. With more aggressive driving (as my wife says, my normal mode), I could watch the six-cylinder in-line engine rise and fall as I accelerated and slowed. Whoa! Rise and fall!

While making a few more hole shots, I could see the engine rise and pull away from one of the motor mounts. With each engine lift, the throttle linkage moved and changed accelerator position. Conundrum solved! As I finished my test drive and headed up my home driveway, the accelerator dropped away and didn't rise. I rolled up to my garage, stopped and looked into the engine bay where the engine took its final set. It had broken completely away from the motor mount and only the oil filter jamming against the frame rail kept the engine off the street.

The Smoking Gun

A close look revealed a failure of the bonding between the rubber biscuit and the mounting plate bolted to the frame. I could see the failed mount let the engine rise and fall more than normal. Excess engine motion carried the throttle linkage with it and caused the described bizarre effect. Evidently, the additional torque of the fresh engine overcame the adhesive bond of the old oil-soaked and heat-cycled motor mount. The purchase and installation of two new motor mounts ended "The Case of the Autonomous Auto." Lessons learned - first, the engine and mounting are a system, and both should have been repaired together; and second, don't play "Racer Bob" with the tow vehicle.