The Case of the Shocking Stall

A friend had an older car with electronic ignition control that tended to stall during right-hand turns. I was asked to investigate. I spent several minutes under the hood poking and prodding to no avail. But I did force a stall when I removed the air cleaner. I discovered that I could reproduce a stall when I held the air cleaner housing in a certain position.

I had never heard of a position-dependent airflow sensor for automobile air intake applications. I was further confused by the fact that shorting or opening the airflow sensor output had stall effect.

Eventually, after re-assembling the air cleaner to the intake manifold with the engine running, I was leaning against the car body and casually brushed against a rubber vacuum hose. The resulting shock through my body welded paint into my hand and stalled the engine. After I recovered (it was a hefty shock), I noted that this would occur if I pushed the vacuum line close to (but not touching) the ignition coil secondary wire using a wooden stick.

The vacuum hose was connected between the airflow sensor on the air cleaner and the spark computer. Raising and moving the air cleaner would bring the hose close to the tower center wire. I found that due to centrifugal force of the car turning to the right, the vacuum hose would swing close enough to the spark wire to pick up an induced voltage carried along the dirt and oil-covered vacuum line into the spark computer and cause it to shut down, stalling the car.

A simple wire tie to hold the vacuum line steady solved the problem, and I replaced the spark plug wiring just in case.

This entry was submitted by John Mitchell and edited by Rob Spiegel.

Roger Hunt is a longtime safety engineer in the consumer products industry, serving in the telecommunications, personal electronics, and appliance fields. He is a skilled auto mechanic of older autos as a side hobby.

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