Today a lady called me about her Camry. She suspected the battery was failing so went to Autozone to have it tested. They said it was bad and we will give them the benefit of the doubt that their store clerk was correct. But as she drove away her car stalled every time she let off the gas. It was better by the time she got home. What did they do to her car? Not exactly anything, but it is what Autozone did not know that caused her worry. Most modern cars are “drive by wire” which is to say between the gas pedal and the throttle on the engine there is no longer a simple mechanical connection like a cable or a rod with swivels. Instead the gas pedal sends an electronic signal to the engine/powertrain computer and the computer sends appropriate commands to a special motor on the throttle which opens the throttle as required. Over time perhaps the throttle body wears and doesn’t close the same, or perhaps deposits build up on the throttle blade causing it to flow less air into the engine when at a certain angle. So the car could idle too fast or too slow. But as was unimaginable in the 60s or even the 70s, the computer keeps track of the idle speed and learns to correct for wear or sludge so that the throttle is always cracked just the right amount for the idle speed built into the computer’s software. This learning was lost when the battery was disconnected for replacement so for a mile or two or three the car kept stalling. By the time this lady got home the computer had largely relearned and things were getting under control again. The Autozone clerk who installed the battery simply wasn’t aware of how this works, but I reassured the Camry owner that she didn’t need to bring her car.
All’s well that end’s well.
When we replace a battery in such a car we clean the throttle body to help the car idle initially and we test drive the car afterwards to avoid the customer experience their car stalling repeatedly.