Dear Car Talk:

Would you recommend that a "stop and go" feature be added to my car, a 2014 Subaru Forester? By "stop and go," I mean a feature that shuts off the engine when you pull up to a stoplight and automatically starts the engine again when you press on the accelerator.

-- Don


No. I wouldn't recommend it. 

First of all, I'm not aware of anybody who offers a retrofit kit to add "stop-start" to a car that didn't come factory-equipped with it.

And it's not simple to add it later, Don. You might think it's just a couple of sensors -- one that detects when you've come to a stop and shuts off the engine, and another to start the engine when you touch the gas pedal. But you also need a much-heavier-duty starting system, because your car will be starting and stopping all day long.

The starter motor that came with your 2014 Forester is designed to start the car half a dozen, or maybe a dozen, times a day. And if you multiply that by 10 or 20, you're going to wear it out pretty quickly. 

And by using a traditional starter, with a gear that pops out to engage the flywheel, your flywheel likely will die a premature death, too. A flywheel can cost upward of $1,000 to replace.

That's why some stop-start systems use something called an integrated motor generator, which is a much more powerful starter motor, designed to fire up the engine a hundred times a day or more, if necessary. It often uses a belt instead of a starter gear.

There are electronic issues, too. On your Forester, things like the air-conditioning compressor and water pump are powered by the engine. And when the engine stops, those accessories stop. So cars with stop-start systems run those systems off of battery power so your accessories are not interrupted at every traffic light.

So it's an expensive and complicated thing to do, Don. It is a great feature. It does save fuel, and it's wonderful for the environment, especially in cities. But unless you drive exclusively in stop-and-go city traffic, you're unlikely to gain more than 3 or 4 percent in mileage from a stop-start system. 

So from a financial point of view, it's impossible to justify. The money you save on gas will never come close to the cost of the retrofit plus your membership in the Starter-Motor-of-the-Month Club. So you're better off waiting until you get your next car to get the stop-start feature, Don.