Dear Car Talk:
I recently purchased a 2020 Subaru Forester. So far, I love this car!
One of its many features is the automatic start and stop system, which shuts off the engine when you stop at a light and restarts it when you take off. When it functions, I notice a small shake as the engine shuts down and another shake when the engine starts back up.
At first, I found this annoying and thought something was wrong. But after speaking with the dealer and numerous friends who drive Subarus, I think the engine just shakes when it starts.
However, the salesperson who sold me the car told me that if one turns on the rear defroster, the shaking will stop. This seems to be true! However, the rear defroster does not stay on all the time, and with each start of the engine, I must remember to turn it on if I want to use it to control the shaking.
My question is: Why does enabling the rear defroster stop the engine from shaking, and why does the car shake when the rear defroster is off? -- Alan
Most cars now have these start-stop systems to save fuel and reduce pollution. And some manufacturers simply do a better job of eliminating that "restarting" vibration than others. On some cars, you don't even notice it.
Unfortunately, Subaru is at the San Andreas Fault end of the spectrum. It's among the worst in damping that engine vibration on restart. Why is that? We can't say for sure. It may have to do with the unique "boxer" engine that Subaru uses in which the pistons are horizontally opposed, creating greater side to side movement and more vibration.
That design has advantages, like a lower center of gravity. But excessive smoothness is not one of those advantages. That never mattered until recently. Not long ago, you'd start your car a few times a day. But with stop-start systems, you're starting it 50 or 100 times a day. So an engine that shakes on restart is suddenly a real annoyance.
We hope Subaru figures it out soon -- through better damping or whatever -- because it's an unfortunate demerit on an otherwise very good car.
Why does turning on the rear defroster stop the shaking? Because using the defroster temporarily disables the start-stop system. When there's a large electrical draw, like the rear window defroster, the car's computer disables the start-stop system to keep the battery from being drained while you're stopped with the engine off.
As an alternative, if you dig into your touch-screen menus, you'll find an option to turn off the start-stop system. It's not as convenient as pushing a simple rear defroster button (we wish it were, hint hint, Subaru), but it will disable the start-stop system for the remainder of your current trip. By regulation, they can't let you turn it off permanently, so you have to do that each time you drive. But it's either that or driving around with a preternaturally warm rear window, Alan. Your choice.