Dear Car Talk:
Five years ago, I bought a 2012 Volvo S60 demo with only 4,000 miles on it. It has been a great car and now has just 58,000 miles on it. But over the past few months, the car has developed a loud clunking sound and slight jerking of the car when I go from park to drive, or park to reverse. The issue comes and goes. Of course, when I took it to the dealership, the problem disappeared. -- David
Sounds like a classic case of a bad motor mount, David. What you're hearing is the engine shifting position when you change gears.
Motor mounts serve three purposes. The first is to hold the engine in place. Obviously, you don't want it to fall out onto the street while you're driving. But you also don't want it to move around inside the engine compartment. There are lots of counter-forces acting on the engine when it sends power to the wheels. And without the mounts, the engine would be doing the Macarena under the hood.
The second purpose of the engine mounts is to damp the engine's vibrations. So there are rubber bushings in each mount that are there to absorb the vibrations, so you're not doing the Macarena in the driver's seat whenever the engine's running. And the third purpose of the motor mounts is to fail when your mechanic has a college tuition payment coming up for one of his kids.
I believe there are four motor mounts in this car. The most common culprit is the one that goes from the firewall to the top of the cylinder head. If your mount was broken completely, you'd get the clunk every single time. So it's not gone yet. Though it might be by the time you read this. That actually would be good. Because when it's completely broken, it's easy to see the engine moving back and forth when you shift from drive to reverse. It'll move a lot, and your mechanic will have no trouble diagnosing it then. So ask him to check all of your motor mounts, David. More than one may be weak and getting ready to fail.