•Q. I took my 2002 Infiniti I35 to a shop to have the rotors and brake pads replaced on the front. While in the shop, the mechanic told me that I would need new struts in the future. When I left the shop, my car was bouncing all over the place. I was driving on a paved road, and I was actually bouncing off the seat. When I took it back to the shop, I was told that when they had put the car on the lift, air got into the struts because there were tiny holes in them. The mechanic told me it would go back to normal in a few weeks ... as soon as all the excess air leaked out of the struts. It has been three weeks, and my car still feels like I am constantly driving over railroad tracks. Is his explanation plausible?•

RAY: No. He made that up.

TOM: Changing the rotors and pads shouldn’t have had any effect on the ride, so I don’t know what they did to your car to make it ride so roughly — but I have a couple of suspicions.

RAY: The most likely cause, in our experience, is that they inflated your tires, or overinflated them.

TOM: If, before the repair, you had been driving around with tires that were low on air, that could have covered up the poor ride you’d get from bad shocks.

RAY: And then, once they properly inflated your tires, you felt the full effect of your worn-out shocks — in which the springs are undamped and the car kind of rides like a basketball down the road.

TOM: Of course, it’s not safe to drive with underinflated tires (or worn-out shocks), so you can’t just let some air out of the tires and keep driving.

RAY: But they also could have grossly overinflated your tires. If they weren’t paying attention, or had a tire gauge that was faulty, they could have put 60 or 80 pounds of air in your tires. And that certainly would cause you to bounce off the seat.

TOM: One other possibility is that they’re actually bad guys and they sabotaged your shocks in hopes of getting more repair work from you. But I’m going to discount that possibility.

RAY: Me too. In our many years of experience, we’ve found that most repair mistakes flow from gross incompetence rather than outright dishonesty. Doesn’t that make you feel better?

TOM: So I’m guessing they overinflated your tires by accident. Have someone check the pressure for you, and if the pressure is correct, then go somewhere else for a second opinion about the shocks. Good luck.

•Q. Recently I was driving my 2001 Ford Focus in cold weather on I-70 in the mountains west of Denver. There are two long, steep grades (10-15 miles at 7 percent to 8 percent) on either side of the Eisenhower/Johnson tunnels. After completing about half of the downhill run on the west side, I noticed that the engine temperature had gone completely cold and I was getting no heat in the passenger area. When I opened the throttle slightly, the engine temperature began to come up some (so did the speed — not what I needed). When I stepped on the gas to go up the other side of the hill, using the engine for propulsion instead of braking, the engine temperature returned to normal and I got heat again. The same thing happened on the return trip. What’s causing this? Thanks!•

RAY: I think you have a classic case of low coolant.

TOM: The way you get heat is that the coolant circulates through the engine and sucks up the heat created by the combustion process. It then releases that heat in one of two ways: either by going through the radiator, or by entering the heater core and releasing heat into the passenger cabin.

RAY: So the heater core is like another small radiator that’s only active when you crank up your heater control. And on most cars, the heater core is the last thing to get coolant — it’s the farthest away from the water pump, and often the highest thing in the system, too.

TOM: So if you were low on coolant, the first way you’d notice it in the winter is from a lack of heat in the cabin.

RAY: If the coolant were low enough to fall below the coolant temperature sensor and leave the sensor exposed, the engine temperature would read “cold.” So it all makes sense.

TOM: The reason it came back a bit when you went uphill is because when you’re revving the engine, you’re making the water pump turn faster, and that’s getting a little bit of warm coolant into your heater core. At least temporarily.

RAY: So you need to do two things. You need to see if you’re low on coolant. And when you find out that you are, you need to find out why. It could be anything from a loose two-dollar hose clamp to a $2,000 cracked cylinder head.

TOM: You want to catch it and get it fixed while it’s still a hose clamp — before you overheat the engine and make things much worse. Good luck.

Got a question about cars? Write to Click and Clack in care of this newspaper, or email them by visiting the Car Talk website at www.cartalk.com.

Reach Jim Parker at 937-5542 or jparker@postandcourier.com.