QDear Tom and Ray:

I just bought a new Toyota Corolla last year, and I love it. I'm a visiting nurse and have to find the homes of new clients all the time. I'd love to have one of those newfangled navigation systems that tell you how to get to specific addresses. But it wasn't an option on the Corolla. Is there any way to get one put in? -- Jane

ARAY: There are several "aftermarket" navigation systems. The one we've been testing lately works extremely well. It's called the Magellan RoadMate, and it's a little box about the size of a 4-by-6-inch index card, and about an inch deep.

TOM: You can have it mounted permanently in your car or, if you're part of a family with several cars or you travel with friends, you can move it from car to car as needed.

RAY: It works just like the factory navigation systems. You enter a state, city and address, you push a button, and it guides you to your destination with a list of directions and a map on a video screen. There's also an optional electronic voice that says, "Hey, jerk, your next turn's coming up!" Or something like that.

TOM: It has all the extras that you typically get with a factory-installed system. It tells you which road you're currently on, how many miles to your next turn and how many miles to your ultimate destination. It lets you save destinations, such as "Home," so you can always quickly set it to get you home from wherever you are. And it covers the entire continental United States.

RAY: It worked almost flawlessly for us. Our one complaint is that when you unplug it and move it from car to car, it often takes a few minutes to "locate itself." It has to check in with the GPS satellites and lock onto its own position before it can guide you. This sometimes took several minutes, while we sat around and twiddled our thumbs. There is a way to tell it your location, to speed up the process, but it's still an inconvenience.

TOM: The Magellan sells for about a thousand bucks. That's a lot. But it's less than the $1,500 to $2,000 that most new-car systems cost. And it has the advantage of being portable.

Dear Tom and Ray:

My wife drives a 1991 Ford Escort. About every six weeks or so, I have to replace the air filter, because oil builds up in the box that holds the air filter. No mechanic to date has been able to solve this problem. Any suggestions? -- Michael

RAY: I'm surprised no other mechanic has been able to fix this, Michael. It's very simple. You just change one part, and this problem will completely disappear. Unfortunately, that part is the engine.

TOM: You have what we call "blow-by." That's when combustion gases from inside the cylinders sneak past old, worn-out piston rings and get into the crankcase, where they don't belong. Soon, there's too much gas and pressure in there for the crankcase ventilation system to handle. So, to relieve pressure, oil gets blown back into the air-filter housing, where it ruins your air filter.

RAY: Assuming your crankcase ventilation system is working, blow-by is basically the beginning of the end for an engine. In your case, maybe the middle of the end. But the good news is that you won't do any more harm to the engine by driving it. So, you can keep driving it until it dies, thereby creating a severe air-filter shortage in your region.

Got a question about cars? Write to Click & Clack in care of The Post, or e-mail them by visiting the Car Talk Web site at www.cartalk.com.

(c)2004 by Tom and Ray Magliozzi

and Doug Berman