Drive it forever
Q I have a 1995 Celica ST Liftback with 245,000 miles on it. It has been having transmission problems -- or at least I think it's the transmission -- for about the past year.
After long interstate drives, say 45 miles at 75 miles per hour, the clutch pedal becomes very "spongy." It engages near the bottom of the throw, close to the floor, but most of the time I can still shift gears. Once I exit the highway and drive at normal speeds, say 35 to 50 miles per hour, the clutch-pedal operation returns to normal after a short period of time. The symptoms seem to be limited to the heat of the summer and were not noticeable during the past winter.
There was one instance on the interstate when the clutch pedal was unresponsive and would not engage when pushed to the floor, making me unable to shift gears. I had to pull off the road, shut the engine off and let it cool. After about 15 minutes, I restarted the engine and there was just enough of the clutch pedal to shift gears. I then got back on the interstate, reduced my speed and normal clutch operation returned.
A local garage misdiagnosed the problem last year and replaced the clutch, so I have a new clutch but the problem still persists. Any ideas?
A Sounds like the slave cylinder is overheating and boiling the hydraulic fluid. I'd start by flushing and bleeding the clutch hydraulic system.
But I wonder why it's overheating? It could be a throwout bearing that's spinning and getting hot because the slave cylinder is misadjusted. Or the exhaust system, which is routed right nearby, may be radiating heat onto the slave or the hydraulic line.
At 245,000 miles you were due for a new clutch, even if it was still working. Too bad they didn't catch the real problem.
Q I bought a 2007
To my dismay I discovered that dealers seem to have a tendency to add more motor oil to the engine in routine service than Toyota specifies. According to what I have read, the engine is supposed to hold 3.9 quarts. However, according to Prius users, 3.5 quarts will bring it up to the full line, and the dealers are actually putting in 4.0 quarts, raising the level anywhere from one-eighth of an inch to one-quarter of an inch over the full mark.
Is it a real problem for the Prius if it is overfilled by the indicated amounts?
A A slight overfill isn't that big a deal. Any more than about a half-quart, however, stands the chance of frothing or having oil pumped into the engine through the PCV system.
Q I recently watched a news clip about the Aqua Tune Hydrogen modification to a car engine, and was impressed with the results.
The clip also mentioned that Popular Mechanics has reviewed some of these "mileage-increasing" techno gizmos, and I wonder whether you can direct me to your reviews on your Web site, since I can't seem to find anything about this.
By the way, they also claim that your installation was "bound to fail," of course, and I'm wondering if you have received any positive feedback regarding this system?
A For a review of my tests, go to www.popularmechanics.com/automotive/new-cars/1802932.html. You can also find these by searching our Web site for "Fuel Saver."
"Bound to fail" as a result of following their instructions? Whatever. I haven't heard from many readers who have installed the Aqua Tune system, good, bad or otherwise. Any time gas prices spike, this whole field of gadgets comes up again.
Q When I switched on my car's air conditioner yesterday, cold "steam" came out of the air vent. It stopped when I changed the air intake from fresh air to recycling cabin air.
I was told recently by a mechanic that I should replace the filters on the A/C, but I didn't, as everything seemed to work fine. Could this be the problem?
A That's not steam, it's fog. The A/C is lowering the temperature of the hot, moist outside air below the dew point, forcing excess moisture to condense out as tiny droplets.
When you switch to air recirculation, the air going through the A/C evaporator has much lower relative humidity, so no moisture condenses out as fog. Moisture will condense on the cold evaporator plates, lowering the humidity of the car even further. That's why you see a puddle under an idling car on hot, moist days.
It's normal. Changing the filters won't make it go away.
Mike Allen is a senior editor of Popular Mechanics magazine. Questions should be sent via e-mail to driveit@nytimes.com. 
