Q I have a 1996 Chevy Corsica with 107,000 miles on it. It has an idling problem that happens only during warm weather, whether the engine is cold or hot. It starts right up, but about 45 seconds after I back out of the garage, the engine begins to chug and sputter, and the car shakes. It feels as if it's about to quit. Finally, after I pump the accelerator for five to 10 seconds, the engine will catch. Then the motor really revs up, and I need to brake to slow the car down. Computer diagnostic analysis is always normal -- but I do find that if I start driving the car hard right away after starting the engine, this idling problem doesn't happen. Please help.

A There are many possibilities, but start with the basics: old or fouled spark plugs, inaccurate coolant temperature or manifold absolute pressure (MAP) sensor, sticky throttle position sensor, leaking fuel pressure regulator/fuel injector or low battery/system voltage.

Three professional tools may help pinpoint the problem. A scan tool connected during start-up lets a technician monitor signals from the engine sensors in real time to determine whether the signal values are accurate. A fuel pressure gauge can easily identify low fuel pressure or a leaking fuel pressure regulator -- any liquid fuel in the vacuum hose to the regulator would also confirm this. Finally, try connecting the engine to a professional electronic engine analyzer to monitor ignition and combustion with the engine operating from start-up through the sputtering.

Q We have a 2002 Camry with 115,000 miles on it. On Sept. 22, the engine light went on. We checked that the gas cap was on tightly. The dealer did a computerized evaluation, replaced a vacuum valve and manually turned the engine light off. We paid $214 for this. On Oct. 4, the engine light came on again. This time, the dealer replaced a canister assembly for another $515. On Oct. 17, the engine light came on again. The dealer said they couldn't do a smoke test because the gas tank was too full. They told me to bring it back when it was less than a quarter full.

We pointed out that since neither the first nor the second attempted repair managed to fix the problem we should get our $729 back. No dice. Again, they manually turned off the engine light, and we drove the car until the gas tank was below one-quarter. The light had not come back on by then, so I didn't take it in. We filled the gas tank and continued to drive it. On Nov. 15, the engine light came on for the fourth time.

Have we been taken for a ride?

A Only if the dealer didn't really give it his best effort to find the problem. If they just threw parts at the car based on diagnostic fault codes, you have a valid point. But then again, do physicians refund money for a misdiagnosis?

Although you didn't share the specific fault codes the dealer found, I'm assuming they relate to the evaporative emissions system. A vacuum leak -- which they may find with the smoke test -- may well be the problem, so why not give the dealer one more try at solving it? If they're successful, negotiate the total cost of all their repair efforts.

Q Our 2007 Mitsubishi Endeavor was totaled in a crash. I loved my Endeavor and want to buy another one. If you were given the choice of a 2008 with 35,000 miles for $14,600 or a 2010 with 26,000 miles for around $16,000, everything else being equal, which would you opt for and why?

A I'd go with the newer vehicle with 9,000 fewer miles for $1,400 more. A quick look at the National Automobile Dealers Association (NADA) used-car pricing guide shows about a $3,500 difference in value between the two vehicles and a retail price of almost $20,000 for the 2010 base model with 26,000 miles.