Dear Tom and Ray:
I have two things: a son who is a shade-tree mechanic and does a good business out of his backyard, and a 2003 Honda Civic that keeps overheating. The car runs fine and never overheats — unless I am in the drive-thru line at fast-food restaurants! Even when I turn off the air conditioner, the needle continues to rise up to the hot level. My son checked it out and found the radiator cap to be leaking, then replaced it and thought that solved the problem. He ruled out the fan, as it was running the entire time he left the car running. However, the next time I was in the drive-thru, the needle started rising — again! It didn't get as high as it had before the new cap, but nevertheless, it was on the way up. Once I get back out on the road, the needle slowly drops back to normal. What could be the cause of this? When my son checked it out, he drove it and let it idle for an hour, and it never heated up for him! I threatened to take him with me next time I went to a drive-thru, so he could see what I am talking about, because I know he doesn't believe me! Do you have any idea what is going on? I really enjoy your radio show and column. Your humor is helpful in relieving the stress that goes along with car trouble. Thank you! — Marlene
Tom: I think this is a cleverly disguised message from Michelle Obama to get you to cut down on the Big Macs, Marlene. This wouldn't happen if you were at the drive-thru at Organic Beet Puree King.
Ray: There may be nothing wrong with the car, Marlene. You say that after the radiator cap was replaced and you stopped at the drive-thru, the needle started to go up again. The question is: How far did it go?
Tom: Right. After you drive the car, particularly on the highway or at higher speeds, when you come to a stop, the engine will get hotter temporarily. There's a lot less air being pushed through the radiator when you're stopped, so the engine heats up some before it cools back down.
Ray: So if the needle simply went up to the red mark, and soon came down, there may be nothing wrong with the car. And that may be why it didn't overheat for your son after idling for an hour.
Tom: On the other hand, if the needle went way up, near the hot zone, then there is still something wrong, and it requires further investigation.
Ray: In that case, it could be something as simple as a bad thermostat. It could be a water pump with a loose impeller. Or it could be the ever-popular and dreaded leaky head gasket.
Tom: Either the failing water pump or the bad head gasket could work fine when the car is idling, but could cause trouble after hard driving. That may be why the car doesn't overheat for your son.
Ray: So I'd suggest that you do take him with you to the drive-thru. Tell him the burger and fries are on you. Then drive the way you normally drive, and let him see where the needle ends up. He may confirm that there's no problem.
Tom: Or he may realize that he hasn't fixed it, and that he has to check out our other theories. Or he may just enjoy the free lunch and want to repeat this test-drive experiment 10 or 15 more times. Good luck, Marlene.
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)