Paul Brand: When a Cruiser bucks, check air-fuel mixture

Q My wife's 2001 PT Cruiser with 48,000 miles is driven primarily short distances in city traffic. The problem is an intermittent jerking or bucking at 50 to 55 mph. On a recent trip, it acted up on Saturday but did not act up on the return trip Sunday. The car's engine is probably warm at that speed, but the bucking occurs in warm or cold weather, with or without using cruise control, and while driving on flat terrain. It is almost as if it wants to downshift but doesn't.

The dealer had the car for two days and couldn't find a problem on the diagnostic computer, nor did the car act up when they took it for test drives. It is not a real violent jerking, but I'm afraid it could be causing damage. The car has been routinely maintained, including new plugs.

A Your description sounds like a lean air-fuel mixture. If this is the case, there are several possible causes: an air or vacuum leak allowing unmetered air into the induction system, low fuel pressure or volume, or perhaps a carbon buildup in the induction system that's absorbing a percentage of incoming fuel. Follow up on your maintenance with a decarbonizing of the induction system. This service is available professionally for a modest cost, or you can try to do it yourself with SeaFoam. Check for air or vacuum leaks and have the fuel pressure checked. The system should develop 58 psi while running, and hold 25 psi or better for 20 minutes after shutdown.

A weak ignition component might contribute such a misfire. An engine analyzer at a diagnostic shop like Precision Tune would help identify this.

The slight hesitation or jerking you're experiencing isn't likely to damage anything, but as you've discovered, it sure is annoying!

Q On cold days, zero or below, before our 2003 Ford Econoline Club Wagon has warmed up inside, the dome lights will flash on and off during sharp left turns. The dealer suspects that a switch isn't opening or closing all the way.

Considering the cost of taking the steering column apart, this is a nuisance I can live with. My concern is that there is a bare wire rubbing somewhere that could cause a fuse to blow, resulting in engine failure. And if the dome light ever stays on, I obviously must find the problem. Do I have anything to worry about, or do I just have a nuisance on cold days?

Rather than the simple spring-loaded grounding switch on the door post of older cars that turned on the interior lights by grounding the light circuit to the vehicle body when a door was opened, many of today's vehicles feature lighting control modules that offer a delay in interior lights turning off after exiting the vehicle. The switches for the lighting control modules are inside the doors and are activated by lifting the door handle.

Because the interior lights come on when you turn left, I suspect that the switch in the passenger door or side cargo door is the culprit. And since the actual dome light rotates down to manually switch it on, the problem might be in this assembly.

The problem might be at the headlight switch since this switch also allows you to manually turn on the interior lights. Unplugging, checking and reconnecting this switch might solve the problem.

Q I have a '99 Buick Ultra that has a minor problem. My headlights are sometimes on during the day when it's bright out. It is sporadic; the problem comes and goes even while I'm driving. Then it works fine for hours. When it turns on and off, I hear a loud click. There are two sensors on top of the dash; I have cleaned them, and there are no obstructions on the dash. If one of the sensors were bad I believe I would see problems different than I'm experiencing.

A This twilight sentinel system switches from daytime running lights to full-intensity low-beam headlamps by sensing the level of ambient lighting. Try turning off this system to see if the sensor is the problem. When it's dark and the vehicle is running in park, push the twilight sentinel switch to "off." Make sure the car is in park, otherwise pushing this switch does nothing. If turning off this system stops the problem, the sensor in the middle of the dash is somehow the cause. A review of my Alldata database shows that there's only one light sensor located in the center of the dash. The sensors on either side of the dash are "sun load temperature" sensors for the heating and air conditioning systems.