Q: I own a 1997 Nissan Sentra GXE and the “Check Engine” light comes on. The dealer has worked on it several times and even though they have changed the canister and canister valve the problem is still there. They can’t seem to solve my problem and I need your help. Please reply. -- M.S., Des Plaines

A: M.S., a little background information will help you better understand the possible problem. The canister is a central part of the vehicle’s evaporative emission control system. This system is designed to prevent fuel vapors (from inside the fuel tank) from venting to the atmosphere. To accomplish this, the canister is partially filled with charcoal, which absorbs the fuel vapors.

During certain conditions, these fuel vapors in the canister are drawn out of the canister and burned in the engine during vehicle operation. When the vapors are being collected by the canister from the fuel tank, this is known as “venting.” When the engine is drawing these vapors out of the canister it’s known as “purging.” There are a number of valves and hoses in the evaporative emission control system that are needed to control venting and purging. Also, this system has to be monitored by the vehicle’s Powertrain Control Module (PCM) to make sure it’s working correctly.

The “check engine” light will come on if there are any problems with this system. Problems range from leaks to possible flow issues due to restrictions in the hoses. Now, here’s what may have happened. According to a service bulletin, some of the charcoal in the canister may have leaked out and contaminated the system. You mentioned that the dealer changed the canister and a valve. There may be more valves contaminated by the leaking charcoal. There may also be contamination in the various hoses.

According to the bulletin, all the evaporative emission system hoses need to be thoroughly cleaned. Any leaking hoses should be replaced. Also, other valves may have become contaminated from the leaking charcoal. If the dealer isn’t willing to spend the time to thoroughly clean the hoses and inspect all the valves, take your Sentra to another Nissan dealer or to an independent repair facility that specializes in emission control repairs.

The Illinois Environmental Protection Agency has a list of repair facilities and it includes how successful these shops are at fixing vehicles that failed the emissions test. You can find the “Repair Facility Performance Report” by visiting www.epa.state.il.us/air/vim/ and click on the link at the bottom of the screen.

Q: I was told that Ford products are notorious for having a low idle and then when you’re at a stop they have a tendency to die out. I was wondering if that was true. I was also told to put a bottle of fuel injector cleaner in my 1993 Ford Escort’s gas tank. Is that advisable? What do you think? -- P.B., Steger

A: P.B., I wouldn’t say that all Ford products are notorious for low idle problems. Your Escort’s low idle and stalling could be caused by a dirty throttle housing. If that’s the case, using the injector cleaner won’t help. Have a technician check your car’s throttle housing to see if a cleaning is in order. By the way, adding the correct amount of fuel injector cleaner to a full fuel tank usually won’t hurt anything.

AutoAdviser: Ira Siegel is an Automotive Instructor and ASE Certified Master Auto Technician. If you have auto-related questions for Ira Siegel you can call and leave your questions at the Auto Adviser Hotline at 708-633-6839 or e-mail a question to Autoadvisr@aol.com.