Subject: 1991 mitsubishi galant failed emissions
Question:

Hi Anthony,

This is follow-up to my previous question about my car failing emission test with worst number. The same car passed the last year. As I told,I took it to the repair shop which costed $530 dollars for replacing spark plugs, distributor rotor, gasket, wires ..etc. but failed test again.
I changed mechanic and went to a certified emission repair shop. He tested my car and all the sensors except the MAF sensor as he could not get the specs for my model. He disconnected MAF sensor and told me that it cars runs better without MAF. He suggested to see mitsubishi authrised dealer as he suspects problem in MAF sensor. But when MAF sensor is connected, I did not get any "check-engine" warnings. Also, without MAF sensor, the amount of smoke seems to be reduced but the fuel efficiency is did not improve. I got an appointment with one mitsubishi dealer next week. I am worried as there could be some other problem. You suggested me to replace the O2 sensor and clear EGR value, but the mechanic told O2 seems to be fine and EGR value cleaning is not major. I asked to clean any dirt/corrosion and try and but he says that he is confident about MAF sensor problem. I am worried as MAF is costly. Can you please suggest me something about my problem ? Would it really be a MAF sensor problem or O2 sensor problem or something else ?
Answer: Siva,
The Mass Air Filter (MAF) a sensor that tells your car's computer several items about the air that is entering your engine. It tells the computer the air temperature, the barometric pressure and the amount of air entering your engine. When that sensor is disconnected, your car's computer will just make a best guess as to how much fuel to added to your engine.

From the info you gave about your mechanic telling you your O2 sensor is fine and EGR value is ok, then the problem could very well be in your MAF sensor. And you're right, it can be expensive. When you take your car to the dealership, ask them to actually check the MAF sensor. They can do this by completely removing it from the car and using a simple multimeter to check the voltages and resistances of the all the various components. They might determine that your MAF is fine.

If your MAF is fine, have the dealership inspect the car's computer itself. Read this article for further information:
http://90gsx.com/eclipse/cap/caps.htm

If your ECU is malfunctioning, it can seem as if various sensors on your car are the problem. But replacing your ECU may remedy your problems. When you disconnected the MAF sensor off, and your check engine light went away, that is very strange. There is a good chance your ECU is the problem. Be sure the dealership mechanic checks it.
Good luck!