I have a 2002 Lancer OZ w/stick shift... My car used to idle rough (700rpm) and so I decided to take it in for a check up... The shop checked the throttle body and found it to a little dirty and cleaned it up. They checked the MAF sensor and it looked good. The scanned the ECM and found that the Long term fuel trim showed a reading of +10% whereas it should be closer to +/-2%. My mileage had also dropped by about 2/3mpg.

After the throttle body clean up, the mileage is back up again but it still behaves odd at times... It runs fine and smooth initially but later on after about 20odd miles goes rough and at high speeds about 80mph vibrates a lot when accelerating but if I leave the accelerator, it rolls smoothly again... I guess it is running lean but not sure what is the exact reason for it... Does anyone know what could be wrong???

Spark plugs were changed about 50k miles ago with platinum ones and i know they are due now but is there anything else that might be an issue for the car running lean? (the shop checked them also and they looked fine).

 

A vacuum leak will do that. It will be easier to find if you just pressurized the system and looked for leaks that way.

Alternatively the o2 sensor could be on its way out.

 

What would be the best way to determine which one of the o2 sensor (upstream or downstream) is on its way out...

 

Only the upstream sensor could do it the rear o2 is backup in case the upstream one fails.