I have the same problem and I thought just cleaning the IAC (Idle Air Control Valve) would do the trick. But the check ingine light came back on. So if cleaning it dosen't work, just replace the IAC.

 

I have had a similar problem out of my OZ Rally I just thought id would throw in some advise my car was having an erratic idle it would surge between 1k rpm and 500rpm and almost cut out at times, my car has thrown numerous codes for sestem lean and fuel temp sensor perf range low and i got a wild hair to clean my EGR as well as MAF and Throttle Body, lets just say the only thing i didnt clean on fuel to air mixture is the surge tank on the intake manifold, so far this costed me around 10 bucks to fix took 30 min of my time and the car idles fine now right at the 700+/- 100 idle range for the 03 OZ Rallys and base lancers with the 4G94 SOHC this would be a very cost efficient and simple fix if this cures your problem like mine now i do still have a slight rough idle and a fuel cuttout on accel at low rpms but that i am almost 90% positive is the sending unit in the fuel tank where these cars have an integrated fuel temp sensor on the sending unit itself and seeing as this part is almost 1000 bucks from mitsubishi 800 with my shop discount im pretty sure i can deal with a small cutout occasionally on my salary lol

Hope my short novel has helped anyone feel free to critisize!

James ( 1st time lancer owner love the car but its not an EVO! )

-- 03 Lancer OZ Rally--
cold air, borla race exhaust, larger injectors, ECU Upgrade, and full custom interior

 

I realize we're replying to an OP that is over a year old here..but I thought I'd add my two cents for anyone who reads this thread searching for a solution to their rough idle problem.

I'd recommend checking your vacuum lines as an easy initial check. I had one with a hole in it at the top rear of the engine and it made my car act all kinds of crazy.

At idle it would suddenly surge and rev up, then it would cut out, stumble and almost stall out before revving back up. It did it to me first coming out of a parking lot and I of course panicked assuming major expensive repairs. I popped the hood, noticed the hole in the vacuum line and plugged it with my finger. The idle immediately smoothed out. Luckily the hole was toward the end of the line and all I had to do was pull it off, trim it back with a pocket knife and reconnect it.

Long story short, check your vacuum lines if your car has an erratic idle!

 

Good day to everyone I'm from the Manila,Philippines I am hoping someone can help me on my problem. I have a '93 lancer with 4g15 engine non-turbo, the problem is when i turn on the a/c the idle stars to drop at apprx.700rpm so i had an IAC and TB cleaning done DIY using carb cleaner. After the cleaning i warmed up the car then idle fluctuates at 1500rpm-3000rpm and from 2000rpm-3000rpm when a/c is on when depressing the pedal. I think my iac is working properly because before i assembled the gear kit inside the iac i set it in open position meaning air can pass through, after i ran the engine and opened it again for inspection the gear is now in closed position meaning the iac motor is running.
Can somebody please help me on my problem, it was not fluctuating before i cleaned it. Thank You in advance.
and sorry for my English !!

 

I have the same problem too... i have Mitsubishi Lancer GL 96-98 (here in Philippines i dont know how it become 96-98 model but to cut the confusion the body of evo 4)

so i have this car a lancer GL 96-98 when i got this to my hands i experience the same problem, cant sustain itself, having rough idle between 500-900, but when the compressor is ON.. not the aircon it stables itself at 1000. even if i have the a/c on if the compressor is not running it always have rough idle.

they said its the electricity. i changed the sparkplugs, high tension wires and the distributor. they found out that the sparkplug is too old to do their duties, high tension wires have cracks on the end part which leaks electricity, and the oil seal cap is damaged that leaks oil to distributor, also the distributor cap has crack that leaks electricity.

they also clean the air filter, they said that there is no problem with it, checked the carburetor they found it clean. adjusted the idle when a/c is on an a/c is off.

after those checking and repairing and replacing sometimes i experience that again and again and again, which brought me to theory that there was a time that it just stalled and after that doesnt turn on as if it has a dead battery so we decided to push it and give it a kick start, after doing that fan belt that connects to compressor, powersteering and main engine began to produce noise and after a couple of feet it stopped, same time with powersteering, and we found out that the compressor is stuck and wont rotate at all. this might be another possible reason, the compressor is being dragged by the engine that results to rough idle. but that's just an theory.

hoping that somebody will give me a solution. thanks