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Doug
Doug, ASE Certified Technician
Category: Mitsubishi
Satisfied Customers: 8611
Experience:  Mitsubishi employed and Factory trained ASE certified technician
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Here - 2000 Mitsubishi Lancer cedia - when cold lurches and

Customer Question

Hi Doug Dave here - 2000 Mitsubishi Lancer cedia - when cold lurches and runs rough - once warm all good ! Any clues
JA: Thanks. Can you give me any more details about your issue?
Customer: As I said when engine cold idle is high - when in Drive it lurches and runs rough in Nuetral idle bad - when warmed up seems fine
JA: OK got it. Last thing — Mitsubishi Mechanics generally expect a deposit of about $18 to help with your type of question (you only pay if satisfied). Now I'm going to take you to a page to place a secure deposit with JustAnswer. Don't worry, this chat is saved. After that, we will finish helping you.
Submitted: 1 year ago.
Category: Mitsubishi
Expert:  Doug replied 1 year ago.

Hi,

This condition is present even sitting still right, as an engine operating condition and not something motion dependent like a transmission issue?

Customer: replied 1 year ago.
Seems to be yes feels (if I can relate to older type vehicles) that the choke is on !
Revs seem high at idle and car wants to take off
Expert:  Doug replied 1 year ago.

Great, thanks.

What would you say the engine is revving at cold idle? At hot idle?

Customer: replied 1 year ago.
Sorry about delay I was working on the car an battery went flat ! About 1100 rpm
Funnily enough when I started the car after flat battery it didn't do the "lurching" thing !
Wouldn't be a computer thing would it ? Doubtful I know
Expert:  Doug replied 1 year ago.

No problem, and me too as I had left for the night when you returned.

There are only two real considerations here and that would be an unmetered air leak (vacuum leak etc) which will cause the idle to go high, or a IAC (idle control valve) failure.

Since you are seeing (if I understood correctly) high idle and lurching etc when cold, but once it is warmed up it gets better, this would most likely not be vacuum related since that would not change when warmed up, but would likely be IAC related as this is incredible temperature dependent.

Specifically, the IACs purpose is to adjust the idle to compensate as needed for different temperatures of the engine. Having it idle high when cold them more normal when warmed up just indicates the IAC is likely stuck opened too far, and once the engine warms up and can use more air it is now in a better operating position where it is stuck, so drivability improves.

There is a relatively easy test you can do on this as well. With the engine idling (preferably warmer so it is lower), turn the AC on and watch the engine speed. Under normal operation when you turn the AC on the computer should idle-up the engine about 100-200 RPM, then when the compressor engages the load will drop the speed down again.

When an IAC is not working the dead giveaway is that when you turn the AC on the idle-up does not happen, and then when the compressor kicks in the RPM goes down as there is no compensation for the load occurring.

Having an improvement when you had the dead battery makes sense for this too... when the battery is depleted (or disconnected), the computer loses its idle adaptation logic and readapts to a zero point condition (and normally the IAC follows suit if it is working). If the adaptation was considerable and it was lost, it is going to assume there is more air flowing in that normal (since it is expecting a "clean" throttle body rather than obstructed with gunk it adapted for). In your instance since the idle is high and there is too much air going in, this looks to the computer like you just cleaned the throttle and cleared adaptations and it would run better temporarily.

I would be doing that IAC test and see how it responds, and ultimately I would probably end up replacing the IAC (mounted on the throttle body) and giving the throttle a good cleaning inside while I am at it.