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350matt, Engineer
Category: Mitsubishi
Satisfied Customers: 22000
Experience:  Mechanical Engineer BEng Hons 22 years experience in car engine industry, 10 years in formula 1
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Are you a real person? I drive an evolution lancer 5. I

Customer Question

Are you a real person ?
JA: I'm an assistant to the Mitsubishi Mechanic. I work with them to help customers like you.
Customer: I drive an evolution lancer 5. I changed the alternator out. Now I have a rough idle and the car dies when under 1K rpm
JA: Is the rough idle intermittent or consistent?
Customer: Consistent . It starts fine and run smooths but soon as I let off the throttle it rpm blimp then the car stalls. I was told I have to let the car re learn itself
JA: What year is your Lancer?
Customer: 1998
JA: Are you fixing your Lancer yourself? What have you tried so far?
Customer: It just started yesterday.
JA: Anything else you want the mechanic to know before I connect you?
Customer: My car drove normal before brining it in for an alternator swap. Now it idles out and dies under 1K rpm
Submitted: 1 year ago.
Category: Mitsubishi
Expert:  350matt replied 1 year ago.


I'd suggest to remove the auxiliary belt again and turn the new alternator over by hand feeling for any rough or tight spots as you can sometimes get a faulty part from new

if thats OK then I'd suggest to check for an air / vacuum leak

after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.

As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Expert:  350matt replied 1 year ago.


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