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Looking to buy a used 2003 mercury sable ls premium wagon. It only has 97k miles on it but it idles rough in drive while stopped and has white smoke coming from the hood. What could be the issue?

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Don't buy it. Seriously, it could have major problems and there could be many causes, perhaps multiple causes.

White smoke indicates coolant, probably steam from overheating or a coolant leak spraying over a hot engine. It could be cracked radiator, split hose, bad hose connection, or an engine seal.

Rough idle is likely a separate issue, and could have many causes, here's just some of them:

  • Fuel supply problems: bad filter, bad pump
  • Injectors failing or clogging
  • Ignition system issues, bad HT leads, weak coils, bad plugs
  • Sensor problems: MAF, air temperature, engine temperature, lambda, etc.
  • Computer malfunctions: the Engine Control Unit (ECU)

There's more on top of that. The problem is you don't know what you're getting, it could be something really cheap and simple; replace the spark plugs and tighten a hose connection and you're good. It could also be expensive and complicated, if you have a blown head gasket it's probably more than the value of the car to fix, unless you can do it yourself and want a project.

Also consider if someone is selling this car with known issues they probably haven't been maintaining it, so there would be many other problems you'll discover later.

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