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Let me start by saying it is a Mercury Sable, I couldn't find help on a Mercury Forum, so I'm here trying to get help. Ill start at the beginning. It's a 2001 Duratec DOHC 3.0 with 140,000 miles on it. I haven't had a spec of trouble with the car since I purchased it with 39,000 miles on it except i changed the coil pack about 4 years ago. Now 3 years ago, in the summer time, I wanted to change the plugs, and plug wires, so I took the upper intake plenum off to gain access to the back three plugs. Changed everything and reused the upper intake gaskets. Everything was great. As winter approached and the temperature dropped below about 30 degrees, I noticed the RPMs on initial startup would jump up to about 2 thousand RPM's and immediately drop down to about 750 and lope a little bit. As the temp dropped more it got worse. When the temperature outside is down around 10, and you start it up first thing in the morning the RPM's would jump to about 2500 and drop immediately down to 1000 and fluctuate between 750 and 1100, and lope to the point that it sets a trouble code. I assumed that the intake gaskets were leaking and I changed them. No fix. Come spring time it was fine. As long as the temperature is above 35 or so it runs fine. if you let is sit and lope and idle erratically for about 10 minutes, it straightens up and runs fine. If you start it up and just take off. it's fine,, it seems like if you keep the RPM's above 1500 as soon as you start it up, it runs fine. I did find a hose underneath the throttle body, (a 90 degree hose) that was sucked shut. I changed it with a new one. I've also replaced the coolant temp sensor. I've cleaned the MAF sensor. The codes that are coming up are a lean code bank 2 and a misfire bank 2 and one time bank 2 upstream O2 sensor. I don't know what else to do. It runs bad in the winter and great in the summer. Please help.
 

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I didn't read about replacing the PCV valve (on the end of the elbow you replaced) or replacing / cleaning your IAC valve? Could be a vacuum leak, but on my 00 duratec spraying throttle body cleaner into the intake opening where the IAC valve mounts seemed to be the cure for the erratic cold idle problem. Try spraying the existing IAC valve and the opening (librally), re-install and let sit overnight as a first step.
 

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Vacuum leak. Likely PCV / rotted PCV hoses related. Common problem. Looking at fuel trims when engine is in warm up mode would help diagnose.
 

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Bad gas......see the other posts (recently this week) about poor idle quality during winter months. Gas stations sometimes run into trouble getting high quality "winter" fuel (the winter fuel is more volatile than summer or spring fuel and helps with combustion when the engine/combustion chambers are cold).

Plus when the weather changes rapidly (like sudden winter cold weather in February) the gas stations just cant keep up with the weather changes.

If the car runs/starts and idles fine after its been warmed up and driven in the morning....then its probably has gas in it that isnt well suited for cold engine temps. The longer the car sits in cold temps (like overnite) the harder it will be to start and idle with poorly suited fuel for the temp the engine starts at.
 
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