Rough cold start '98 Mercury Mistique 2.5 L v6

jjdrewski

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#1
Glad the site is back, but my old posts on this topic seem to be lost. The problem was a rough engine, misses, shaking on cold start any time of year. All kinds of fixes were offered. I noticed that the rpm at startup never seemed to get as high as my two other cars of various makes. With 90K miles, I wondered what would prevent the startup rpm from rising into the mid teens and also cause rough running. Guessing that the ECM is pumping a rich mixture through the injectors for the start, I figured the idle air control must have to open up more for the start than for any other cause-like A/C or elecrical demand. Maybe the valve was binding at higher openings. So, knowing I would break a few of the nearby vacuum tubing, I got a foot of silicon model airplane fuel line from my shop and proceded to remove the IAS motor and clean it out. A half teaspoon of carbon grit came out until it was clean. Reassembled, patched the vac lines, and started it up at initial 1600 rpm idling down to a smooth 600 rpm no skips!! fixed! I thank you all once again for the background info leading me to this fix. Happy Holidays! PS I wonder where all the carbon came from--its supposed to be filtered intake air!
 

billr

wrench
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#2
I'm pretty sure the carbon is because oil returned to the intake by the PCV line back-streams all the way back to the TB and further; even though the general air flow is "the other way".
 

nickb2

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#3
Might want to soak some carb cleaner in that area and empty a can of the same stuff through a pre-throttle body hose at idle. May make some heavy smoke while and after cleaning, just go beat the sh%t out of the car after cleaning. I have never used seafoam, but some ppl here seem to really like that stuff. My own mixture is mopar combustion chamber cleaner in liquid form mixed 1pt cleaner for 3 pts gas. Engine after cleaning comes out really pristine inside. Again do not do this in a closed space, need lots of ventilation or a gas mask. :ROFL