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Discussion Starter #1
My 2008 focus idles a bit rough when first warming up and gets some serious vibration when doing so. It revs up to 1200rpm for a minute then drops down to 700 and shakes while idling until it warms up. Sometimes at idle the rpms drop to nearly 550 and it begins to shake. This mostly happens if i roll the windows down at idle when the increased electric load seems to cause the RPMs to drop.
So far: Ive had motor mounts checked, had it checked for air leaks, air filter looks good, idle control valve cleaned, fuel system cleaned and fuel filter changed, spark plugs look good, valve cover gasket replaced because it was leaking.

Put a couple thousand miles on it since this issue started (issue started a few weeks after purchasing car) with no problems driving except maybe slight vibration and acceleration seems a bit slow, even if I floor it, but it could just be this model of vehicle. Mechanic says don’t worry about it, its just a high mileage engine (94k miles) but I can’t help but want to fix it so it idles smoothly. Could it be an alternator problem or is it the engine? Or should i ignore it and keep driving?
 

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These Focus are designed for one reason ……. MPG. The computer constantly checks idle and lowers it to the point of stall... then increases it slightly. This goes on constantly in order to tweak out every last bit of MPG. It is able to do this by monitoring all the sensors. if a sensors goes bad the whole process fails. The good news is that the system troubleshoots itself constantly. If you have a issue there should be a code saved and your check engine light should be on.

The vibration is 95% caused by your passenger side motor mount. I had 150,000 miles on my 2008 and it drove like crap. Vibration from hell to the point the passengers in the back seat would get car sick on long rides. I replaced my passenger side motor mount..... and it was a drastic improvement. I'm sure you will get a few more comments but the passenger side motor mount is a hydraulic mount that is going to fail. Just replace it and we will go from there. Do not go cheap and just buy the rubber mount pieces.... get the whole OEM assembly... mine ran about 100$ it was by far the best thing I ever did to my Focus. There are a ton of cheap Chinese knock offs on Amazon for 30$..... they will work... but not last 95,000 miles that the one that came from the factory.
The next best was a cold air intake.
 

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Mechanic says don’t worry about it, its just a high mileage engine (94k miles) but I can’t help but want to fix it so it idles smoothly.
94K is high mileage, so just deal with it? Get a new mechanic.

Like the comment above, I'd suspect your passenger mount is failing. I wonder how exactly your mechanic "checked it." Mine failed around 100K, and that's probably longer than most go without dying.
 

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Discussion Starter #4 (Edited)
I had a great mechanic check them they had really good reviews, they said some or one of the motor mounts had already been replaced but i guess i will get a second opinion. This is not the same mechanic as the one who said to not worry about the idle. Also there are no codes except an emissions test that never completes.
 

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If it wasn't a dealer, they probably used aftermarket mounts. Those tend to be crap and fail earlier vs OEM.
 

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Not familiar with a emission test that never completes? Go to a Autozone and ensure it is not throwing a code … any code is bad. Is the check engine light on? It might be a O2 sensor... where the computer is thinking you are running to rich and adjusting idle to correct.
 

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Discussion Starter #7
No there is no code. When i look through readiness tests though one of them is never able to complete.
 

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as long as its not failed then you just need to keep driving it. 35-35 miles per hour for 3-5 minutes should set it.

That's nearly impossible to do safely though.
 
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