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The vibration issue is almost certainly due to a carbon buildup on the intake valves, which all direct injection engines are known for. It's a common well documented problem. Using a Top Tier gas will do nothing to remedy the problem either, as the fuel is injected directly into the cylinders, bypassing the valves, as opposed to a port injected engine, where the fuel is sprayed across the valves, thus cleaning them. Adding a fuel cleaner additive to the gas tank will do nothing either. There are several easy methods to clean the valves yourself, using SeaFoam, etc. I've done it on my XTS and it works fine.
I beg to differ. We have had the vibration at idle in several brand new XTS's we've rented long term for travel over the years all with +/- 10-20 miles on the odo, and we just had another rental XTS with 14k and 0 vibration (no doubt as a rental the cheapest gas was used). Now we have another 19 XTS with 9k which has the vibration. As I posted above I believe it's the cooling fans not being properly balanced or perhaps the assembly not secured properly by the rubber grommets which are there to reduce any vibration the high speed fans cause (which can be significant if not balanced properly), or the fans may be out of synchronization with each other.
 

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Have no idea why yours only vibrates in the winter, but the vibration in ours only comes on when the cooling fans go into a higher speed mode, within a minute of the fans slowing back down the vibration is gone until they go back into high speed mode. It appears the fans are out of synch with each other...If you've ever piloted a twin engine plane or boat it's the same type of resonance. If you want to be sure you can unplug the cooling fans connections and see if it goes away...it does on ours. This vibration has been plaguing many new XTS's since they came out.
 

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Same with my 2018 xts awd... see my earlier posts about vibration at idle... now that it’s warm out no vibration at all and nothing has changed, not even the oil. (Only have 3300 miles on it since November).

Only thing I can think is winter gas versus summer gas mixtures...
 

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The winter mix was something the service dept mentioned to me. They said only XTS owners are complaining about the problem.
 

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I'm starting to think my April oil change and idle improvement coincidently coincided with the seasonal fuel changeover. Car still idling smooth as silk. We'll see what happens in the fall.
 

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The vibration issue is almost certainly due to a carbon buildup on the intake valves, which all direct injection engines are known for. It's a common well documented problem. Using a Top Tier gas will do nothing to remedy the problem either, as the fuel is injected directly into the cylinders, bypassing the valves, as opposed to a port injected engine, where the fuel is sprayed across the valves, thus cleaning them. Adding a fuel cleaner additive to the gas tank will do nothing either. There are several easy methods to clean the valves yourself, using SeaFoam, etc. I've done it on my XTS and it works fine.
I just cleaned my valves, I don't believe it's been done since new (I bought it used) and it has 109,000 miles on it. I used a can of the CRC GDI cleaner first and then the next day used the Seafoam cleaner and lube. It seems to have cleared up the really low idle the makes the car shake but I think it's time to do the spark plugs as I can't see in the history of the car of it being done.
 

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At 10,000 miles I don’t think I would have that much carbon buildup to create my problem. Besides, it idles smooth in the summer and rough in the winter. Weird.
 

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I can feel a very consistent, slight low frequency vibration in the wheel at idle. Feels like 10-20 Hz. 10 HZ would be 600 RPM if it is once per revolution. Could just be a characteristic of the V6? Haven't owned a V6 for many years. It is nothing like a miss on a V8.
 
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