I have a 2006 nissan quest that idle is high all the time went driving step on the breaks it filds it is never is goin to stop the transmission works verry hard

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check your wheels balance,If the problem still check your rims whether you have any slight bend,

Nissan quest has a huge proble. I searched with google and many others to fidn this same problem. The over drive can cause the torque to slowly break the cheap motor mounts. If you replaced it yourself you know how cheap and weak they look. There are mroe expensive brands with lifetime warranties in which they last a good 4-5 years even with a lead foot aka my wife slamming on the gas everywhere lol. So look to a more expensive motor mount with a lifetime warranty. When someoen guarantees it for life they build it not to break :) sorry for spelling errors im on a tablet. Im havign a bundle of trouble with my quest, the vcr built in stopped working, blower motor went out replace fsu and blower switch in dash and next up is blower motor. Have fun nissan quest are great vahicles, cheap parts but a hell of a time diagnosing the problem. Good thing is its about 50 bux for a life time warranty front motor mount. A way to help stabilize it i did when racing is put a heavy duty chain around the bracket as well, wont hurt a thing and will save the life of your motor mount. Sounds rigged but works like a charm. I had to change a motor mount on my mustang form racing 1 time a month untill I put the chain on there with the new one.

Check and see if your cooling fans are working. At least one should come on at all times when the a/c is turned on.

Misfire?

It is possible that the fluid level is low. If so then fill it because this will cause the transmission to slip. The next step is to change the fluid and filter. Hope this helps. Good luck.

Any part can break at any time. Even new ones. See this all the time

Have 2004 Nissan Quest. After 40 mph Van runs rough and there is a loud rattleing noise that sounds like an old 4 bbl carb waaaah, but ther is no code