I know the 3 cyl mode helps warm the engine quicker due to the load put on the engine, so...

If I put the truck in gear and keep my foot on the brake am I doing any harm to the trans if I let it idle like that for 5-10 minutes?

What about if I do the same thing but bump the idle to 1200 rpm or so? Is that hard on the tranny or any of the parts?

Sorry if it's a dump question. I only know the basics when it comes to how auto transmissions work.

Thank you!

 

putting it in gear and giving throttle while on brakes will make it burn out... not good. what i do is leave it in park and hold throttle at 1150 rpms which is where 6 cyl high idle is at and it warms up in about ten min but no more than ten minutes cause then you risk diesel fuel washing over your cylinder walls. hope that helps