I recently gave my son my used C840 for college and he called the other day and said it was running hotter than normal, additional heat emanating from the keyboard.  He brought it home and I was just going to clean the CPU heatsink/pipe/fin/fan and see if that made a difference.  Before the cleaning it was running at 54C with 100% usage on the CPU, after the cleaning it dropped to 48C.

 

Since I had it apart I decided to check the flatness of the heatsink where it mates to the CPU.  The factory heat pad on the bottom of the heatsink was all dried out and hard like glue.  I checked the heatsink with a stainless steel straight edge and I about fainted.  You could see daylight under the straight edge in quite a few places on the bottom of the heatsink.  I lapped it with 400 minus and then finished it with 600 minus (15 minutes) and re-checked it several times with the straight edge and the imperfections were gone.  After I finished I very carefully cleaned everything up with alcohol swabs and was careful to wipe down the sink/pipe/fin surfaces with alcohol also, I had powderless latex gloves on so no oil from my fingers got anywhere on the assembly.

 

I put a thin-thin coat of Artic 7 on the CPU with a sponge tipped swab and put it all back together, booted it up, and ran Sandra on it for five hours.

 

The results were pretty good the full load temperature dropped all the way down to 42C with the idle now at 37C.  If this is the first time you have had your C840 apart you will notice quickly that the CPU cooling assembly is really puny, the cooling fin assembly is puny also, and the twin fans for cooling are minimal at best.

 

You might want to try this if yours is heating up and you have already cleaned the fans and fins.  If you have questions let me know......