Dimension 8400 thermal event/overheating

I need help with overheating in a Dell Dimension 8400.

I suffered a damaging power surge so I installed a new (used) mother board, some new (used) memory, a new heat sink (new from Dell) and upgraded the CPU from a 3.0 to a (used) 3.8. I’ve also installed new retaining clips for the heat sink. Ever since I’ve installed the new mother board I’ve had overheating problems and thermal events. I’ve tried to solve it with blowing the machine free of dust, installing a video card fan, getting a new heat sink with new retaining clips and reapplying arctic silver thermal paste. I still have the original heat sink fan and shroud.

What I’ve found is that it seems to be a matter of the pure physical pressure of the heat sink on the CPU. If I start the 8400 with the machine upright which is the default position it shuts off immediately. If I turn it on its side it runs normally. However, the fan is as noisy as a vacuum cleaner. (The boot up screen also tells me that the last shut down was because of a “thermal event”.)

With the machine on its side the heat sink is now upright with all its weight pressing down on the CPU. If I press on the sink the fan calms down. The more I press the quieter it becomes. The 8400 sink is large and heavy. If the machine is upright in its normal position the heat sink is effectively “hanging off” the CPU and its weight is falling away.

I don’t want to run the 8400 on its side. I’m bothered how this would affect the hard drives and the fan noise is unbearable. I also don’t want to place weights or otherwise increase the pressure on the heat sink to improve sink and the CPU contact.

Is there anything I can do about this? Do I need to buy a new fan and shroud? Will a new fan/shroud have more springiness and therefore press down on the heat sink to increase sink and CPU contact?

Since I can decrease fan noise/speed by pressing down on the heat sink, I assume this is just a physical contact issue and wouldn’t be solved by a new mother board. True?

I just don’t get it – how can a new motherboard lead to a physical contact problem that can be solved by applying physical pressure/weight? What’s changed?

Thanks in advance for your help. It is very much appreciated.
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More about dimension 8400 thermal event overheating
  1. try re-installing the Dell heatsink, use a good thermal grease to ensure that no space is between the CPU and the heatsink itself, if you're running good and no overheat coming up from the CPU but you suffer from a DELL fan noise, i suggest you to do a BIOS update since i had one of these problems and fixed it since i had a Dell machine
  2. The problem is with the CPU/heatsink installation. On certain Dell models, the heatsink pivots off of 2 support points and is secured by 2 screws. You may need to do some customizing in order to get intimate contact between the CPU and the heatsink. Try the following:

    1) Check the fit-up between the CPU and the heatsink with no thermal compound. With a slight clamping force (by pressing with your hand) see whether the heatsink 'rocks' from side to side.

    2) Add layers of aluminum foil to take up any slack under the pivots. Check fit-up again.

    3) When you have the right fit-up, apply a thin film of thermal compound (per established industry practices) and re-assemble the heatsink to the CPU. Tighten the screws in a diagonal pattern, in small increments. This will ensure uniform contact pressure.

    When the CPU/heatsink contact is correct, the heat transfer will be right, and the fan will run at the normal rated speed.
  3. Thanks for the advice. I rechecked the connections and found I had screwed the CPU base in tight enough. Once I'd done this I had no problem. The simplest thing.

    Thanks for your suggestion. Without it I'd never have gone back to the fundamentals.

    Thanks again for taking the time to give me your advice. I very much appreciate it.
  4. Glad it worked out!
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