I have a PC that used to shut down randomly, but recently as I hit the power button it turns off in seconds.
I tried to clean dust off. take it apart and connect again, I even changed PSU and it didn't help.
then I noticed if I unplug the cpu power (4 pin cable) off, PC will stay on with all fans and leds working, but of course it won't boot up. so I plugged cpu power in again to try to go to bios screen in these few seconds, and I managed to check cpu temperature, it starts at 80+ C and increases in seconds to 100 C then shuts off.
I thought it could be a cpu heatsink/fan problem, but even though it wouldn't start at 80+ C would it?
any help would be appreciated
When you took the PC apart and cleaned it, did you remove the CPU heatsink/fan (HSF)? If yes, then did you clean off the old thermal paste between the CPU/HSF and reapply new thermal paste before you reinstalled them?
I did take it off, but did not change thermal paste.
as I said let just assume the fan is off, is it normal the cpu would start at 80+ C ?
that's what's making me wonder, otherwise I would even change HSF and get an aftermarket one if necessary.
The temp of CPU will run thru the roof in matter of seconds without cooler or really bad conduction. Run down to Radioshack and get a thermal paste. If that does not help RMA your CPU...maybe a temperature sensor is faulty...
with no heatsink or a poorly thermally contacted heatsink the temp of a cpu can reach very high very quickly, in seconds in fact. so if you have not fitted your heat sink properly this could cause the symptoms you are seeing, not replacing the thermal compound is not bad enough to cause this on its own.
You're dealing with a socket 775 processor! If you don't have good contact between the CPu and HSF, then temps will immediately skyrocket. Shutdown is occuring at the typical 100C temp...I wouldn't attempt to start it again until I replaced the thermal paste. Hopefully you haven't damaged the CPU; on second thought that might be a good excuse to ugrade to a Sandy Bridge...
Had a friend with the exact same problem, though it was caused by a poorly seated heatsink rather than bad paste. He'd reapplied the paste as he should, but didn't clamp the heatsink down properly. Computer consistently made it into Windows, then powered off about ten seconds later.
He just kept powering it back on over and over again, and fried his CPU dead. Do yourself a favor and don't start it again until you replace the thermal paste, those temperatures put a lot of stress on the chip.