Fans louder after thermal paste job, despite a lower temp.
I just replaced the thermal compound of my old MacBook Pro (high quality grease, the old one was dry as dirt).
Before that, I had to underclock it from 2 to 1GHz to keep it stable (using Coolbook). The temperature at rest was around 55°C, and at full CPU load, it would reach 72 °C. The fan never kicked in at that temperature (never loud, but I didn't measure the speed at that point), but if it stayed that high for too long, the machine would freeze.
After the paste job, at 1GHz, even under load , the CPU temperatures (Core1, Core2 and CPU temperature diode) don't exceed 53°C. However, in these conditions, the fan runs at 3800 RPM (louder than what I remember it was before).
If I increase the clock speed, the fans accelerate, and reach 6000 RPM under load at 1.5 GHz. The temperature is then around 65-70°C
I measure the temperatures using Marcel Bresink's Temperature Monitor and the fan speeds with Lobotomo Fan Control. The latter is supposed to enable tweaking the fan speed settings, but it doesn't work here.
I also noticed that the temperature is now more volatile than it used to be.
Thanks in advance for your help.
Well yes, if the fan's never kicked in, or ran only at low speed, now that they run at 6K RPM they'd seem noisier. They'll also burn out faster. Maybe you'll want to order replacement fans so they'll be ready. Breakdowns always seem to happen late at night on Sunday a Monday morning deadline.
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Fans do not decide to run fast on their own and replacing the fans will not solve the the underlying problem or make the fans run quieter . The SMC tells them how fast to run, (under normal circumstances) based on what some temperature sensor(s) is telling it. If the SMC can't find some hardware it needs to communicate with it will put the fans on high to prevent damage and let the user know something needs attention. Your temperatures don't sound bad at all. I suggest starting off by resetting the SMC and PRAM to see if that tames the fan speeds down. Click here and click here for instructions on how to do the resets. If that doesn't resolve the problem download iStat by clicking here, a free program/widget, to monitor your temperatures. That program monitors more than the temperature under the CPU. Please post the temperatures (with and without the system under load) it gives here.
Next I suggest you run AHT (Apple Hardware Tests) to see if you are having sensor problems. If your laptop still has the original hard drive and it hasn't been repartitioned you can boot while holding the D down and access it. If that doesn't work the AHT is on the original disk(s) that came with your laptop. If AHT gives any error codes please post them here.
I read you had temperature problems prior to changing the thermal compound, but the fans were running at a "normal" speed. I'd double check all that the heatsink is properly seated, also that all the wires/cables are seated correctly and in their proper place. You did not mention what thermal compound you used, I suggest Arctic Silver 5. Thermal paste needs to be spread out on the chip very then and evenly. Too much can cause problems.
If everything checks out good with AHT, the connections are in order and resetting the SMC did not resolve the issue. I would replace the heatsink along with the sensors on it. If the heat tube on it has been compromised heat won't be drawn away from the chips as it should and the SMC will see the imbalance of temperatures between the chips and the heatsink, then raise the fan speed to prevent damage.