G5 Quad CPU 0 high amp reading

[Krizzo]

Hello fellow Mac lovers. This is my first post here. Nice forum, I think I'll stay!

Let's get directly to my problem/question: I acquired a G5 Quad and already did a flush and refill of the lcs. I assembled everything back, repasted the CPUs and did a thermal calibration. Everything works so far, but: CPU 0 is at 47°C in idle while CPU 1 (lower one) sits at only 37°C. So CPU 0 is 10°C hotter. Pumps and fans are all detected and running at normal speeds.
Voltages are at 1.16V at all cores. But the amp readings are different: CPU 0 draws up to 9 amps in idle while CPU 1 only draws 6 amps.

Is this normal behavior? Does CPU 0 maybe do all the OS background work in Leopard while CPU 1 is truly idling along? Or do you have any other explanations?

Greetings and thanks!

 

[CircuitBored]

Welcome to the forums!

Yes, one CPU tends to run hotter than the other in my experience but G5s are also incredibly fussy about thermal paste too. Have you done an extended load test yet? Run the command yes > /dev/null & four times in a row to give the four cores an intense stress test and let it run for thirty minutes. If your temperatures don't stay comfortably low then you still have work to do on the cooling system.

What was your process for flushing the LCS?

 

[Krizzo]

Doing a load test the hottest core settles at 63°C fans at full blast. This is perfectly fine, I think. What makes me wonder is the difference, the other CPU is at 52°C. But if you folks here can confirm that this is normal, I don't see the problem.

I have the dual pump lcs. I just cut the four hoses connecting the pumps, drained the system, filled both loops with vinegar and let it sit for 24 hours. After that i thoroughly flushed until I didn't see any red crystals coming out. Then I replaced the hoses and filled the system up with PC water coolant (my gaming PC is also water cooled, I have experience and material). I just connected the filled-up hoses, I didn't modify the lcs. The liquid is flowing, no big air bubbles, no grinding.

 

[beachycove]

Some difference is normal. 11ºC is a bit much, in my view, but as full load and 63ºC is good, so that damage is not imminent, I would run it for a while and see how you get on. I have done a number of G5s and can confirm that the heat sink grease is hard to get right on these. I’d give it a chance to cook for a while and measure again in a couple of weeks.

 

[Byrd]

I think it sounds plausible that the top most CPU could be 10 deg C hotter than the bottom part, with heat rising, poor design of LCS, and so on. If it was wildly different you could point a finger at going back to reposition the CPU block once again and/or bleed the coolant on the "hot" CPU.

However, from what you've described - decent load temps, no crashing and pumps check out fine - you should be able to set and forget it for 18 months until you need to flush and add new coolant, with periodic checks for leaks and bubbles.

JB

 

[Krizzo]

I took out the CPU assembly, re-tightened the CPU socket screws - Didn't change the issue. Top CPU still 10°C hotter. I definitely spread the thermal paste over the die as recommended for chips without heat spreader. Ok I think I'll have to live with it then.

I'm now going to swap the rear CPU exhaust fans, those are the noisiest in my system. Project silence continues.

 

[Krizzo]

Update: After refilling the loop twice I'm now pretty certain that the upper pump is dying. Sometimes the CPU idles at 70°C (!) while the lower CPU is at 33°C. Tiny air bubbles in the clear tubing don't move at all. The coolant is very hot though, so I think the pump is spinning, but not as fast as it should.

 

[herd]

I don't have one of these Macs, but from what I've read there are little filters in the lines that can clog up with corrosion particles. It sounds like more maintenance or repair is needed on the cooling system. You could try swapping the two CPU cards to see if the high temperature is something electrical on the card. i.e. f you swap them and the bottom CPU has high amps/temperature then maybe the cooling system is not to blame.

 

[Krizzo]

Update again: I rebuilt and flushed the lcs again completely and interconnected both loops to form a two pump single loop. Both pumps run fine, both processors run fine. Seems like the system was still clogged and needed another thorough flush.