This is a repost from the "G5 Quad w/rebuilt LCS will not overheating thread", but I am posting it here so my notes are easily available for anyone attempting the New Blood Mod. I purchased a G5 Quad from the local classifieds. I was actually looking for an air-cooled dual G5, after reading all the problems with the LCS. The seller had no idea as to the specifications. When I inspected it, it was a Quad model with the dual-pump LCS. I bought it anyways and inherited a small boatload of trouble. The LCS showed no signs of leakage, but wasn't able to effectively cool the CPUs. After running a few days, the system overheat and the red LED came on. I did some research online and after considering all my options, I performed the "New Blood" Mod as described above.
Although my unit still hasn't passed a thermal calibration, I consider the mod to be successful and am sure after some additional steps my G5 will pass calibration. My G5 now runs cool, the fans are quiet, and it doesn't shutdown due to overheating. In short, it's a 100% useable computer. This mod isn't plug and play, but should be doable for anyone with some basic tools and skills. The main thing is to be patient and not to expect everything to work perfectly right away. It seems my cooling system is performing better now than when I first performed the mod, it may take some break in running to get everything running. I only had the LCS apart once, choosing to trouble shoot it rather than re-do it.
Some Lessons learned so far:
1. The original cooling fluid has a lot of sediment / contaminant. If you get an LCS G5 and the LCS hasn't failed, it will fail and will not be able to regulate the CPU temperature under load. This can be due to: dry thermal paste, air in the LCS, mineral deposits on the inside of the heat exchanger, failed cooling pump, blockage in the circulation system. Leakage isn't the only failure mode and a dry LCS isn't necessarily a functioning LCS. Below is a photo of the original cooling fluid that I extracted from my LCS. All that sediment cannot be good for cooling efficiency. Do you want this stuff in your G5?
2. Incorrect assembly can keep the CPU from cooling properly. I damaged one of the spring-loaded pins during reassembly. My G5 overheated immediately. I had to find a new pin and reinstall the CPU. The cooling surface is very small, there are no heat spreaders. You need quality thermal paste and need to apply it correctly. I recommend spreading a thin layer on the CPU. The spring-loaded mounting pins must be evenly and fully seated to ensure even pressure and 100% contact. We are dissipating over 100W in less than a square inch of surface area.
3. The LCS needs to be cleaned out. Mineral deposits can form on the inside of the radiator and act as an insulation layer. Ultrasonic cleaning would get rid of any mineral deposits on the inside of the radiator and heat exchanger. Another simpler idea may be to add 50% vinegar and run the computer for a few days, then replace the cleaning fluid... That's what I would do on an old car radiator.
4. Air bubbles can form in the LCS at any time. After performing the mod, my CPU B was 20C warmer than CPU A. I couldn't figure it out for a long time, suspected all kinds of things from incorrect assembly, see 2, to defective CPU card. I swapped the CPUs A and B, no change but ruled out a defective card. The cause was air bubbles. I thought I purged all the bubbles when I filled the LCS...I was sure of it. Then I ran the system for a day, and found lots of air bubbles on CPU B. Run your computer for a few hours, then inspect for air bubbles. Purge as necessary. Run again and reinspect. Run the system in different orientations to find the air bubbles, i.e. vertical / horizontal. Run it with the lid off so you can see the air bubbles. Put in a guitar pick to keep the fans from going to 100%. Sometimes just turning the system on, the initial suction effect of the pumps will reveal an air bubble.
5. If your system doesn't pass thermal calibration right away, don't get frustrated and tear the LCS apart. If it's not overheating, run the computer and enjoy it. Or put it away for another time. Look for air bubbles, see 4. Check your assembly, see 2. Not passing thermal calibration doesn't mean the LCS doesn't work. It only means it cannot keep the temperature regulated at 70C or less under full load. Don't keep trying to run calibration on a defective LCS. Find the root cause, correct, then rerun calibration. Also ambient room temperature has to be below 77F for calibration to work.
6. A simple mod: I put a strip of tape between the fan assembly and the heat exchanger. This forces all the air into the heat exchanger, because we removed the CPU cover plate and I noticed quite a bit of air was leaking out the sides. This seems to drop the CPU temperature by another degree C or two.
My G5 is a work in progress. I haven't passed a thermal calibration yet, but am very close. My next step is to replace about 50% of the coolant with vinegar, then run the system under load for a few days. Replace all the cooling fluid, then reattempt thermal calibration. If that doesn't work, then I'll consider getting an ultrasonic cleaner. But that's about $150 for the cleaner, but I know if I bought it, I'll find many more uses for it.
The Quadra G5 is an awesome computer and I believe worth saving. I also have a Pentium 4 from 2005 and the G5 blows the Pentium 4 out of the water. It's not even close, and is like comparing a vintage Boeing 747 with a DC-3. I can think of no other computer from 2005 that's as useful today as the G5 Quad. Overall I'm happy with my G5 Quad and expect to get many years of enjoyment out of it.