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G5 Quad w/rebuilt LCS will not stop overheating


Well-known member
In the G5 quad cooling system, there are two little plastic inserts about the size of a pill bottle lid, with microchannels etched into them. In the disassembly/excavation, as I recall, they are next after the O-rings. (It is, by the way, reasonably easy to refill the system once they are out, since fluid circulation channels are then exposed.) Anyway, those microchannels are supposed to distribute coolant at the business end of the heat absorption system. The trouble is that when sludge or debris are present (and they just are present now) they clog up so that coolant cannot pass. The coolant therefore does not cool.

Have you cleaned them up? I don’t see them mentioned here.

Mine is the single pump version, btw.

I actually ran a G5 without those plastic inserts/ mini-filter for a while (ran fine on reduced CPU), because the original configuration was so prone to clogging in my machine. On my latest rebuild, I put them back in and — after a couple of heat sink compound applications — things were a great deal better. (I have found over the years is that the G5 is inordinately sensitive to varieties and quantities of heat sink compound.) However, I also installed an automotive fuel filter in the loop so as to catch those particles before they could clog up the microchannels. So far it has worked.

The quad is a frustrating machine for tinkerers like us. I have found the several air-cooled machines in my possession to be very reliable, once serviced, but the cooling system in the quad was just not engineered for the long haul. And we are now nearly two decades in….


Well-known member
The suggestion of using an ultrasonic cleaner is an excellent idea - I recently bought a 10L unit and didn't even think of using it on what is obviously a clogged loop, somewhere, and I suspect the small lines around the block have filters (that are a right PITA to get to, but what's another afternoon). I think we are all at a similar point where something has solidified and while I can swear I spent a good sunny afternoon blasting hot water and vinegar in mine, it did nothing. The replacement pumps in mine are clearly impeded by something.

And we are now nearly two decades in….

You do make a good point :)


Well-known member
Actually, mine did have a tiny filter at the radiator intake, the filter being just a piece of nylon mesh in a cup shape, with a rim. It had clogged, and came out entirely as an obvious failure point. I forgot about that yesterday.
I've been playing with my Quad for a couple of months now. The main thing about the quad is don't get frustrated. If it doesn't work, just put it away and try again later. Mine has the dual pump LCS and I performed the "New Blood Mod" as described in an earler post. Unlike the single-pump LCS, the cooling block cannot be disassembled and cleaned out.

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.

2. Incorrect assembly can keep the CPU from cooling properly. 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. Run your computer for a few hours, then inspect for air bubbles. Purge as necessary. Run again and reinspect. This has been a big issue for me. CPU B was 20C warmer than CPU A. I couldn't figure it out for a long time. The cause was air bubbles. Run the system in different orientations to find the air bubbles. 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%.

5. If your system doesn't pass thermal calibration right away, don't get frustrated and tear the LCS apart. Run the computer and enjoy it. Or put it away for another time. Look for air bubbles, see 4.

6. A correctly functioning LCS should keep the maximum temperature at 70C or less under full load. If it cannot do that, then it will fail calibration. 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.

My G5 is still a work in progress. I haven't passed a thermal calibration yet, but am very close. The CPUs remain below 70C most of the time, even under load. The fans are quiet unless I load the system. The system is actually very useable. I have been compressing video, compiling Macports, and watching YouTube and haven't been able to make the system overheat or fail. I think running it under load is helping to break up old deposits in the LCS.

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.