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As the subject says, I noticed that the coolant in my overflow tank would, over time, turn from the typical green to darker orange. This was despite my repeated efforts to flush the overflow tank. I had removed the radiator cap several times to observe the color of the coolant in the radiator, which looked green. I didn't have any symptoms of a head gasket failure, and it was a very clear, clean orange instead of a murky, cloudy brown. I was also fighting a battery terminal corrosion issue.

I recently read a few things about electrolysis in the engine coolant and how to diagnose it. Using a multi-meter, I found out that this was an issue for my engine. Specifically, the voltage between the negative battery terminal and the coolant in the radiator was .5volts with the engine off and no accessories on. My understanding is that it should have been closer to .1Volts. Combined with the battery terminal corrosion, I figured I had a grounding issue. So I decided to check the resistance of the negative battery cable. I found that the other end was no longer bolted to the engine block. I guess I forgot to hook it back up when I put a new clutch in 2 years ago. Since then, apparently the only ground from the battery was the smaller wire that attaches from the negative terminal to the radiator support. The orange coolant was a more recent development, but maybe it takes time for the electricity to degrade the coolant to that point.

I hooked the primary ground back up, and drained the radiator and engine block. It was a dark orange too. I guess looking through the radiator cap isn't a good way to check coolant color. When all was refilled, I checked the voltage between the coolant and negative battery terminal, and it was about .1volts. I'm pretty sure that the problem is fixed. At the very least, the electrical system is now properly grounded.

By draining the radiator and opening up the plugs at the bottom of the engine block, I drained about 4 quarts. I put 4.5-5 quarts back in after the initial air bleed. Stated capacity of a 2.2NA is 6.2 quarts. So I'm guessing that the heater core doesn't drain via a siphoning action when the engine block drains, but 1.2 quarts seems like a lot of coolant to be tied up in the heater core. While all of the drains were open, I lifted the car up on all 4 corners to try and persuade as much coolant as possible to find its way to the drains. I'll see how much I have to put in after some driving around.

Hopefully this post will help someone else with their troubleshooting efforts.
Anyone else have a similar experience?

That's interesting to hear. Thanks for sharing.

Let us know if you continue to experience issues with the coolant.

Well, finally figured it out. Although the above post I made is true with regard to voltage in the coolant, and my missing ground, it did not cause the orange color in the coolant..

I just finished replacing my water pump which failed after 30,000 miles. The failure was not catastrophic. I heard a knocking/rattle/growl noise and isolated it to the pump. I'm going to dissassemble the pump to verify, but it appears to have been a bearing failure. The shaft turns very rough and is noisy with no other visual signs of distress to the pump. This probably means that the water pump bearing grease has been contaminating the coolant, possibly turning it orange for some time now. I power flushed the radiator, block, and heater core and re-filled with new coolant. Hopefully it stays green this time.

For those wondering: Yes, the failed water pump was aftermarket. Yes, the replacement pump is OEM.

Took the pump apart, down to the individual ball bearings. There was plenty of grease in the bearings, although there was a trail of grease leaking out of the front seal down the housing which is a possible indication of a slight coolant leak. This pump uses a duplex ball bearing. The bearing closest to the pulley had failed. There were several spalls in the inner raceway, and a handfull of balls were mangled. I don't think that the slight coolant contamination alone caused the failure due to the abundance of grease still remaining in the bearing. It may possibly have been a material or manufacturing defect. It was a great experience taking the pump apart though. If anyone has a spare, I highly recommend it. All I used was a 2-jaw puller and a hydraulic press.