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One summer day on the highway, my ‘79 was running hot, not in the red but riding the bottom of the redline. When I got home, I could hear a hiss from the radiator cap and decided to get a new one, and flush the system while I was at it.
After removing the thermostat, I drained the coolant, added distilled water, ran the engine for 10 - 15 minutes and repeated. I had planned on switching from Dexcool to Green and wanted my flush to be completely clear to avoid mixing. Each flush looked more and more clear and I was feeling good, then came flush number 8. It looked coffee. I flushed 2 more times to see what was happening and they looked the same as number 8.
Someone mentioned potentially having an issue with the transmission lines that run into the radiation so I used the excuse to buy a Dewitt aluminum direct replacement, but as I was taking the old radiator out and disconnecting those lines, the transmission fluid was as pink as pink could be. I’d imagine if that was the problem, the coolant would have turned a more pink color than black. Another search indicated a blown head gasket so I’ll be buying a gauge to test compression and (hopefully) rule that out.
Any thoughts or insights? Am I on the right path? Any other possibilities?
Coolant color after each flush, beginning with the first flush on the right and the last flush on the left
Pull your block drain plugs, to flush out the sediment and old coolant.
They are on each side, about the middle, down low.
They are usually pretty tight. I hit them with an air impact (judiciously) to break them free.
The block holds about 4 gallons.
Some people don't like pulling the plugs, others don't mind.
Also, you can get a block testing kit to check for exhaust gasses in your coolant.
Samples the gasses in your coolant, and changes color in the presence of combustion gasses.
WAG on the dark coolant, it's probably sediment that has been stirred up by you flushing the system.
If you have heavy scale in the block, you could put some CAT engine block descaler in.
You dump the coolant, put in the descaler, refill with water and run it for a month, then dump, flush, rinse as needed, and refill with your preferred coolant.
But, it does require operating the vehicle, something alot of Corvettes don't get.
Mine doesn't have that problem, I drive the hell out of it.
Good luck
Jeff
Ps. When coolant and oil/trans fluid combine together, they make pudding, oil will be grey/black, atf would be pinkish, depends on the color of the coolant.
Last edited by jeffwebley; Jan 11, 2021 at 12:25 PM.
Reason: Forgot a detail
Did You drive the car after each Flush ? You say that you removed the Thermostat but I'm guessing that it didn't open till the last time. So why not flush it some more and see what it looks like.
I had to punch through a block of sediment with a screwdriver, but the coolant ran clean from my block plugs. Hopefully you can get yours out easily. It's a lot easier with the exhaust logs removed.
I agree with the others that the last flush has likely dislodged some sediment. It looks like your on the correct path, and wouldn't hurt to run it for a day or two around town between further flushes. These are old cars and you never know what someone has dumped into it previously to your ownership, so flushing is a good idea. Make sure you have the heat selected on so it flushes the heater core as well (some put in a manual shutoff valve)
Another vote to pull the block plugs. When I took mine out, not a drop of coolant came out until I broke through a build-up of scale inside with a screwdriver. Then, I could flush all the old crap out.
The coolant has stayed a nice clean green ever since.
I got a garden hose on it this morning flushing from the upper radiator hose to the lower radiator hose. Filled a 5 gallon bucket about 4 times and it started coming out clear. Going to take the heater hoses off and flush that separately this afternoon and see how it goes. Also plan on getting ahold of a pressure washer and back-flushing the block again with a little more English, then I’ll hook up the old radiator and try to run her for a while and do another flush.
I looked at the block plugs and would like to maybe save that for later.. seems like a task I’ll get right on the second try, and I need her running soon. Reserving this for next step if necessary.
Like the idea of a gas test kit, plan on looking into this for the next flush session. I’ll post another update when I get a little further.
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Coolant Flush - From Clear to Black
