I went to drain some coolant from the radiator ( warm engine was turned off about 1/2 hour prior) to install a new thermostat .. but when I loosened the stopcock only a dribble came through the outlet. Backed it way ouy without any change. I used clear tubing to drain and could see that the coolant had a lot of gunk and was rusty colored.

The color of the coolant in the overflow tank is normal green.

After about a half hour I stopped draining (got about 2 liters) and let it settle overnight, and next day I could see settled out rusty gunk and more of a green color to some of the liquid.

Do you all think this clogging is normal and could be fixed with a good backflush and new coolant, or would this indicate a more serious problem (badly coroded, new radiator)?

Thanks in advance for any help ..

-rlp

 

I would flush it. Sounds like there is too much garbage settled in the bottom of the radiator.
I've always judged the condition of the radiator by looking down the filler opening and looking for corrosion/deposits on the ends of the water passages.
My coolant was looking really bad when I bought the truck but the inside looked really clean after the first drain.
I used Prestone super flush.
Drained, ran engine until hot, and filled many more times than the directions called for (until the water drained fairly clear.) Then ran the super flush for about a week before cleaning it all out.

There are quicker ways to do all this but I just do what I can in my garage.

 

Did you remove the cap? If left intact, air can't get in, and it just dribbles...

 

Thanks all ... yes Bikeitswift, I had the cap off. That was my first thought, so I checked and I had loosened it. I just took it off entirely. I know the rad needs a flush .. I am curious now if that's what causing my intermittant overheating problem rather than the thermostat.

 

Just my 2 cents....

I had a problem with a radiator a couple times, once It was as you described with the rust gook. I used one of those super flush chemicals and within a few hours after I used it and flushed and refilled I had so many holes in the thing it was like swiss cheese.

I would just go with regular water, run it, drain and just keep doing that until clean.