Somewhere along the line, someone mixed green and red antifreeze in my 4runner. I discovered this when replacing the timing belt, water pump, thermostat and radiator. under the waterpump, there is a little caked on gunk, which I think could be from the two incompatible fluids. Most of the metal was clean, though.

Since I am replacing the radiator and putting in Toyota OEM coolant, there will be no problems there. What about the pathways inside the engine? Should I do a chemical flush, or are they open enough that they would never clog up?

Thanks.

You should be fine, especially since you are replacing all the fluid now with OEM. If you read the OEM manual all it says is about the coolant is that it needs to be ethylene glycol based. I know there is a big scare to deviate from the "toyota' brand but RTFM lol.

I used Zerex Asian formula when I recently swapped mine out.

Thanks.

Off to the dealership to pick up the crank bolt and coolant. Maybe I can finish this tonight!

Without knowing what the other two styles of coolant were, I would plan on flushing it. You don't need to be anal about it, but backflushing the heater core takes ten minutes and hurts nothing.

I would not worry about any damage or plugging. Dried coolant by the pump will always look like that. Good thing you changed WP. However, a thorough flush is a good idea when changing coolant types independent of there having been TWO of them. Ethylene glycol is ethylene glycol and they all work the same, mixed or separate, for 25K or or 30k miles.

Where they differ, is in the effectiveness of the anti-corrosion additive package as you pass 25k. There are several different technologies out there today, and different additive packages to make them work. Many of these are antagonistic to each other, so it behooves you to get all that old coolant(s) out of there as you switch to a known, top-grade expensive coolant. Leaving much of the old crud in there could degrade your new anti-corrosion additives, especially if you want to run it longer than 25k or so.

The heater core is easy enough to flush with a hose and probably needs it anyway after 14 years.

As for the engine, can I put a hose on the top and let it drain out the bottom, or should I use the chemical method where I fill the system up with distilled water and chemical flush, run the engine, drain, repeat with distilled water, then finally fill with coolant?

If your water is very soft, AND you do at least one full flush with distilled, it is probably ok to use the hose flush. Recognize that calcium and metals found in hose water are VERY bad for the corrosion inhibitor chemistry and this is the reason for the trend to pre-mixed coolants. I hose flush my 1966 tractor but not my Toyota, and I have exceptionally soft water. So if you are going to put that crap in there, make sure you get it out before putting your pretty, colored, expensive coolant in. If your water is hard enough to need a softener, I'd stick to fill with distilled, run and drain. Personally, I'm no fan of chemical flushes.