My current squeeze had a '98 mustang with 120,000 miles that runs very well. What is the problem you ask? Well, she had never added, flushed or changed her coolant. Never touched it. My first thought is to flush that baby out and refill. But do you reach a point where you can do more harm than good in flushing out 120,000 miles worth or junk?
What harm could you do? I suppose that if there is a large amount of scale built up in the radiator, backflushing could break some of that away and cause it to flow into the engine, but normal flow through the radiator would do the same.
You should drain the radiator and inspect the what tubes you can see inside the tank by looking down the cap opening. If you see a lot of scale, then consider replacing the radiator. If it looks clean, then flush it out, inspect the hoses and replace if needed, refill and be done with it.
Flush it. The recommended service interval for the coolant is first at 50,000 miles, then every 30,000 miles thereafter. It’s way overdue, and is likely quite acidic and eroding some components.
It’s theoretically possible that flushing might remove some corrosive goo that’s temporarily sealing a leak area, but any actual damage is already done. Waiting just increases the odds that there will be a problem, or that an already existing problem will get worse. And it’s better to discover any such problem now than sometime later in the middle of a road trip.
I strongly suggest an actual flush, not just a drain and fill or simple fluid exchange (which is often called a flush, but really isn’t). Normally clean water works fine for flushing. In this case, it might be necessary to also use a cleaning solution. I would approach it by flushing first with water. If there are signs of rust or other contamination, I would then add the cleaner, fill with water, and run for ten minutes after the engine is fully warmed up. Let cool, then drain and flush again with water before refilling with a fresh water/antifreeze mixture.
Doing this properly involves disconnecting at least one heater hose and if at all possible removing any engine block drain plug(s). Flush through the top of the radiator, the disconnected hose, the pipe the hose went to, anywhere you can get access. Flush through each access point until all exiting water is free of antifreeze. Ideally the exit water will be clean, but may not be if there’s rust in it.
To clarify my above post a bit, when flushing open or remove the radiator drain plug, and if you can’t get to a block drain plug then disconnect the lowest radiator hose joint.
Thanks much for the info. Looks like I have some work to do.