What's the best way to flush a radiator? I used a bottle of radiator flush and flushed it with tap water a few times, but still is rusting brown water. Also, my tear down manual says there are bolts above the oil pan to open to get all the fluid out, I see an allen wrench fitting, but not sure what that's to. Only am getting 2 to 2.5 gallons out, instead of the 4.8 that it actually holds.

 

http://www.prestone.com/products?detail=AF-KIT

 

What if you opened the radiator drain, turned on the heater so coolant will flow through the heater, and ran the engine while keeping the radiator full with a water hose. Feed and bleed with the engine running will eventually flush the water clear. It might be a good idea afterwards to remove the block drains in order to get all of the tap water out since it usually has minerals in it that can precipitate out and plug up the system especially the heater core. I only use distilled water for mixing with anti-freeze for that reason.

 

I recently went through this with my 96 Bronco. The previous owner before me put straight water in. I can say that you will never get rid of the rust if you have an iron block. Sure it will get better, but you will have to replace many things. I'm extremely picky about oil leaks, so I removed the motor and tore it down to bare block. This gave me the opportunity to clean the block of any rust build up, but the damage had already been done. The best thing I did was replace the radiator, all hoses, and heater core. That kept the coolant for the most part green, but still had a slight brown tint to it. Trust me I tried it all before replacing the whole coolant system. All the Preston radiator flush and other brands, but they did absolutely nothing for me. I would drive it to work and back, about 70 miles, with flush in it, and flush it out with a garden hose. It always came back rusty. And yes I removed the thermostat before flushing every time.

 

So I flushed out the radiator and once I got it to be fairly clean, I placed new fluid with distilled water, and then the radiator started leaking. Luckily I was able to return it under warranty, and put another round of antifreeze and water into it. Now, driving it for a week, the heater core has a leak because the windshield fogged up and smelled of antifreeze. I just put in Bars stop leak, which had the best reviews. Not sure what to do, since it's over $1000 to have the heater core replaced at a reputable shop. The truck has almost 200k miles, and not sure if I want to continue throwing money into it. I have to put in fuel injector cleaner almost every 9 months, since my spark plugs begin to misfire.
I'm at a crossroads, and waiting for a big engine repair to happen, then I'll probably throw a new rebuilt engine into it later. I'll start looking for a newer Ford SUV to drive in the meantime. 🤔

 

It's a big job but there is a good video on youtube showing how to replace the heater core and blend door.