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Flushing the Cooling System

I bought a replacement radiator for my Oldsmobile. I want to flush the system so I don’t ruin the new radiator. So, is it better to flush it with the old radiator (which is leaking like a sieve) or to use the new one? I had put two bottles of stop leak into the old radiator which is kicking back into the coolant reservoir, so I really want to make sure that everything is clean, but I am afraid the system won’t flush properly if it’s all just leaking out the old radiator. I also don’t want to ruin the new one by flushing nasty gunk through it from the old radiator. Opinions?

I thought about it. Is there a way to do this without the radiator? Then I wouldn’t have to worry about which radiator to flush with.

I would pull the old radiator and the thermostat and flush like mad through the upper & lower radiator hoses - both directions. Just run a garden hose @ full blast thru there. Then install a new thermostat & the new radiator.

Pull both the heater hoses off the engine (make sure the heater control valve is open) and flush the heater core too. Since you used stop leak it may clog some of the runners there. Flush it both ways also.

I’d flush it first w/ the old radiator since all the additives are in both the radiator and engine and I think putting in the new radiator and then flushing would possibly gunk up the new one’s cooling fins.

The best way I’ve found to do this is to buy a Prestone cooling system flush kit, part # PRE-AFKIT and a can of fast cooling system flush, either Prestone Super Flush AS-107 or Bars Products Cooling System Flush BAR-1211. The Prestone kit is about 6 bucks from most auto parts stores including The kit will make it simple to BACKFLUSH the cooling system which really dislodges rust, funk and gunk instead of foreward flushing. It’s a breeze to use and gives you permanent access for future flushes without fuss. Comes w/ all the hoohas you need including hose clamps, spout, a Tee fitting AND a spout for your coolant bottle for refilling.

Just follow the directions on the packet for installing it. Takes about two minutes and a sharp knife to cut the LOWER heater hose down by the entry to the passenger compartment, add a tee fitting w/ supplied small hose clamps and a cap for the tee. Add the fast flush to the radiator w/ the engine running, top it off and take it for a short drive, 15-20 minutes. THEN after draining the system from the radiator petcock, put your garden hose right on the heater hose tee, add the spout to the radiator neck where the cap was, run the engine and flush until you get a clear return. Drain, close up the system, add coolant, purge the air in it and you’re good to go. Fire it up and check for leaks.

Don’t forget to close the radiator petcock before refilling. Oh, BTW, if you haven’t done so yet, I also suggest buying new radiator hoses and replacing all those at the same time you do the radiator along with a new thermostat and gasket unless yours are pretty new.

One last thing I want to mention is safety for pets. Anti-freeze has ethyl glycol that smells sweet but it’s very dangerous. If they drink even a small quantity, it it can kill them so keep dogs and other critters away from it until you’re all cleaned up, washed down and ready to go for a test spin w/ dog as your copilot. ;>)
Happy motoring ;>)

First, don’t use a chemical flush, I have never had anything but trouble after using them. Hoses give out and seals start leaking after using those chemicals. Once, after using a name brand “Super Flush” (new on the market at that time), all the hoses failed within two months, two were brand new, and then the water pump seal went out, that was the last time I used a chemical flush. That was about 30 years ago and I haven’t had any cooling system problems since.

Since you have had to replace your radiator, I would recommend that you go all the way and replace all the hoses, heater and radiator, as well as the thermostat and radiator cap. While everything is off, open the block drain and flush out the engine with a garden hose. You can also flush out the heater core while the hoses are off.

When all the water has drained, put the block drain back in, put on all the new stuff and fill the system with 2 parts long life antifreeze to 1 part distilled water.

You don’t want to put in a new radiator and fresh antifreeze only to lose all that expensive antifreeze a month or two later due to a burst hose or have an over heating incident due to a thermostat or radiator cap.

I did not mention the water pump as they are usually pretty reliable as long as you don’t use a chemical flush on them. But after investing in all this, you might want to consider a new pump as well, that depends on your comfort level with the old one. I mean, you do have everything else off so it won’t add that much to the labor.

All very good information. Thank you everybody, I have a pretty good base on what I should do.