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I've been trying to remove the coolant drain plug but it's too tight. And I think I'm starting to bend the walls a bit. Are there any alternative ways to flush the system or to remove the bolt without risking further screw destruction?

remove bottom radiator hose will release most, but not all, the coolant.

Then I just flush it out with a garden hose? What about the leftover in the engine? I guess i'll dilute it a few times with distilled water to get it out as much as possible..

read the manual eg Haynes. Many engines have drain plugs and yes sometimes these cannot be removed easily either. Then the next trick is to get it refilled and get all the air out of the cooling system- not as easy as it might appear. Air can occupy a lot of volume and the coolant tank needs to be watched carefully. One thing to do is measure how much fluid you remove - if it is initially refilled with less than taken out then there is air in the system

I never removed the engine block drain plug and followed a similar path to Mr Morlock and yourself.

1. Remove plug from bottom of radiator and drain into a container so you and note the quantity.
2. Flush out with hose and then fill with water (tap or distilled/demineralised) and run engine with the radiator cap off until top hose is warm.
3. Switch off and drain.
4. Repeat 2. and 3. until what drains out is just water.
5. Fill with coolant (and distilled/demineralised water if required) noting how much has been put in and compare with what you got in 1.
6. Drain clean expansion bottle and fill between low and high mark with coolant.
7. Run engine with radiator cap off until top hose gets warm and add coolant as the level drops as it cools - may take an hour or so.
8. Replace cap and take it for a drive near home and get it up to temperature, go home and switch it off and wait until fully cooled down. Check/add coolant to radiator/expansion bottle. Have you put back the same amount that came out?
9. Monitor closely over the next few days.
10. Don't let pets (cats/dogs) anywhere near spilled coolant - they like the smell/taste and it will kill them.

spot on JBT

@JBT, ideally I want to do that too. But the drain plug is on too tight and attempts to unscrew it have just resulted in me warping the walls of the Philips pattern. I'll try again with a bigger screwdriver. Then follow your method.

Maybe when it is hot (or cold) it may be a little looser?
My suzuki has a plug with wings on it, so you just grab it and twist. Easy-peasy.

zephyrus17 wrote:I'll try again with a bigger screwdriver.

That would be best. The only alternative is to undo the lower hose at the radiator or destroy the the plug and get a new or replacement radiator bung.

while your at it you might as well clean out the system with some radiator flush such as nulon or winns. You don't do it that often so you might as well do it right.