Mixing old and new coolant

[masospaghetti]

In general, when you do a coolant change, what happens if you mix old depleted coolant with new fresh coolant? For example, if you drain and refill just the radiator, you might only change out 50% of your coolant.

Another way to ask: If you have a even mix of depleted coolant (0 years left) and fresh coolant (5 years left), is the result a 2.5 year coolant?

 

[Hacra]

As long as you don't mix up different coolant types, you should be fine, you will just have less effective coolant.

If you mix up different types of coolants (propylene glycol \ ethylene glycol) you might end up having gel like substance which naturally will fully or partially block coolant flow, causing eventual engine failure (over heating).

 

[koda2000]

There are no block drains on our engines. If you just drain the radiator you're replacing about 1/2 the coolant. The only way to replace all the coolant is by flushing the engine with water multiple times (preferably distilled water). Then your left with approx 50% of your system being filled with just water. Filling your empty radiator with concentrated antifreeze will get you close to the recommended 50/50 mix, but you'll have to test the mixture and adjust as necessary if you don't get necessary protection level against freezing. I shoot for around -34 degrees F, which is a 50/50 mix. Another alternative is to pay to have have your system professionally flushed at the stealership.

The thing I've seen with old coolant is that it never looses it's ability to not freeze, it just eventually looses it ability to prevent corrosion and acid builds in the fluid over time. Chaining 1/2 your coolant is better than chaining none of it. If 5 year change intervals are recommended and you only replace 1/2 the coolant, I'd recommend just changing it at 2-3 year intervals. I suppose it comes down to how **** you are about vehicle maintenance.

 

[974X4BLACKSPORT]

If you really need to get ALL the old coolant out of the engine, the easiest way is to suck it out with a wet/dry vacuum. I have a CRAFTSMAN 4hp 10gal. I have used for years on my cars,
It goes like this:
Remove the lower hose from the radiator and let it drain, then remove the upper hose from the radiator and gently fold over the hose to block it off, secure with a clamp or such, then use a wet/dry shop vacuum and secure vacuum hose to open end of lower radiator hose, configure vacuum for "wet" duty, start vacuum, it will suck out 99% of the coolant out of the engine, block, heads, and all.
If you are using same type coolants, it really is not necessary to get all the coolant out of the block. Only when you are changing to a different type of coolant is it really necessary to get ALL the old coolant out.

 

[C420sailor]

I strongly encourage flushing. Not because of the age of the A/F, but because of all the sediment and nasty stuff that is floating around in there. You really want to flush it all out.