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Radiator flush or not to flush?

I drive an ‘05 Toyota RAV4, which I have owned since 2009 (bought “new” from a dealer). Sadly, the radiator has started leaking after 11 years (my first major issue not counting routine maintenance such as tires/brakes/battery). I’ve gotten four quotes, two of which included a coolant flush and two of which did not; for the shops that didn’t include it, they both basically said you don’t need it because you lose 80% of the coolant with the radiator replacement and the rest is in the bottom of the engine block. I’ve tried googling to see if a flush is really/absolutely necessary, to no avail. I have a few thoughts on the subject as an uneducated layman but would be interested in others’ perspectives and the rationale behind doing it as well as arguments for/against it…

If you’re replacing the radiator, then yes, most of the coolant will be removed.

It depends on the vehicle/cooling system how much that is.

Ask the shops who just want to do a drain and refill how much more it’ll cost to open the drain cock plug from the engine block so it gets drained too.



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I agree with Tester. I would add, when I had mine replaced, as preventative maintenance, changed the thermostat, upper and lower radiator hoses.


If the coolant and the radiator passages look clean I would not flush.
I change the thermostat and radiator cap every 10 years, so yes, I’d have them change those too, with OEM parts only.

I could ask the shops that quoted with the flush included the same question; their quote without the flush is pretty much in line with the other two shops. My concern is not so much that there will be coolant left in the engine block (at least, not directly). The primary question, as stated in my original post, is whether a coolant flush is actually needed, i.e., what purpose does it serve in this particular scenario and could not doing it potentially lead to negative/detrimental effects down the road or is it really just overkill?

If the drain cock on the block is opened, and rust is seen in the coolant, then a coolant flush should be performed to remove the rust.

Otherwise, it’s not required.

Coolant flushes are performed as maintenance service indicated in the owners manual.

It’s easier/faster to hook a machine between the upper radiator hose and radiator to replace the coolant, and replace all the coolant than it is to raise the vehicle to maybe get the drain cock on the radiator open, catch most of the coolant in a pan, close the drain cock, lower the vehicle, and refill the cooling system.

Here’s the machine I use.


We routinely do a cooling system flush when replacing a major component of the cooling system like a radiator or water pump. One reason is that simply draining the radiator leaves all the old coolant in the engine block and many cars do not have accessible block drains (or block drains at all).

Also we need to make sure that the cooling system is filled with the correct fluid. If a system has been leaking the owner may have added some kind of stop leak or used some kind of “universal” coolant that needs to be removed. For example if your car requires Toyota Pink Long-Life Coolant then we need to make sure that there’s no other kind in there.

If however your car was in for a 100,000 mile service that included a coolant service and 6 months later your radiator springs a leak we would simply drain and fill.


Is the radiator leaking at the seal between the tank and the core? If so, then it is just due to aging rubber and not because the metal corroded because the coolant wasn’t;t changed per schedule.

Now if the rubber seal has started leaking, how much longer do you think the rubber hoses will last. If the are the original ones, they are 16 years old now. Might be a good idea to change them and the thermostat. Flush not needed unless the cooling system has been neglected.

Charging for a “flush” is a way of padding the labor bill, the tech receives one hour of labor for this.

My co-workers often charge for a cooling system flush during a cooling system repair however these vehicles are drain and fill, I have not seen anyone touch the cooling system flush machine in the last 10 years.

Using the flush machine is a less effective method of replacing the coolant, the water pump mixes the old coolant with new during the flush process.

BTW you won’t find rust in an aluminum engine.

@asemaster Had not thought to check my records (or with my regular shop) to see if/when I’ve had a major service performed that would have included a coolant flush, so I will definitely check on that. If it’s never had one done or it’s been a while, might be worth considering. Definitely have not been adding anything on my own, so that’s not a concern as it should only have Toyota’s pink coolant in there. If it’s already losing most of the coolant due to the radiator replacement, is what’s left of the old coolant in the engine block (let’s say 80/20 for argument’s sake with 20% being what’s left) going to contain enough contaminants to have a negative effect or cause issues?

There are two types of contaminates that can get into the cooling system. Those generated internally from metal erosion, corrosion and breakdown of the coolant and its additives. The others are external such as dirt, minerals from water (especially hard water), dissolved metals (lead, copper, iron) from not purified or distilled water etc.

A few internal contaminates will not harm the cooling system. They are accounted for in the maintenance schedule. Toyota knows that the original fill of their coolant can last 7 to 11 years and/or 150+k miles before losing its ability to protect the engine. Since subsequent drain and fills are less than 100%, they will not last that long, so they just recommend a coolant change every 100k miles. As long as you stick to that, you will never run out of protection.

What if the engine has cast iron wet sleeves?


The 2AZ-FE engine has thin-wall cast iron cylinder liners that are cast into the aluminum block.


Do all aluminum engines have cast in-place iron liners?

Or are some wet sleeve lined cast iron?


Good point.
Are wet sleeves more often found in aftermarket race engines, instead of street cars?


I’ve installed many sleeves in engines in past years, long ago. But none of them were “wet sleeves”.

I had thought that wet sleeves tended to be used in racing engines, vs dry sleeves elsewhere.

If wet sleeves tend to be used more in racing engines, then the radiator and/or coolant probably last far longer than the engines, which renders the flushing discussion on those mute.