Thread: Replace radiator, flush cooling system?

Originally Posted by Bohica

I'm thinking yes, before the radiator is removed. Or should this be done after the new radiator goes in?

I reckon you answered your own question before you asked it. Why flush old crap into a new rad?

I don't believe the flush gets everything but still useful. Last time I added a serviceable coolant filter on the top hose and for months I got sediment. Well worth it.

Cheers

Like John said .. old one in and flush.
Also flush after old one is out too.

But!(if you have the time).
I can highly recommend the use of CEM's coolant flush stuff.
A bit expensive, but so far from what I've seen, it does do something.

So prior to new rad and flush, add some(I think 100ml does 10lt or whatever .. so not much is needed) and it 'softens' whatever gunk may have built up inside all the coolant galleries.

After I got my 300 Tdi and before last summer I'd flushed it just with straight water for a good hour or so, engine and rad, old coolant out .. etc. and clean water finally flowed and I thought I was 'flushed'.
But then just a few weeks ago, I thought .. better to be safe than sorry for the upcoming summer, I'd give it a proper flush with some stuff(whatever flush stuff I could get).
So I took the punt and ordered the CEM stuff(I also got the oil flush stuff too). Came in a couple of days of ordering too.

Late last year I had to change water pump, so I ran the hose through the block while I had access, and straight clean water come out right away.

Then, armed with the CEM flush just a few weeks ago, I just added to cooling system for a day, as is. ie. no need to clean it out first or anything.
After running it for a day, I then flushed the system again. Block was clean, other than some of the old green coolant from a year ago
Rad came out filthy tho. Mainly when back flushing it, and allowing whatever to drain back out. From the top it pushed that brown gunk out the bottom.
But from the bottom, it seemed to flush out more brown gunk. Obviously some brown gunk was getting baked into the bottom of the rad(from the top the water was clean looking, and I could easily see the bypass hole in the TDI's radiator design.

Then filled with straight water again and have been running it like that now for about 4 weeks or so now. Filled with straight water and added the recommended amount for about 10 lt or so that the Tdi holds.
What's happened now after so many weeks, is that a brown build up of steamed up goop(maybe rust or whatever) is slowly adhering to the inside walls of the coolant reservoir above the water line.
So I'm 'guessing' that the flush stuff is still doing a job of loosening whatever build up is inside the rad.

Have done a test run (and a bit .. actually a lot!), and found it's definitely helped with my heating up problem when running the A/C(which is why I've tried this stuff out).
Can't say how much lower coolant temp has dropped as I used to chicken out when the temp gauge kept rising above 100°C, A/C off and highest recorded temp was 105°C before it finally started to drop.
So far, pushing it hard(labouring engine, more so than hard revving) up the Pentland Hills with A/C on, I've had to back off due to 710°C EGT reading, but coolant temp maxed out at 97°C for any length of time more than about 10sec or so .. it'd drop down to 95, then back up to 97 and so on.

I'd love to get the brown goop buildup in the coolant reservoir analysed to see exactly what it is .. but it's not important, I'm sure it's whatever was blocking the lower rad.
Whatever it is, I just have to clean it out, do one last flush for a period of time again.
Then install my new rad, and finally fill with coolant.