After having my old radiator changed with a brand new one (because of a crack), it seems as the engine (coolant) temperature is not as stable as before.

Old radiator 'stats': Almost always stable at 90°C (194°F)
This includes standing still in traffic for a couple of hours. It would maybe change a few degrees. When cruising at 120KM/H (74.56MP/H) it would also stay at almost exactly 90°C.

Now with the new radiator it feels as if when cruising it stays somewhat stable at 85°C. (185°F) and standing still in traffic it bumps to 93°C ish (199°F)
These numbers aren't scaring me, but the fact that the second I can start driving after standing still, it goes down rather fast.

It's never really stable, if that makes sense.

Could this be because some air-ways were blocked on the old one?
Would a new one cool better or faster? (I assume this is the one)

Again, the numbers are 'fine', but the fluctuation is what caught my eye.

Vehicle: Audi A6 (C6 4F), 2005. 2.4L

Image of old radiator: enter image description here

  • Did you replace the thermostat also?
    – Grantly
    Commented Mar 16, 2018 at 11:08
  • @Grantly Nope. Only the radiator has been replaced.
    – Paramone
    Commented Mar 16, 2018 at 11:29
  • 1
    Assuming you flushed the entire system to get all the crud out of it, I would say you are seeing the fluctuation because the radiator is flowing more freely now.
    – CharlieRB
    Commented Mar 16, 2018 at 12:33

2 Answers 2


I agree that it is something to not get too frightened about.

The answer to this lies within the recent cooling system changes and the answer to the following questions.

  1. Is the replacement radiator aftermarket or oem? How about the coolant and the mixture?

  2. Did you follow manufacturer recommended cooling system bleeding procedure.

  3. At any point before the repair, did the vehicle get hot at all?

  4. Has the climate changed much from before, and now you are not using ac or defrost?

It's common for an engine to increase temperature when idling for a length of time since you don't have the airflow from moving and most radiator fans won't turn on until a bit above the middle mark of around 195 F.

  • 1
    My engine never gets to 195 but then I don't use the Fahrenheit scale... Do be careful with units...
    – Solar Mike
    Commented Mar 16, 2018 at 15:05
  • Indeed, not frightened at all. Just wondering why it would fluctuate :). I'm 95% sure it's OEM. Atleast, the retailer claims it is, and it looks pretty much the same as the old one, including engraving positions etc. I've done bleeding myself, but obviously it isn't as perfect as a mechanic/Audi I drove my car down the highway quickly after an accident (hence the repair), while some coolant has drained out. It must've heated up more than usual, but I doubt it overheated. Climate, maybe ~5-10 degrees celcius. I'm pretty sure it's because of the airflow increase that it fluctuates :)
    – Paramone
    Commented Mar 16, 2018 at 15:43

You need to finish bleeding the air from the system. Make sure you constantly check your fluid level in the radiator after every 10 - 20 miles and fill it. Make sure you check it when car is cooled down. Also make sure you use the recommended coolant for your car. Honda's are known to run 100% better on their type 2 fluid, its blue. Chevy trucks and GM vehicles run with Dex-Cool which you do not want to mix with other types of coolant. Good luck.

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