Got the dreaded P0597 - Thermostat Stuck Open error code a few weeks ago. Unlike other cars, modern BMWs have the sense to design the t-stat to fail open. This prevents your precious engine from going KABOOM in a short time span. A call to my local BMW dealership they quoted me $223 for the t-stat alone plus another obscene labour rate of $300. So…. DIY it is. Picked up a MAHLE unit from Amazon for $90.
The interweb has a few of BMW thermostat change instructions and videos. However only one youtube video pertians N20 on a 528i. And it was a bit grainy. Also I was a bit stingy wanting to minimize draining the fresh (less than a year old) coolant. That required some out of the box planning. That said, if your coolant is old enough, you should just drain it.
Be forewarned, this is a messy job. There's no way around it.
Tools needed:
- 8mm socket wrench
- T30 Torx with extension socket
- Pliers
- Flat head
- Nitrile gloves
- Big drain pan
- Funnel with small hose attached into a bottle (I used to this to capture the inevitable coolant hose splash)
- With 8mm socket wrench unscrew the bolts for the underbody tray. Cant' remember how many.
- I used turkey baster to suck out coolant from the expansion tank
- Unclip the PCV hose. Gawd that thing smells. I stuck some newspaper to protect and keep smell away
- Flat head away to remove the air intake plumbing. Cover the inlets/outlets to prevent coolant spilling in
- Flat head away the clamps that held 2 of the 3 coolant hoses. The other one requires a clamping plier. OPTIONAL: I replaced that clamp with the worm-screw style clamp.
- As I removed each hose, I put the funnel underneath it to catch coolant spill. There are lots of space for a good size bottle. Use your creativity!
- T30 to remove the twin torx screws holding the T-Stat. I find this is the hardest because I can't see jack squat despite removing the hoses and cables away. Some coolant spilled. Arrghh!
- Replace malfunction t-stat with MAHLE unit and reverse action on everything.
- In the end I had to replenish about ~3L worth coolant (BMW finest concentrates $30 and Walmart's finest distilled water @$0.99)
- Perform the e-water pump bleeding step (lots of guides abound). Make sure you have a battery charger hooked up as it takes 10 minutes for it to complete the process.
- After that I just hosed down the engine bay.
- Reset the CEL code. Go for a drive.
That's about it.