My TStat Story

[93 525 Paul]

Took the wife's 525 on a biz trip to Santa Barbara. First time out of town in a loooong while. Temp in town was always straight up, but it seemed to warm up slowly. Temp ran way cool on the freeway, but when I got there and was doing the in-town shuffle, temp went back to the normal straight up.
Thought to myself, either the gauge/sending unit is going goofy or the thermostat is hung open. Leaning toward the tstat guess. Ran fine cool that way, on one stretch of highway with no cars in sight rolled her up to an easy 115mph. Smooth as silk. Weird thing was that with the guage dropped all the way into the blue, the heater wouldn't blow warm. It was 50*F out and it was blowing cold air. I didn't think it was possible for the water to get that cool in a running engine, so I thought I might also have a problem with the heater core or controls. What are the odds of that happening at the same time? No way, it's got to really be running that cool. Hard to believe.
Got home and promptly read the Royce manual. Right there it says that a thermostat that is stuck open will lead to low highway temps. Taa-daa! Love that manual.
Was in a hurry, so I didn't order from BMA or Autohaus AZ, but in retrospect I should have. Paid the stealer $15 too much for the tstat and after I get it all apart, I paid a local parts house $6 too much for the TMC aluminum tstat housing (my original plastic one was cracked.) Small change, but it's the principle of the thing.
Pulled the old one out and it was not just hung open, it was broken and was stuck WAY open.


Replaced the thermostat, housing, o-ring and profiled gasket. The surface of the head where the thermostat housing bolts was slightly corroded. Since it was right where the profiled gasket sits, I cleaned it up thoroughly and put a very very thin coat of RTV on it. Let that set up for 20 minutes and put it all together.
Wish me luck! So far (knock on wood) seems to warm up quickly and operate at "straight up" on the gauge on the freeway and in town.
Searched the archives and nothing similar with follow up. One fellow was going to let us know how he ended up and that was back in October.

[TC535i]

Nice. My new-to-me 540i drove in the blue the whole way down from Oregon where I bought it (1200 miles), and everytime I hit the freeway. Around town it would stay in the middle (when it finally warmed up after ~15 minutes...), but once you hit freeway speeds, it went to cold.

The next weekend (only time to work on the car...) I got a new thermostat and aluminum housing, and went to work. After 2 hrs, I now had a car that warmed up within a mile or so in the morning, and stayed in the center at any speed!

[SnakeyesTx]

HA! Mine looked EXACTLY like that last weekend too! Since I thought it was covered by my shop that installed my motor, I took it to them. In the process, they managed to drop a bolt, install one too short, cracked the t-stat housing, broke the fan blades when the fan and clutch fell into their drip pan and crack the new shroud they put on when they installed the motor the month before.

No... I didn't have to pay for any of it, but I have a bunch of shiny new parts under the hood

And I'd REALLY like to have that aluminum t-stat housing part number. I was planning on having my machinist make me one to offer people on the forum here, but its good to know it already exists.

[93 525 Paul]

HA! Mine looked EXACTLY like that last weekend too! Since I thought it was covered by my shop that installed my motor, I took it to them. In the process, they managed to drop a bolt, install one too short, cracked the t-stat housing, broke the fan blades when the fan and clutch fell into their drip pan and crack the new shroud they put on when they installed the motor the month before.

Hmmm. Can you say ham-handed?

Part number...

11531722531 MTC Aluminum replacement/upgrade $21 at Autohausaz.com

http://www.autohausaz.com/search/pro...Water%20Outlet

[93 525 Paul]

Mine was an OEM Wahler unit replaced a couple years ago - definitely not the original. Look at that friggin' spring! Any chance the German engineers over did the spring rate relative to the steel used to construct the body? Jeez, it looks like a Big Block valve spring, well not really, but hey c'mon.

[bigtisas]

Good story. I might have similar problem. My car takes 15 mins to pass the blue mark under 60F outside temperature. I'll check my tstat this weekend.

[TC535i]

What's causing all the broken thermostats?

Probably lack of flushing the cooling system. Fluid changes pH, causes corrosion/deterioration of metals. Just imagine what it's doing to your head/headgasket...

Flush them systems every 2 years, kids!

[93 525 Paul]

Probably lack of flushing the cooling system. Fluid changes pH, causes corrosion/deterioration of metals. Just imagine what it's doing to your head/headgasket...

Flush them systems every 2 years, kids!

No problem with following the factory advised coolant flush, that's always good. However, take a close look at the pictures. There's no corrosion on the stainless that the thermostats are made of. My tstat is not corroded (although the head is and that transferred to the gasket around the tstat) and the 540 pic shows zero corrosion.