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Old 01-01-2009, 10:04 PM
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BlackBMWs BlackBMWs is offline
Check twice, fix once...
Location: Sacramento, CA
Join Date: Nov 2008
Posts: 732
Mein Auto: 540it 533i 318ic 325i

While I have a 1999 540it, I believe our Water Temp (WT) sensors are the same.

I can reach the sensor from the top of the engine compartment. I'd do this with the system coolant cold. I'd remove the belly pan and use a collection pan to collect spilled coolant. Personally, I'd probably drain the system before I pulled the sensor, just cause I would not want coolant in places it might spill/shoot into. Don't want to create a different electrical problem. I suppose if you leave the coolant tank cap on the flow might be lessened.

Someone may have a better procedure, but if you didn't want to drain the cooling system, here's what I'd do. Let us know which way you proceed.


1. Remove the belly pan. Belly pan removal requires clearance and a Phillips screwdriver. 7 to 9 screws around the perimeter of the pan. Once all the screws are removed, shift the belly pan to the rear to clear the bottom of the front bumper cover. Place a suitable pan underneath the lower radiator hose to collect coolant loss. (Whether you are totally draining the system or just gonna try to be quick.)

2. Prepare the o-ring on the new sensor. I lightly coated mine with dialetric grease. I believe someone else used Vaseline.

3. Remove the airbox for access clearance to the sensor.

4. Remove the sensor wiring connector on the old sensor by pulling back the sensor retaining clip, then pulling the wiring connector upward.

5. Prepare for coolant loss. Protective eyewear would be good here. Squeeze the WT sensor ears and pull upward. Be mindful you may have a small geyser of coolant.

Coolant will flow out of the hose as this is the lowest coolant point in the system. I suppose if you're quick and ready you could do it with a minimum of coolant loss.

6. Put the new sensor in quick and careful.

7. Reverse connect everything back together. (Except Belly Pan)

8. Refill loss coolant with 50/50 mix of new coolant and distilled water. Turn on ignition, but not engine, set heater to highest temperature with fan on lowest position to turn on aux coolant pump to get flow in system while topping off.

9. Wipe down what you can and check for leaks. If there are none, reinstall Belly pan.

10. Cruise around a bit, then when cooled off, recheck coolant level.

11. Enjoy suitable beverage...

When was the last time you changed your coolant? If more than a few seasons, might be a good time to flush it out and change it anyway. There are various opinions on coolant choices, I just went with the OEM coolant when I did my overhaul recently. 1.5 gal coolant with 1.5 gal distilled water.

I'm certain someone will chime in if I've missed something... Good luck!

1999 540it - Jet Black/Sand Beige, style 5 rims, Zionsville Cooling kit
1984 533i - Schwarz/Schwarz, 17" style 65 reps
1997 318ti Sport - Schwartz II/Schwartz Anthratz, style 68 rims
1995 318ti Sport - Schwartz II/Schwartz Anthratz, style 27 rims
1994 325i Sedan - Bostongrau/Tan
1991 318ic - Schwarz/Anthratz Stoff
1985 635CSI - Schwartz\Sand - Sold
1984 318i - Champagne/Tan, Stock - Sold

BMWCCA# 160411

Last edited by BlackBMWs; 01-02-2009 at 12:34 AM.
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