temp drops from 12oclock when going downhill.. Thermostat?? - 98 318ti

[e36elf]

Hi folks,
Just noticed today a few minutes into the downhill run of a 500m (1500ft) hill in 3rd gear doing about 1,500 - 2,000 rpm with foot off the gas, outside temp about 13 c (55F), the needle started dropping below the middle mark. It didn't go down to the blue, but went down until just a fraction above the white mark at 1/3. Once back on the flat, it returned to the half way mark after about 3 minutes driving.

Does this sound like the thermostat is on its way out?

Car has done 140,000km (87,000 miles) and usually takes about 8-10mins of driving (on the flat) until the needle reaches the 12oclock position.

I've done the thermostat on my previous e36 (325i) so I'm sure I can do the job. Would replacing it be indicated here? And if so, would you also recommend the water pump at the same time?

Many thanks in advance for any replies!

[robcarync]

have you noticed any other things such as a decrease in gas milegage? I am convinced my thermostat is stuck open because it takes a long time to heat up to 12 oclock, the heating in the cabin is intermittent, and my mileage has dropped ( i have had this problem before). I have a thermostat and a water pump waiting for me to put it in....One may say "if it aint broke, dont fix it" but if you havent changed the water pump before, and you have the money to buy a new one...you may as well grab one while you are emptying out the coolant and are working in the area.

if the only thing you are worried about is that one downhill run, you may want to wait a bit before you replace it and spend money...but it could be a thermostat causing the "over cooling" issue

[cooljess76]

definitely a stuck thermostat. Replace it and be sure to PROPERLY bleed the system.

[chudzikb]

I have the same situation, it is the reason my car is in 1.2 billion pieces now. I am doing the whole deal. The water pipes, new radiator, every hose, sensors, vacuum lines, you name it I am replacing it. But, to do just thermostat and the water pump? Not so bad. Getting that water pump out was a PITA. I tried the thread method, it just cracked. Then the claw hammer on a piece of wood, and large channel locks. Messed around for a couple of minutes before it finally gave way. I just hope it all goes together correctly in the end...