Best Thermostat Choice?

[rmorin1249]

I live in western MD where the weather can range from close to 100 degrees in the summer to well below freezing in the winter. I want to replace my thermostat and note that they are available in a range of temperatures, 71, 75 and 80. Which one would be best to ensure cooler operation in the summer with the A/C operating? Right now my car tends to run a bit hot in slow traffic but is fine at highway speeds. I thought I would start with the thermostat as I have a relatively new radiator and water pump. Thanks.

[Bert Poliakoff]

Thermostats are an inexpensive item. Start in the middle with a 75. It's not a big thing to changr it out if it doesn't do what you want

[eric1ono]

Looking at RealOEM http://www.realoem.com/bmw/showparts.do?model=5531&mospid=47202&btnr=11_0765&hg=11&fg=35 It looks like the standard is 80 for the Euro M635. However, keep in mind this car was spec'd for a European climate which is cooler than where you are. I tend to agree with Bert, I'd go 75, see what the results are then go to a 71 if needed.

Hope this helps.

Eric

[rmorin1249]

Makes sense. Now I also see there are 2 O ring sizes. Are they size matched to the stat or the housing?

[TJC1]

Crap, I know I'm going to get in trouble again but here goes!

The temperature designated on a thermostat is the temperature that said themostat is designed to open at. If you have three thermostats with three different opening temperatures they will (given that all three thermostats open to an equal amount and allow the same amount of coolant flow through the system) not have any effect at "maintaining" a set temperature at the system's operating temperature. The cooling system as a whole, designed for a particular engine, transmission, chassis, body etc., will dictate the operating temperature.

If your car overheats with a properly operating 80 degree thermostat it will most likely overheat with a properly operating 71 degree thermostat as long as they are the same design and flowing the same amount of coolant.

Simple question based on the following scenario:

You are driving up a 6% grade for 10 miles and you notice that the temp gauge rises from half to 3/4. You then crest the hill and drive down a 6% grade for 10 miles and the temp gauge drops from 3/4 to 1/4. Do you think the thermostat changed it's position from open to close or vice-versa at any point during this drive?

Also for those in the colder climates (going from my ancient experience living in Wisconsin here!!!), how many times have you heard of folks blocking off air flow to the radiator in winter time to get the heater to work even though they may have the highest temperature available thermostat installed? And how about all those 18 wheelers that have the shuttered radiators in winter time to keep their engines from running too cold and having the resultant problems with their diesel fuel gelling?

[rmorin1249]

TJC1, I agree with your hypothesis. The reason for replacing my thermostat is to see if perhaps it isn't opening all the way and allowing coolant to freely pass in and out of the radiator. My car tends to get a bit hot, if my gauge is accurate, when driving slowly and especially with the A/C on. Given that a tstat is a fairly inexpensive part, I figured I'd give it a try since both my radiator and water pump are both fairly new. Driven at highway speeds even in 100+ degree temps with the A/C on, the needle stays below the midmark but at slow speeds, below 20 mph, the needle moves to about halfway between the midmark and the red mark. Perhaps this is normal?

[bkats]

I personally, would replace it with a Tstat that is the stock rating. If you use a lower rated one, your engine may have trouble getting to operating temperature (i.e. the engine will be cooled too soon after startup). A higher one might lead to overheating quickly after startup for the opposite reason. I believe proper M30 coolant temperature is around 85 degrees C. Which means all of the above-mentioned Tstat ratings would be fully open by operating temperature, and that your current Tstat is either malfunctioning OR rated for a temp higher than the operating temp OR functioning properly and unrelated to your overheating. I don't believe any of those would suggest you should install a lower-rated Tstat.

I personally, would think that sitting in traffic on 100+ days (probably higher off the tarmac) with the A/C on would push the needle to the right. How much/far is highly variable and dependent on lots of things that affect heat transfer like humidity, air circulation, and various engine loads/efficiencies. In the worst conditions, two needle widths would not concern me. I have a similar problem and I am looking at the auxiliary fan and/or the temperature sender as a possible culprit. The fact that your problem is not present at speed suggests that it is not the coolant-engine-radiator system. At low speeds, the fan systems are usually a prime suspect.

[rmorin1249]

I personally, would replace it with a Tstat that is the stock rating. If you use a lower rated one, your engine may have trouble getting to operating temperature (i.e. the engine will be cooled too soon after startup). A higher one might lead to overheating quickly after startup for the opposite reason. I believe proper M30 coolant temperature is around 85 degrees C. Which means all of the above-mentioned Tstat ratings would be fully open by operating temperature, and that your current Tstat is either malfunctioning OR rated for a temp higher than the operating temp OR functioning properly and unrelated to your overheating. I don't believe any of those would suggest you should install a lower-rated Tstat.

I personally, would think that sitting in traffic on 100+ days (probably higher off the tarmac) with the A/C on would push the needle to the right. How much/far is highly variable and dependent on lots of things that affect heat transfer like humidity, air circulation, and various engine loads/efficiencies. In the worst conditions, two needle widths would not concern me. I have a similar problem and I am looking at the auxiliary fan and/or the temperature sender as a possible culprit. The fact that your problem is not present at speed suggests that it is not the coolant-engine-radiator system. At low speeds, the fan systems are usually a prime suspect.

I guess I should try the "newspaper test" of my auxiliary fan. Perhaps I'll try this at the Vintage this weekend in the presence of some experienced E24 owners. My temp gauge has never actually gone into the red, so perhaps what I am seeing is perfectly normal. Even on hot days, the car runs slightly below the midmark at highway speeds even with the A/C on.

[TJC1]

TJC1, I agree with your hypothesis. The reason for replacing my thermostat is to see if perhaps it isn't opening all the way and allowing coolant to freely pass in and out of the radiator. My car tends to get a bit hot, if my gauge is accurate, when driving slowly and especially with the A/C on. Given that a tstat is a fairly inexpensive part, I figured I'd give it a try since both my radiator and water pump are both fairly new. Driven at highway speeds even in 100+ degree temps with the A/C on, the needle stays below the midmark but at slow speeds, below 20 mph, the needle moves to about halfway between the midmark and the red mark. Perhaps this is normal?


I think the way our system is designed the temp moving up a bit at idle or in traffic situations is perfectly normal. The question is once it moves up will it come back down to your car's "normal" position again? If it does, then I think I wouldn't worry about it too much, but like you said t-stats are cheap and a little experimentation can't hurt!

BTW when I went down to Bert's last week the temps were in the high 90's at least when I was driving and traffic was heavy and slow and some lights were ungodly long. My car's temp would creep up from just under 1/3 to just over 1/3 ('80 635 so it's a different gauge setup from yours), but as soon as I got moving again it would come back down to "normal" very quickly and stay there.

[bkats]

The auxiliary fan is electric and is controlled by two temperature switches on the side of the radiator. You cannot test it with the newspaper test. The engine fan uses a viscous clutch, and the newspaper test can be used on that.

The auxiliary fan setup should be tested a variety of ways:

If the fan turns on with the AC, the fan, the resistor, and the fan motor are all okay. If not, one of the three is bad. Probably the resistor.

Next, short the two plugs for the temperature sensors. The lower one is supposed to be the low speed setting. The upper one should be the high speed. (someone with two temp sensors please confirm.) Shorting them should activate the fan at low and high speeds, respectively.

If they both work, the temperature sensors are likely bad. I would replace both.