How low an ambient temperature should an S52 thermostat maintain a minimum temp?

[melville]

I had the M Roadster out in low, sub 20 deg F temps today for several hours. The temp needle spent a lot of time about half way between the 'cold' range and the lower hash mark. Going 80 up a long freeway slope, it would touch the lower hash mark. Once over the top, it would shrink back. I noticed an effect with blower fan speed also--like I was getting most of my cooling from the heater core. I'd turn the fan from '2' to '1' and the temp needle would creep up a tiny bit.

In ambient temps over 50 deg, it's usually toward the higher hash mark in similar situations.

What I noticed is that with the lower temps it also used a lot more fuel. 21.5 mpg where most similar tanks have been 24ish to 25ish mpg. Similar tanks being all the others on this 8000 mile trip that I've been on these past few weeks.

So I spoze I have a couple questions:

1. Where does the temp gauge take its reading? Is this the temp of the coolant returning to the motor?

2. Would the lower ambient and motor temps cause a change in fuel trim to the tune of 10% more consumption? All comparable tanks have had ethanol in them, so that's not a factor unless the last tank was E15 and all the others were E10.

Here's my sad little roadie today:



The weather came in hard the last 50 miles today. It was time to get off the road when I drove past a Wrangler and an XTerra that had spun off in separate incidents. Me in an M Roady with Pilot Super Sports! When I got off the interstate, the brake pedal gave me a massive ABS foot massage. Leaving the stoplight, the TC was in full engagement, even with a second gear start. The light turned red before I got it across the intersection!

[z3forlife]

Sounds like you thermostat is stuck open. This happened to my 99 2.3

[Mr Bingley]

Sounds like a thermostat to me--the needle should be pegged at 12:00 when warmed up. Running with a malfunctioning thermostat will certainly effect fuel economy 10%. It does look somewhat the worse for wear in that pic.

[shogun]

coolant temp sensor is here #14 http://mfans2.com/parts/catalog/Z3-R...er_head_vanos/
Double temperature switch = 1 signal goes to the DME, the other to the cluster for the gauge. It is in the small cooling cycle. Most probably you have a stuck t-stat. Also check the visco fan clutch.

Under such condition it should just move slowly and not at high speed. The fan clutch is a mechanism that will clutch the fan on and off depending on the need for more or less cooling air to flow thru the radiator. It is a thermostatically controlled device that when operating normally will vary the fan speed independently of the engine speed. When cruising down the road at freeway speeds, with outside temperature less then desert conditions, the fan should be merely be idling along, turning just fast enough to add a little air flow when needed, in this way the fan noise and drag on the engine is greatly reduced. When reducing speed, the fan clutch will sense higher temperatures thru the radiator and “clutch up” thereby increasing the fan speed to help maintain constant engine temperature. It may in fact, cycle as the temperature of the air thru the radiator changes depending on airflow. If the fan clutch operation is normal, when first starting the car, the fan clutch should “clutch up” and an increase in noise and airflow should be obvious. After about 60-90 seconds, the fan will un-clutch and the noise and airflow will drop. The fan will continue to turn but at a much reduced speed. As the engine warms and the thermostat opens to regulate the temperature, the air thru the radiator gets hotter and the clutch will sense this, thereby increasing the speed of the fan to maintain a normal operating temperature.
First signs of trouble:
A normal temperature indication at freeway speeds and an increasing temperature as the vehicle slows is one of the first indications of trouble. Many other things may give this indication but if the temperature seems to be stable at speeds but climbs in traffic or while stopped, this is a good indication that the fan clutch isn’t working correctly. As the temperature continues to climb, the auxiliary electric fan should start but may not provide enough air to keep the engine from overheating. Another sign of trouble is if the fan noise is high and never decreases after starting, and is there anytime the engine RPM is higher then idle, this means that the fan clutch is “frozen” and is not releasing. Although this will not result in immediately serious trouble, it will load the engine continually and gas MPG will be reduced. Load on the fan belt(s) will be higher and shorten the life of that component also.

Testing the fan clutch:
If you have reason to suspect that the fan clutch is defective, here is the recommended procedure to verify the condition of the fan clutch.
1. Start the car (cold) with the hood open and note if the fan is turning, increase the engine RPM and note if the fan turns faster and the noise increases, if it does, first good indication, if it does not increase speed/noise, clutch is bad and needs to be replaced. (Remember, this must be tested after the car has been off for and extended period, over night etc.)
2. Leave engine running and note if the fan starts to slow down after 2-5 minutes, speed/noise should diminish and even raising the RPM, the fan should not make as much noise as when first starting, if it does slow, this is the second good indication. If speed/noise does not decrease, clutch may be “frozen” and should be replaced.
3. Leave the engine idle and watch the temperature indicator. When normal operating temperature has been reached, some increase in fan speed/noise should be noted, in particular when the RPM is increased. If temperature is fairly stable and the fan noise/speed increases or cycles, third good indication. If temperature indication continues to increase, with no increase in fan noise/speed, clutch is defective and should be replaced.
4. After the engine is at normal operating temperature or above, is the only time that the “rolled up newspaper” test that many people talk about should be performed. Take some newspaper and roll it up into a long narrow tube. Be carefull, keep hands and fingers away from the fan while performing this test! With the engine at full operating temperature and idling, take the rolled up paper and insert it on the back side of the fan and try to reach the hub of the fan avoiding the blades until close to the hub. Push the rolled paper at the fan increasing the friction to the hub area of the fan. If the fan can not be stopped easily this is the fourth good indication, if it can be stopped the clutch is defective and should be replaced. Again, this test can only be performed when the engine is at or above full operating temperature.Testing can be performed in any order but just make sure the conditions during testing are those that are specified for that specific test.

[melville]

Thanks all. If I ever get moving again, I'll see what's happening. Here's the situation on the ground 15 minutes ago:



Going nowhere today.

[jclausen]

Yuk

[melville]

Here's how things looked when the snow was nearly done:



Once I got moving, it was clear that I was about 150 miles short of getting through the snowstorm that first day. No accumulated snow after that point. If I'd left about two hours earlier.........

Since then I've been traveling west and south into increasing ambient temps. It's quite possible my thermo has been stuck open for a few years and that it never made itself obvious where I live. Ambient temps are rarely much below 40 at home.

Readings are normal now. I do have a project for when I get home.