Confused - again!

[adamm]

Can anybody explain this little mystery to me please?

My car's only been on the road after a complete rebuild since June/July, there have been a couple of hiccups up until now but she's mostly been well behaved - certainly so for the past couple of months. That's her background.

Last weekend I'd been out in her for the day when I noticed that the temperature was starting to rise. Fortunately I was only 10 miles from home at this point and the rising temperature was slow (but steady). When I arrived home she was just under the red. When doing a preliminary check I noticed that the lower radiator hose was cold as was the bottom of the radiator.

So the next day I gently took her over to my dad's (where I work on her) and the temperature ran upto '3 o'clock' but not over. We took the thermostat off and tested it where it appeared to work ok so put it back in. Topped up the system and did our best to eliminate any air. Low and behold after running for 10-15 minutes the needle started to creep up past the half way mark again with the bottom hose remaining cold so we assumed it must be the thermostat and we'd not tested it right! Other things to check mentioned in my various manuals included head gasket (replaced with refinished head less than 600 miles ago), radiator cap (holds good pressure), blocked radiator (new last year), water pump (new and double-checked this week to make sure it's still ok - I know new things CAN go wrong sometimes!) and a couple of other things I can't recall right now. Basically everything seems to be working and HAD always been working so why this problem now?

I did some digging using the search function and it seems a lot of people's cars run with a cold bottom hose and I could live with this too if it weren't for the temperature now starting to creep up as it does. However surely BMW didn't intend for this when they designed the system all that while ago?!

If anybody can help it would be most appreciated as I'm now running with a gutless thermostat body to get me about (the car's my daily drive) which isn't ideal as it's now running too cold! Life would be so simple without these little cars we love so much... TIA

[Grover]

Is your fan coming on? Are you sure it's not a bad temp sensor for your dash indicator?

[Mike Self]

If you have an aftermarket thermostatically controlled electric cooling fan, of course test it to make sure it's functioning properly.

That being said...

Those "double" 2002 thermostats have been known to stick. My '69 had a random running hot pattern some years ago. Not particularly air-temperature related, but periodically (couple of times a week) it would act as yours did--start to run hot and when felt, the lower rad hose was cool.

A BMW tech at the time suggested I add some "water pump lube" to the coolant, and it solved the problem. Apparently those 'stats can stick intermittently.

To test yours, remove from the car, tie a string to it and suspend it in a pan of water on the stove so that it doesn't touch the pan's side. Observe its opening temp with a thermometer in the water to see if (1) it opens at the proper temperature and (2) it does so consistently. Cycle it a half dozen times (run it under cool water to make it close, then pop it back in the pan again) to see if it opens consistently.

If it does, you'll have to look elsewhere for the problem. If it doesn't, try another 'stat.

cheers

mike

[Guest Anonymous]

Mike is always right (except for the best colors). So I would follow his advice. But consider a couple of other things he did not mention. Number 1, you temp gauge is a little more reliable than reading tea leaves. If you really want to know what's going on temperature wise, get a heat sensing gun or try a different aftermarket temperature gauge. Even an oven thermometer placed on or in the radiator can go a long way to allay your doubts.

I have personally never experienced the trapped air syndrome mentioned by others. If I were terribly concerned about it, I might drill a 1/8" hole in the thermostat to promote freer flowing with the understanding that the thermostat may be a little less quick to react to temperature changes.

You mentioned that you did some work to improve the cooling recently. I helped a friend with a new radiator in another BMW. He installed an aftermarket custom monster all-aluminum model. The new radiator is so efficient at exchanging heat, he has had to cover one half of it with cardboard/plastic to get the engine to operate at normal temperatures. He has an electric fan which wont even turn on after prolonged idle.

Cheerio

[Chris_B]

Here are a few more things to think about.

You might want to check what is happening temperature-wise when you see the temp gauge reading go up. 02 temp gauges can read incorrectly due to poor grounds and sensor/gauge problems.

This symptom could well be the result of a subtle problem with your thermostat, particularly since your motor runs cold with a gutted thermostat. Checking a thermostat outside the car is not entirely reliable. A small decrease in flow can cause the cooling system to heat soak over time.

It is easy to use an infrared thermometer to check the temp of the top of the radiator and upper hoses. You can buy one for <$100, or perhaps a friend has one...? Also, you can (carefully) release the radiator cap and stick a thermometer in the coolant to verify the coolant temp. BTW, hot at the top and cool at the bottom is ordinarily a sign of a well functioning cooling system.

Most shops have equipment to test your coolant for the presence of hydrocarbons (and many will do so on the spot at no charge)- a sure indicator of exhaust gases in the coolant and a leaking head gasket or cracked head. This test mostly eliminates those two possibilities.

[Chris_B]

Some quick research shows that infrared thermometers are now available for quite a bit less than $50. I found a number for ~$35 + shipping. Well worth the investment IMO as an automotive diagnostic tool.

[adamm]

Thanks chaps, all helpful as ever! Can anybody tell me to save looking it up what sort of temperature I should be reading in the top of the radiator?

I think that outcome of all this is that I pull my finger out and get on with installing some auxiliary gauges - I've been planning to for a long while but never get the time - perhaps that time has come!

Cheers!

[Chris_B]

If you have the normal 80C thermostat, that would be around 175-180 degrees F.- that would be the normal operating.

[02Les]

Thanks chaps, all helpful as ever! Can anybody tell me to save looking it up what sort of temperature I should be reading in the top of the radiator?

I think that outcome of all this is that I pull my finger out and get on with installing some auxiliary gauges - I've been planning to for a long while but never get the time - perhaps that time has come!

Cheers!

The pic shows your in car temp gauge in celsius.

Can't remember who originally posted this pic, but it's come in handy many times. Thanks to who ever you are.

[colorobo2002]

running hot with old t-stat. Replaced with open t-stat and runs cool. I'd try another proper temp t-stat.