Engine Temp Drops When Heater Is Turned On

[wulley1]

I've got a 97 328iS that I bought in may. Since it's mid fall in upstate NY, the heater is starting to get some use for the first time since I bought the car. When I turn the heat on, the engine temps drops down from the middle of the gauge down to the blue section. The heat being blown out also gets cooler and cooler as the engine temp drops.

At first I thought that maybe my thermostat was fried open. But if that were the case, my engine temp would be all over the place. Ever since I bought the car, I've never seen the temp move from the middle of the gauge oncethe engine was warm. It doesn't matter if it's 110 and I'm sitting in traffic, the temp stays constant.


What the hell is wrong?

[Fadec40]

Well, if the thermostat was stuck open, which is usually how they fail. your temp would never get that high because coolant is always flowing. , Hence no heat. You might want to look at the coolant temperature gauge senders (sensors) as well. I would first take a look at the thermostat, then take it from there.

[wulley1]

I thought the middle of the temp gauge was were the engine was supposed to be at. It's always there and never moves....until you crank the heat.

[NeverLeaned]

Originally posted by wulley1@Oct 2 2004, 07:58 AM
I thought the middle of the temp gauge was were the engine was supposed to be at. It's always there and never moves....until you crank the heat.

[snapback]203395[/snapback]

I would expect a modest drop in the temp if you crank the heat.
Cranking the heat is a common technique for helping engine overheating problems until you can get the car serviced.

Not sure where to look first the problem though. I had an old toyota in college. Changed the coollant and the temp was always too low. (had to put a piece of cardboard in front of the radiator to keep the temp high enough to make heat in winter. (ya it was pretty ghetto )

The next year when I changed the coolant again, the problem disappeared and it went back to normal operating temps. Still have no idea what happened there, but I guess it had something to do with my coolant mixture. (only thing I ever changed) This is a needle in a haystack idea, I would suggest you look elsewhere before you change coolant.

[wulley1]

I've thought about draining the coolant.

[Brian325i]

I read that when you turn the heater on, it opens up the thermostat because the engine is having to work a little harder to heat the air.

[wulley1]

driving home from rochester this morning, it was pretty cool outside, maybe in the mid forties. I was traveling at about 85mph wit hthe cruise set. The engine temp just made it out of the blue, and that was with the heat turned off.

While I am glad the engine is running to cool instead of too hot, I still really want to fix the problem. But I'm pretty baffled now.

[billB]

Wulley1

change the thermostat. It is wide open. and thermostats don't heat the air inside the car and engines don't work harder to heat inside the car either. At least not the way it is described above.

This is how it works:
thermostat normal position is closed when engine is cold. you start engine and after a little while thermostat opens up (the timing of when to open up is temp. dependant i.e when you take thermos. out read the fine lettering on it you'll see numbers these numbers are the temp. at which this thermostat must open when reached (spring loaded mechanism type of a deal ) ; allowing coolant to circulate). Now if it does not open (closed path/no circulation))=overheated engine; if it stays wide opens (open path/full blown circulation like your car) = engine has to heat the entire coolant before heat reaches certain tem. and your interior in the form of hot air. Heating the inside is another matter without details: though it is hot coolant dependant the heating core under you dash is responsible. Hot coolant flows through it (in and back out controlled by valves NOT THE THERMOSTAT) blower hits it then hot air gets inside through your vents.
Now think: you did 85 MPH on NY thruway coupled with temp. of 43 degrees while the entire coolant is circulating to radiater (wide open thermostat)......there is no way on earth your engine will be able to heat the coolant WITH A WIDE OPEN THERMOSTAT in fact the faster you go the cooler it will get. Try this, stop and idle for 20 minutes or so you'll see a rise in the temp and thus hotter air inside.

p.s. there is also a temp. controlling switch which inputs some info. in the above process

I hope this helps you out to sort your problem but this is another story
regards
billb

[NeverLeaned]

Maybe its just coincedence.......... but perhaps your thermostat went at on the same day it started getting cold.

[wulley1]

it's certainly worth trying to replace it. It's an inexpensive part, and certainly wouldn't to have a new one on even if it's not wore out yet.

I've never changed one on an e36, anyone seen a DIY?

[97alpineM3]

yes.. go to pelicanpart.com click through the first couple of pages.. and then look at some of their tech articles.. there are many DIY pages.. good luck... trust me.. there is a thermostat page too...

Good Luck

Its not hard..

Les

[wulley1]

Doesn't seem like it would be too hard. It'll give me an excuse to drain the coolant and change that at the same time.

What coolant am I supposed to use?

[93 325i]

Just adding my 2 cents..My old car 1995 318i, had the same exact problem. Turn the heat on- engine temp drops & heat gets cooler and cooler..It was the termostat. Changed it and problem was gone

[bad man working]

I haven’t read all the replies, but I can tell you BMW had a major problem with their thermostats this effected M43, M52 and M54 engine for a long period of time the thermostats don’t just fail open they break and have no effect. Depending on climate road conditions driving style ambient temp etc the engine may run at normal operating temp some times.
I’m an English BMW tech and the rule coolant in this country is – 50ºC normally a 50% antifreeze to water mix but check the bottle

Hope this helps

Bad Man Working

[hyper_cool]

Hi Wulley,
I'm just curious, did replacing of your thermostat fixed your problem? I'm having the same issue & I just want to confirm the fix suggested such as replacing thermostat fixed the issue. Please kindly reply back. Thanks.