'99 540i - Lose heat at higher speeds

Did a search and could not find anything about losing speed at higher speeds. So here is my problem.

While city driving (mainly to work and back) i have heat blowing from all vents. But when i get on the highway and travel for 30-40 minutes cold air begins to blow...and i have not adjusted any of the heat settings.

Any ideas? The car is at my local indy shop now and they are confused.

Does the cold air blow through the visible vents, or from the floor or lower windshield area? I sometimes get cool air all of a sudden. I look at the adjuster on the center visible vent, and remember I had changed it to cool. It seems very simple, but have you checked it?

yeah, i checked the visible vents and cold air was blowing. Indy shop did not really have an answer either.

Did you move the adjuster on the center vent?

What I meant to ask, and I may have asked it wrong before, but my question is, does cold air blow from the floor and windshield?

What does the heat guage do when you are on the highway? Does it go down? If the thermostat is stuck open, the coolant may be getting too cold from the extra air going through the radiator.

Does it take longer than ususal for the car to warm up in the morning?

I suspect that it is unlikely that your thermostat is stuck open, but you never know.

Originally Posted by Petes540i6

What does the heat guage do when you are on the highway? Does it go down? If the thermostat is stuck open, the coolant may be getting too cold from the extra air going through the radiator.

Does it take longer than ususal for the car to warm up in the morning?

I suspect that it is unlikely that your thermostat is stuck open, but you never know.

This explains exactly what my car does. It goes sometimes when on the highway, and takes a long time to warm up in the morning. However, if I sit at a stop light, or a drive-thru some where, for a couple of minutes, the temperature guage will eventually go straight up. Once it's straight up, it usually stays there, no matter what it seems. However, before getting there, like I said, it will go back down.

What does this mean?

its your THERMO!! Go replace it, do it NOW!!!! Go, Go!!!!
(BTW, i had the same exact problem, dont worry about it)

The center adjuster is on "red" when it is blowing the cold air. I am having the thermostat replaced as we speak. It never took long to warm up in the mornings and the temperature gage always stayed in the 12 o'clock position.

I will update when i get the car back. I am having the waterpump and the tensioner assembly and new tires put on as well.

thanks for your help.

Wareagle, any update on this issue?

My 2000 528i is starting to exhibit similar issues. - My car warms up normally and the temp guage does not flucuate (stays one lick below dead center) so i am doubting it is the thermostat?

Just got the car back today. Will test out the next time i am on the highway (it may be a few days) . I am hoping for the best. Will let you know

thx

wareagle72, was it the thermostat? I am having the exact same problem with my 99 540i.