Totally weird heat problem

[iamcreepingdeath]

So in my 1991 318is, the heat blows hot when the car is idling (and warmed up, obviously), but when I rev the engine and hold the revs, (or start driving) the heat cools down to ambient temperature.  So in other words, while driving, the heat is cold, when idling at a red light, the heat is hot.  

Totally opposite of what you may think, and its throwing me for loops.  I just did the "mess under the intake" delete, and took a look at the blower motor and stuff, and visually, everything looked good.

And on a totally separate note, only the 4th speed on the blower motor works.  for the first three speeds, nothing.  I even replaced the blower motor AND the little brown resistor pack below the blower.

thanks guys!

[RED IS 91]

If I were to guess I would say that your thermostat is partially open.

[DesktopDave]

Check to be sure your heater hoses aren't backwards.  The thermostat is another likely culprit.

Check the fan speed switch for continuity on all speeds.  It carries full power to the fan & can melt a little over the years.  I'd have been sure it was the resistor pack too.  You replaced the motor so it shouldn't be the connector block either.

[BlueBMW]

I had the same problem and it ended up being my heater core lines were backwards and the little bypass valve was installed backwards.

Here's the post I made about it a while ago..

http://www.m42club.com/forums/showthread.php?t=10904

[BlueBMW]

Lol looks like you already posted in that thread.  Any luck with any of that info?  If that's not the problem I'd say probably start with a thermostat (cheap and easy)

[iamcreepingdeath]

I really think I just may have another bad resistor pack, because I did the mess under the intake delete, and while I had everything apart, I took a good look at the heater hoses going into the firewall, and two things were evident:  first, there was no bypass valve.  Second, It appeared that the hoses weren't installed backwards, because of the pre-bends in the hoses appeared to be bent to fit and they could not have easily fit the other way.

One thing I can do is I can try swapping the resistor pack from my other 1991 318is which I know is good, into this (black) 1991 318is and eliminate that.  Also, several people have been saying it was the t-stat, but how can that be so?  it just doesn't seem to make sense.

[BlueBMW]

At idle the coolant flow is less so it warms up but while driving the flow is more and there is more airflow over the radiator cooling the coolant too much.  That would be a stuck open t-stat.  Sometimes that can cause poor heater performance.  Although I'd think you'd notice it in the temp gauge too.

[iamcreepingdeath]

At idle the coolant flow is less so it warms up but while driving the flow is more and there is more airflow over the radiator cooling the coolant too much.  That would be a stuck open t-stat.  Sometimes that can cause poor heater performance.  Although I'd think you'd notice it in the temp gauge too.

yeah my temp gauge doesn't work at all, so that throws another wrench in the gears....

Also, the temp change is gradual from idle to revving, but not THAT gradual.  It actually happens over a time of like 15-20 seconds, from hot heat at an idle to cold heat at a constant 2.5k rpms.  still think it could be the t-stat?  if that is still the consensus, I may as well order one and slap it in...

[BlueBMW]

They're so cheap and easy to do... I'd just put one in.  I'd consider them a maintenance item.  Ground nut loose on the temp gauge or something else?

[dinu.negrean]

I have experienced the same situation. Once it was a low coolant level. Once it was the t-stat blocked open.

You may have both if the coolant drops temperature that fast when you start runing.

[enildeR]

Yeah, along the lines of  what dinu said: have you checked your coolant level? That, and have you bleed the coolant system of air?

I actually have the exact reverse of your problem lol

[BlueBMW]

yeah my temp gauge doesn't work at all, so that throws another wrench in the gears....

Fix that temp gauge!