Calling All Shop Gurus

[jimbemotumbo]

Weird problems with HVAC. The dealer says all is normal. Even an indy BMW specialist can't find the prob. I've done the searches - Nada. So here's the deal:

95 525 5sp. Car has 82k and is in pristine mechanical condition (except for this thread issue). Careful and thorough maintenance.

I get hot air coming from the vents, even with the A/C. First, I checked all fuses and connections. Perfect. Replaced the control valve on the firewall. Took apart the old one and it was OEM and still in perfect condition. No diaphragm prob & clean as a whistle inside.
Hmmmm.
HVAC head unit seems to function fine. Dealer vists netted nothing. Not only could they not reproduce the problem, all diagnsotics were normal.
Hmmmm.
Independent specialist checked it out. Same as above. A/C worked fine for them. They also checked voltages and found all in order. On the way home it got hot again. They offered to take it all apart and check flaps, but said it would be 12 hours of labor just to get it apart.
Hmmmm.

Only other thing I can add is that the A/C blows cold at first. real cold. Eventually, the heat comes on. If I hit the A/C button (off) and shut off the fan, wait a few seconds and then turn on the fan and hit the A/C button (on), it blows cold again. Sometimes it will be fine for 30 minutes or more, other times it gets jiggy right away.

Engine seems quite hot, though temp gauge shows middle reading (it's been upper 90's too). Also, and probably unrelated, I am starting to hear a little pulley rattle up front. Recent water pump/tstat replacement & all new coolant. This just for full disclosure.

Any help would be greatly appreciated, since I have exhausted my "professional" resources in the area.

Thanks all!

[DanDombrowski]

First thing I would do is find out of the compressor stays on when the air gets warm. You want to find out of the problem is inside the cabin or in the engine bay before you do anything.

[632 Regal]

if its low on charge and the microfilter is clogged the evaporator could be freezing over.

[jimbemotumbo]

Cabin Filter is fresh. AC Cycles normally and stays on when the heat comes .. just can't keep up with the heat pouring through the vents. Forgot to mention that both pass and driver side are exactly the same, so I doubt it's the sensors under the dash or a bad control dial.

[winfred]

one cheep experament you can try is clamp off the feed line for the heater and see if you still have the same issue

[Paul in NZ]

Replaced the control valve on the firewall. Took apart the old one and it was OEM and still in perfect condition. No diaphragm prob & clean as a whistle inside
does this mean the two heater valves on the firewall?The default position is OPEN 12 v closes them.

[jimbemotumbo]

Yes, Paul. I initially thought the unit failed since its so common, and swapped in a new one. 12v is there at the connection.

I've scoured Bentleys and searched the forums and the net, but have not seen this twist on the old hot air out the vents problem.

Thanks for the idea re: clamping the feed. I guess this may be one of those puzzles that you circle in on, eliminating everything else on by one. At least the dealer made me feel smarter. ;)

[632 Regal]

pinching the intake hose sounds good to me too, just cause the regulators new dont mean its working.

still think it is an evaporator issue...

[Paul in NZ]

does it run hot without the A/C?

[jimbemotumbo]

does it run hot without the A/C?
Yes. When it decides to heat up it does it whether the AC is on or off. In fact, I'm not at all sure that the AC stays on when it gets hot. I'll need to look under the hood next time it does this and check the compressor.

Evaporator issue? How so?

[Paul in NZ]

if its low on charge and the microfilter is clogged the evaporator could be freezing over.
what jeff said previously

[winfred]

possible furball on the face of the evap blocking airflow due to a clogged or long missing micro filter


Evaporator issue? How so?

[saconnect]

I know this is a silly stupid suggestion, but did you rotate the little knob in the center dash vent? You know, the one with blue and red dots on it? I'm sure you have, but I thought I might mention it just in case....

[jimbemotumbo]

Thanks all. I think were on to the solution. Given that it's highly unlikely both sensors in the cabin went south at exactly the same time, same for the dash control rheostats, and that everything else is in good shape and apparently functioning, I'm going to investigate both the clogged evaporator idea, or perhaps that the AC compressor charge is causing the evap to freeze up.

Seems the most likely. I'll report after I check it out.

Again ... THANKS!!!!!