No cabin heating at fwy speed

[bzoli]

Hi,
I have a problem with cabin heater.
The engine warms up quickly after start, stays at the right temperature, no overheating problem at all. Cabin heating works if I’m driving slowly in city traffic but the hot air cools down if I’m driving 70-80mhp.
Last time I drove 30-40 min. in a freeway not having heating at all. Close to the office at the stop light while I was standing it started blow the hot air.
If I’m moving fast I have no heating, if I’m driving slow I have hot air.
I might be important that I have replaced the radiator in the beginning of the summer, but I haven’t used the heating on freeway, just in slow traffic where it is fine.
The car is ’95 530i, M60 engine.

Thanks
Zoltan

[brosher]

Sounds like your thermostat is stuck open.

[bzoli]

Sounds like your thermostat is stuck open.
Well, regardless of ambient temperature the engine warms up quickly within 3-4min. Once it is warm it stays right on temperature, as it should be.
The cabin heater blows cold on highway speed only, if I slow down it warms up, than at 70-80mph the air cools down again, but does not effect the engine temp.
Why do you think it is thermostat?

Thanks
zoltan

[Blitzkrieg Bob]

under the heater valves.

It may not be working

[632 Regal]

because thats a tell tale sympton of a bad stat.

Why do you think it is thermostat?

Thanks
zoltan

[stx133]

try the air on recycle setting too, may help in the short term?

[NovceGuru]

I assume at 80mph your coolant is being cooled a bit better than at say, 25? </3

[zhandax]

Although these cars are supposed to be self bleeding, if you park downhill you may have gotten an air bubble in the heater core. More likely is that your heater valve has/had some crap in it. My heat went out coming back from Atlanta last Feb just as I crossed Monteagle in a foot of snow. I will find out which when I rebuild the cooling system next week (before it gets cold again).

[bzoli]

I assume at 80mph your coolant is being cooled a bit better than at say, 25? </3
Today in the morning the ambient temp. was 43degF (6degC - yeah its getting cold), engine coolant temp stared rising less than a minute, and fully warmed up to operational temperature within 4 min.

If I go the highway there is not coolant temperate drop (as far as I can see from the temp gauge) just because I drive fast and more coolant air is available.

How much coolant temp. drop do you expect at 80mph if the thermostat would stuck open?


thanks for the response!

[brosher]

Well you can't entirley trust the gauge as it's not very accurate. There are no numbers or scale. I think it's just there to show impending doom.

What happens with a stat that is stuck open is coolant keeps flowing over the radiator no matter what temp it is at. Around town there isn't enough air flowing over the radiator to make a differance in temp and the stat would prob be open anyways. When you get on the highway the stat should close to keep the engine and coolant at operating temp. If it stays open then the coolant will continue to be cooled below the desired temperature, ussually 85C.

Your heater works by blowing air over the heater core which is basically a small radiator. If the coolant temp lowers enough from the above scenario than the heater core will not get hot and you have no hot air. When you get off the highway there is no longer air flowing at a high rate over your radiator so the coolant and heater core warms up again.

On most cars you could look into the open radiator cap. If you saw coolant flowing right away than your stat is stuck open. I don't know if there is any way to verify on the M60 in such a way. It's $30 for the stat and a new O ring, I'd start there.

[tdgard]

Ahhh. The classic BMW heater valve failure. This is extremely (read yearly) common on the E28--haven't seen it on an E34 but it's identical in theory. You need new guts for your heater valves up on the firewall. If you reach over, is there hot air coming out of the passenger floorboard? Haven't opened up the ones on the E34, so I'm not sure if one controls the other.

You've got a torn diaphragm in the valve. The reason you have heat while moving slowly or stopped is that there is not enough pressure to open the tear and coolant can go into the heater block. At highway speeds, the coolant pressure is high enough to open the tear and go right back to the engine.

I know they make a kit containing the guts for an E28. Not sure about the E34.

[632 Regal]

sounds good to me!

Ahhh. The classic BMW heater valve failure. This is extremely (read yearly) common on the E28--haven't seen it on an E34 but it's identical in theory. You need new guts for your heater valves up on the firewall. If you reach over, is there hot air coming out of the passenger floorboard? Haven't opened up the ones on the E34, so I'm not sure if one controls the other.

You've got a torn diaphragm in the valve. The reason you have heat while moving slowly or stopped is that there is not enough pressure to open the tear and coolant can go into the heater block. At highway speeds, the coolant pressure is high enough to open the tear and go right back to the engine.

I know they make a kit containing the guts for an E28. Not sure about the E34.

[Blitzkrieg Bob]

http://www.bmwe34.net/e34main/Maintenance/Electrical/HeaterValve.htm

http://bmwe32.masscom.net/

From what I have experianced with my heater valve system, stuck leaky valves means heat all the time, and for this set up no repair is available for the valve pistons and seals. (I tried it)

I'd still check the AUX pump, since you get some heat at low speeds and none at higher.

[tdgard]

http://www.bmwe34.net/e34main/Maintenance/Electrical/HeaterValve.htm

http://bmwe32.masscom.net/

From what I have experianced with my heater valve system, stuck leaky valves means heat all the time, and for this set up no repair is available for the valve pistons and seals. (I tried it)

I'd still check the AUX pump, since you get some heat at low speeds and none at higher.

The way I understand the Aux pump is that it is active when the car is stopped or moving slowly to increase the coolant pressure to the valves. Since he has heat during those instances, I would wager the pump is doing it's job. Stuck and leaky are two different things. If it was stuck closed, there would never be heat. If it was stuck open, there would be heat all the time. If it's leaky, the coolant will take the path of least resistance--which at highway speeds is back through the failed valve toward the engine via the larger hose.

I would bet your right about the kit. It was a bastard finding them for the E28 & they failed in this manner all the time (e28 heater valve on google gets about 18,000 hits). Maybe the valves can be swaped to check. If not, simply take them apart--if the o-rings and diaphragm are rotten, there's your problem.

[bzoli]

Thanks guys. I got some idea!
Over the weekend I going to check these thinhg out.
If I solved the problem I will post the solution.

Thanks again.