Heater what's it doing?

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Niner
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Joined: Sun Apr 02, 2017 8:52 am
Location: Manchester

Heater what's it doing?

Post by Niner »

Hi, it's probably been covered already but my heads nipping with trawling through all the posts so I'm just gonna ask. From start up the heater blows very hot when it's set to cold then after driving 10 miles it then blows cold ( correctly) also the temp gauge reads zero then after a while snaps to and registers normal, connected? Is it the thermostat? Thought I'd ask before I start taking things apart unnecessarily
Thanks in advance for the help I know I'm gonna get cos yr that sort of people :D
84 csi
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Brucey
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Re: Heater what's it doing?

Post by Brucey »

you are talking about a post 5/82 car, right? On these the heater valve is electrically operated; do check but IIRC it needs to be powered to be open, and it needs to be open to get any heat in the cabin. So this would point towards there being a fault in the heater valve control circuitry .

The erratic temperature reading on the dash gauge is more likely to be a fault in the instrument cluster than anything else. You can check what the actual temperature reading ought to be by using a multimeter to check the resistance of either the dash sender or the motoric CTS; both are in the thermostat housing. Compare readings between cold and hot.

However having said that, it is also not uncommon for there to be a real fault with the thermostat/cooling system;

a) the coolant flow through the heater (from cold) is arranged unlike other cars (and is also different again on post 6/87 builds)
b) if there are airlocks then anything can happen, more or less; you need the bleed hose from the rad to the header tank to definitely be clear and flowing and both self-bleeding and manual bleeding usually don't work at all well until the thermostat is open
c) a small blow in the head gasket can cause the cooling system to behave weirdly and especially different between hot and cold running conditions
d) thermostats don't last forever; when they fail they tend to go sticky in action
e) there are two different thermostat types (specific to the housing which is marked accordingly); if the wrong one is fitted then all kinds of weird behaviour is possible; the rear part of the thermostat is different between the types and this operates a separate valve that controls the coolant pressure to the heater.

So there are at least four places where you could have faults and you probably have multiple faults rather than just one; the symptoms you describe are not 100% consistent with a single fault. Possible fault locations are

- heater valve controls
- heater valve (failing valves close themselves at engine speeds over ~1500rpm)
- thermostat
- head gasket
- temperature gauge parts

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Niner
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Joined: Sun Apr 02, 2017 8:52 am
Location: Manchester

Re: Heater what's it doing?

Post by Niner »

Wow, thanks Bruce for a very comprehensive reply, it's an 84 model, being new to 635s and working on bmw can you tell me what IIRC IS?
Thanks again
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GRNSHRK
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Re: Heater what's it doing?

Post by GRNSHRK »

Hah, funny :lol:

IIRC = If I Recall Correctly \:D/

:mrgreen:
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Niner
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Re: Heater what's it doing?

Post by Niner »

IIRC ?? DOH! :oops: :oops: still I'm not as hip as I used ta be :roll:
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sansouci
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Re: Heater what's it doing?

Post by sansouci »

Niner,
We can be more helpful if you add the vehicle description and the last 7 of the VIN to your signature. The VIN is usually key to ordering the right parts, but caveat emptor....
Sansouci
84 E24 633Csi Auto, Bronzit/PearlBeige 6997510
93 E32 740il M60 Auto, Alpenweis/Ultramarine
60 528i M30 5-speed Green/Beige (crushed)
71 240Z 4-speed White/Blue (rusty & sold)
65 396 Chevelle 4-speed, Marina Blue/Black (stolen)
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