can a bad thermostat cause no interior cabin heat?

[colin86325]

Hi all,
my 1991 318iS was overheating, so I turned on the cabin heater (on a hot July day) so I could limp home and figure out the issue.  But the problem was, that there was no heat coming from the vents. I found this odd, since the car always had good heat. 
So I'm wondering whether a stuck thermostat can cause this (no cabin heat)?

The water pump is not making any noises, so I'm focusing on the thermostat for now.

[bmwman91]

If the thermostat in the head is stuck closed, then you would get heat. The heater is basically a bypass for the thermostat, so if it was stuck closed all of the hot water would be going through the cabin. If it was stuck open, I would expect luke-warm air.

If the engine was overheating and no heat was coming in the cabin, my first guess would be that the water pump impeller broke. Some water pumps have plastic impellers, and they are known to come apart. If this is the problem, pay a little extra for the OEM pump, or at least a reputable brand OE part. The cheaper ones do not last as long.

If it is not that, then there is a thermostat in the heater lines, and it could also be stuck. However, if the heat went from working to not working (plus overheating engine) all of a sudden, my money is on the water pump.

[colin86325]

Thanks for replying, definitely was helpful.  I had some brain fade when I was posting--since I had done the "mess under the intake" I was no longer sure about the revised coolant path.

I believe the thermostat had failed, so I replaced it.  And my sister was driving the car so I think the car was low on coolant as a result of overheating.  I replaced the thermostat (I drilled a very small hole at the top to facilitate bleeding) and it's been keeping temperature and the heat also works now.

Yes, I agree with the water pump impeller--I always use metal impellers since I've heard bad things about plastic impellers failing, especially with the E36.  And it was a bastard of a job--involving a 4' pry bar--to replace to original water pump.  I've since used anti-seize when installing water pumps on the M42s I've worked on.