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Car Forum / Mercedes-Benz Cars / October 2005

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91 300E Heating Problems

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Brian Yates - 25 Oct 2005 03:22 GMT
In the cold weather, the heater is sporadic. Once the car is warm,
especially when it is raining, the only way to get it heat up the inside, is
to floor the gas pedal. I'll get some mist on the windshield, and then the
hot air comes out and it gets nice and toasty.

That will last for a little while, it gets cold and then I have to do all
over again.

Any ideas?

Shivering in Vancouver,
Brian
T.G. Lambach - 25 Oct 2005 04:09 GMT
What's the engine's usual operating temperature?? It should be about 80
degrees C..

Have you checked the coolant level?

Have you felt the heater hoses to check if both are warm after driving a
few miles?

It could be that the heater is, in fact working, but that a vacuum leak
is causing the warm air to be constrained to the heater by a (vacuum
powered) duct door.

Then there's the heater control valve a/k/a the monovalve, that may not
be working correctly.

You need to look at some of these to find the problem.
Brian Yates - 25 Oct 2005 05:55 GMT
Hi,
Engine operating temperature and coolant levels are fine.
I'll check the hoses and the other items.
Thanks,

B.
> What's the engine's usual operating temperature?? It should be about 80
> degrees C..
[quoted text clipped - 12 lines]
>
> You need to look at some of these to find the problem.
Andy Smith - 25 Oct 2005 16:34 GMT
> It could be that the heater is, in fact working, but that a vacuum leak
> is causing the warm air to be constrained to the heater by a (vacuum
> powered) duct door.

Sorry to 'butt-in' on this thread, but I also have a problem with the
(screen) heating on my 300E - it heats up ok, but I can't get anything out
to the screen - consequently, it mists up. It sounds like there could be a
vacuum leak, but I don't know where to look!
(It blasts from the side window blowers and I get cold air from the central
blower, so I know the heating is working fine - controller seems to work
fine as well)
do you know whereabouts of, or how to check for non-functioning duct doors?
I have looked at all sorts of diagrams, but I can't find anything that gives
me a view of the heater ducting!
many thanks for you help
andy
T.G. Lambach - 25 Oct 2005 18:19 GMT
I believe you have diagnosed the problem generically but I can't help
you pinpoint its cause. I'd start by having a serious read of the
diagrams and trace the vacuum lines first on the paper to understand
function, then in the car.

Hope that no one has ever "repaired" the system for their "repair" may
be the real problem!
Guenter Scholz - 25 Oct 2005 19:41 GMT
First, let me ask, do you get 'screen' heat when you push the defrost button
?  this directs all the heat toward the windshield.

cheers, guenter

>> It could be that the heater is, in fact working, but that a vacuum leak
>> is causing the warm air to be constrained to the heater by a (vacuum
[quoted text clipped - 12 lines]
>many thanks for you help
>andy
Andy Smith - 25 Oct 2005 20:48 GMT
> First, let me ask, do you get 'screen' heat when you push the defrost
> button
> ?  this directs all the heat toward the windshield.
>
> cheers, guenter

no - (there is no defrost button) - even if I switch all the heat to the
windscreen (and the fan on maximum), I get nothing - no heat, no air at all
Cheers
Andy
Guenter Scholz - 25 Oct 2005 21:05 GMT
There's got to be a defrost (or whatever you want to call it, defogger button
maybe?) button, I've got one on my '89 300E and all car's I've ever driven
have one of these.  In any case, assuming that's the button you pushed when
you directed the heat at the windshield, then there appears to be a real
malfunction of the system.  Probably the vacuum hoses.

good luck

>> First, let me ask, do you get 'screen' heat when you push the defrost
>> button
[quoted text clipped - 6 lines]
>Cheers
>Andy
T.G. Lambach - 25 Oct 2005 18:22 GMT
Another possibility if the cooling system was drained since last winter
- an air lock. Some cars and your model may be one - I don't know - get
an air bubble in their cooling system when its drained and refilled.
Experienced mechanics know which ones do and act to purge that air. Just
a thought.
Martin Joseph - 26 Oct 2005 06:08 GMT
> In the cold weather, the heater is sporadic. Once the car is warm,
> especially when it is raining, the only way to get it heat up the
[quoted text clipped - 8 lines]
> Shivering in Vancouver,
> Brian

I do not believe that any Canadian with self respect could complain
about the "cold" in Vancouver, in october none the less?

What the hell?  DId it hit 40 over night?

What are you a transplanted californian?

Sheesh,
Marty

PS Sorry no help on the technical problem
 
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