Curious Heater Problem

Hello,

I have a 1993 900 CS ( Non Turbo) with a curious problem. Car starts and heats up well. When engine is at normal operating temperature there is plenty , I mean plenty of heat , but as you sit in traffic - not moving- and the engine temp rises up to the point that the radiator fan comes on the heater goes cold. But once you start moving and the engine temp starts to drop back down to normal the heat comes back. The temp knob seems to have no impact during the no heat period , is effect again once the operating temperature drops back to normal. The radiator fan comes at the right time and works well.

Any ideas?

Allan Schneider

Reply to
Allan Schneider
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Obviously something is happening to restrict the flow of hot water to the heater core. The thermostat must be problem. Some Saab thermostats have a "bypass". Maybe you have an incorrect themostat installed or one that does not have the bypass. Mystery it is!

Reply to
darthpup

Check your coolant levels?

Reply to
Nasty Bob

It is working exactly as it is supposed to. The idea is that if you're running so hot that the radiator fan needs to come on, you really need maximum cooling. To achieve maximum cooling, you circulate all your coolant through the radiator, bypassing the (far less effective) heater matrix.

Reply to
Grunff

Maximum cooling would be obtained by circulating the coolant through both the main radiator and the heater core. More surface area more cooling.

Reply to
darthpup

Nonsense. Time spent in the heater core is wasted, because the heater core is less good at dissipating power.

Reply to
Grunff

Ohm's law works for thermal calculations too...

Reply to
Dave Hinz

It does sometimes, but not always - in a system undergoing periodic fluctuations, it doesn't. It works fine in steady state systems.

The engine will cool down faster if you ignore the heater core and circulate all the coolant as quickly as possible through the radiator.

Reply to
Grunff

We can therefore assume from your logic that the cooling system will loose more heat by restricting the flow of fluid and decreasing the cooling surface area? Interesting.

I think something is installed backwards in the cooling system, probably closing off rather than opening up a flow way. Probably the thermostat is installed backwards.

Reply to
darthpup

Look, that's just the way it's supposed to work, and for the reasons I gave. I don't have the inclination to argue about it, nor can I be bothered to dig up a technical reference that explains it.

Reply to
Grunff

There is an error in darthpup's logic; he is asuming that flow thru the radiator is at maximum. (1) Heater core is designed to transfer heat to passenger compartment, not cool engine that's secondary (2) Radiator is designed to eliminate waste heat from engine.

If flow thru radiator is not limited at this point then increasing coolant flow thru it, rather than heater core, would increase heat loss. Assuming a radiator is more effective heat exchanger for a given flow than a heater core.

Reply to
Steve E.

Yes, this is spot on.

What's more, if anyone needs more convincing, examination of the thermostat housing makes the situation very clear - you can see the port in the cylinder head which gets blocked above the high temp point.

Reply to
Grunff

Saab designed an automobile that fails to properly heat the interior from the heater core when at an idle? Decreasing the flow path increases the flow rate? Try that again somewhere else. Pump rate is constant over time at idle.

Reply to
darthpup

Decreasing the flow path area will *always* increase the flow rate through the decreased path (assuming a constant pump pressure). In some situations this is known as the "venturi effect".

The key to Grunff's assertion is that, when the heater is blocked off the flow through the *radiator* will be increased (not the total flow which would be somewhat less), but since the radiator is a far superior heat exchanger the total heat dispersed will be greater.

Get it now?

Reply to
Fred W

Hello NG,

I have learned a lot s "When the engine reaches full operating temperature , the thermostat closes off the lower outlet in the thermostat housing to substantially reduce the coolant through the heater core"

I am assuming that at the above full operating temperature the cooling fan comes on and the flow is also further diminished.

This is consistent with my experience and I wonder if others have seen this as well. As the cooling fan comes on ( or the car begins to move ) the engine temp goes down and the heat comes back on.

There are times when I am stuck in traffic and it is also very cold and this feature seems unhelpful at best .

Any way to net it all out

1) The SAAB appears to be engineered that way and 2) It is operating in the manner intended.

Any ideas ? or just live with it ?

(Another thing I have observed is that once the engine is warmed ( not the highest operating temp- the temp needle just moved up a bit) up a LOWERING of the temperature adjustment back to 2/3 increases the amount heat coming out of the vents. Give the discussion in the post following my original post it would suggest a lower coolant flow rate and HIGHER residence time in the heater core . Thus more time for heat transfer to take take place. I may be all wrong about this.)

Reply to
Allan Schneider

Yay!

If you really want to disable this functionality, the easiest way to do it is by fitting a non-OEM thermostat. This will open in the normal manner, but not go as far as blocking the heater core port at the higher temperature. Where you get one depends on where you are - in the UK, a Unipart thermostat of the correct size would do the job.

Reply to
Grunff

Hello ,

Can you tell me more about this thermostat? I am assuming that OEM means Original Equipment Manufacturer. Sometimes it is also called "other equipment manufacturer " but you do mean SAAB-- correct?

Why would a non- SAAB thermostat be different than the original ? If they claim computability shouldn't they operate the same way as well? How does one know ?

And would defeating this feature have a possible negative consequence? As it appears to be quite intentionally designed that way , it could be of some necessity or importance? Going around it may have be risky with unintended consequences - like over heating and serious engine damage.

Thanks again

Reply to
Allan Schneider

No way... For a constant pump pressure/flow rate Q (volume/time), decreasing the path area (A) will increase the flow _speed_ (v) as Q = v * A

Venturi effect is entirely different; at increased _speed_ the _pressure_ will decrease.

Reply to
MH

Hi!

Original Equipment Manufacturer - so either Saab branded, or made by the manufacturer who originally made the part for Saab. As opposed to pattern parts.

A non OEM one is the same shape and size, and works in much the same way

- but it fits lots of other cars too. Most cars don't have a "shut off the heater core at high temp" feature, so those thermostats don't do that.

That's a tricky one, and depends a lot on where you are and what pattern parts are available to you.

I'd say if your cooling system is in good condition, and you don't tend to run hot much of the time, then it will be fine.

It is important, in that it puts the engine's safety ahead of the occupants comfort. This modification would reverse that priority.

FWIW, I've bought 900s with non-OEM thermostats, and some with no thermostats at all, which were running fine without any overheating problems.

(I'm not recommending having no thermostat at all - that's not good practice)

As I said, depends on the overall condition of your cooling system (radiator efficiency, how clogged up the water ways are etc etc.).

Where do you live, and what are the seasonal highs/lows?

Reply to
Grunff

I have north of New York City in the US. The temperature ranges from 0 F to 100 F . Does this help?

Reply to
Allan Schneider

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