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Car Forum / Saab Cars / January 2006Curious 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 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! > Hello, > [quoted text clipped - 11 lines] > > Allan Schneider Check your coolant levels? > 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 [quoted text clipped - 7 lines] > > Any ideas? 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.  SignatureGrunff Maximum cooling would be obtained by circulating the coolant through both the main radiator and the heater core. More surface area more cooling. > Maximum cooling would be obtained by circulating the coolant through > both the main radiator and the heater core. More surface area more > cooling. Nonsense. Time spent in the heater core is wasted, because the heater core is less good at dissipating power. SignatureGrunff >> Maximum cooling would be obtained by circulating the coolant through >> both the main radiator and the heater core. More surface area more >> cooling. > > Nonsense. Time spent in the heater core is wasted, because the heater > core is less good at dissipating power. Ohm's law works for thermal calculations too... > Ohm's law works for thermal calculations too... 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. SignatureGrunff 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. > We can therefore assume from your logic that the cooling system will > loose more heat by restricting the flow of fluid and decreasing the [quoted text clipped - 3 lines] > probably closing off rather than opening up a flow way. Probably the > thermostat is installed backwards. 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. SignatureGrunff 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. >> We can therefore assume from your logic that the cooling system will >> loose more heat by restricting the flow of fluid and decreasing the [quoted text clipped - 7 lines] > 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. > There is an error in darthpup's logic; he is asuming that flow thru the > radiator is at maximum. [quoted text clipped - 6 lines] > radiator is more effective heat exchanger for a given flow than a heater > core. 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. SignatureGrunff 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. > Decreasing the flow path increases the flow rate? Try that again > somewhere else. 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? Signature-Fred W > Decreasing the flow path area will *always* increase the flow rate > through the decreased path (assuming a constant pump pressure). 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. SignatureMH '72 97 '77 96 '78 95 '79 96 '91 900i >> Decreasing the flow path area will *always* increase the flow rate >> through the decreased path (assuming a constant pump pressure). [quoted text clipped - 5 lines] > Venturi effect is entirely different; at increased _speed_ the > _pressure_ will decrease. You are right about the venturi effect. That's a different concept. But I still think that if you reduce the parrallel paths from 2 to one, and maintain the same pump pressure, the amount of flow through the remaining path will be increased. Signature-Fred W The Venturi effect is a special case of the Bernoulli effect, in the case of fluid or air flow through a tube or pipe with a constriction in in it. The fluid must speed up in the restriction, reducing its pressure and producing a partial vacuum via the Bernoulli Effect. The Bernoulli Effect is what gives lift to an airplane wing. >>> Decreasing the flow path area will *always* increase the flow rate >>> through the decreased path (assuming a constant pump pressure). [quoted text clipped - 10 lines] > maintain the same pump pressure, the amount of flow through the remaining > path will be increased. As the manual says" the valve closes off the heater core reducing the flow" obviously the valve is not working correctly if it shuts the heater core off completely. Swedes would have had this corrected long ago with temps below zero degrees in Scandinavia. > The Bernoulli Effect is what gives lift to an airplane wing. Only partially. If Bernoulli's principle is the only thing that makes airplanes fly, they could never fly upside down... What gives lift to an airplane wing is its angle of attack with the relative wind and Newtons third law (action = reaction - if you stick your flat hand out of the window of a driving car, at an angle with the wind, it will push your hand up or downward, regardless of Bernoulli's principle or the venturi effect. Saab content? They build aircraft... SignatureMH '72 97 '77 96 '78 95 '79 96 '91 900i > .. if you reduce the parrallel paths from 2 to one, and > maintain the same pump pressure, the amount of flow through the > remaining path will be increased. I do not know how you define 'amount of flow', but flow rate Q is defined as a volume passing throug a surface area in a certain time; e.g. litres/sec or m3/hr. With the same pump at same rpm and the same flow rate, if you close one leg of the parallel path, the same flow Q will go through a smaller area, at higher speed; if you half the area, the _speed_ will double. If hot water flows through a radiator at higher speed, its retention time in the radiator is less, and it will release less heat to the passing air. For the flow speed to be as slow as possible, you would want the radiator to have an as large as possible flow through area (for both water and air). The thermostat changes nothing to the pump's flow rate, it just opens to divert part of the flow out of the block to the radiator, instead of directly back to the pump. SignatureMH '72 97 '77 96 '78 95 '79 96 '91 900i Fellow Travelers, This has been a most enjoyable thread but it's time to dispel all the myths with the introduction of some real wisdom.....mine, of course. ; ) Simple water flow models are easily analyzed with Joule's Law...Power = Current x Volts. This axiom, together with Ohm's law, shows us that if you have enough Voltage (Pressure), small changes in the Resistance (Passageway options) will make no meaningful difference in the Power (Heat) delivered to the Loads (Radiators) because the Current (Available Flow Volume) part of the equation becomes insignificant to the Voltage (Pressure) part. As pointed out in a parallel thread, it's only a problem at low RPMs. Low pressure. Low Voltage. We have a classic Voltage sag resulting in inadequate Power delivery to the Load with the higher Resistance. The bathroom light goes dim when your wife turns on the hair dryer. With triple the Voltage there would be no problem. So...Is the water pump developing enough pressure (Voltage) to avoid these problems? A worn/defective impeller could do all these things. Does noone remember changing the thermostats in a SAAB 93 from a "Summer" thermostat to a "Winter" thermostat? It's the same problem. An inadequate (or malfunctioning) heat exchange system being asked to operate beyond it's abilities to perform at the extremes. Let's also remember that the divergence of coolant flow through the heater core on a hot summer day is unlikely to be a problem in a vehicle that controls its heat with a water valve!! Thanks to everyone for their input on this one! I've enjoyed the ride. Samhot > > .. if you reduce the parrallel paths from 2 to one, and >> maintain the same pump pressure, the amount of flow through the remaining [quoted text clipped - 16 lines] > divert part of the flow out of the block to the radiator, instead of > directly back to the pump. Dear Woofer , A concise and I believe accurate analysis. But what does one do with this problem or how does one deal with it ? ( Within reason of course) . When engine is at idle it warms up and the two stage thermostat progressively diverts more hot coolant to the radiator and away from the heater core. The cabin temp drops and passenger complaining begins. Before air flow through the radiator is the answer of course , but not always possible when in stop and go traffic. The car is a 900 cs and has a single ( two stage thermostat) unlike the 99 which had a summer and winter model . Any ideas? ACE > Fellow Travelers, > [quoted text clipped - 48 lines] >> divert part of the flow out of the block to the radiator, instead of >> directly back to the pump. I really think a water pump replacement (SAAB original part) would be a good gamble. The thermosat replacement (SAAB original) would be smart too. Maybe start with the thermostat. Cheaper and you'll have to replace it someday anyway. Woof. > Dear Woofer , > [quoted text clipped - 65 lines] >>> divert part of the flow out of the block to the radiator, instead of >>> directly back to the pump. >> Decreasing the flow path area will *always* increase the flow rate >> through the decreased path (assuming a constant pump pressure). [quoted text clipped - 4 lines] >Venturi effect is entirely different; at increased _speed_ the _pressure_ will >decrease. Velocity pressure goes up, static pressure goes down, total pressure remains constant when flow passes through a veturi (i.e. a narrowed portion of the tube or pipe. this is how the detergent is sucked into the wash water in a washing "wand". The lower static pressure at the restriction which is causing the velocity pressure to rise sucks in the "soap". Malcolm > Hello, > [quoted text clipped - 11 lines] > > Allan Schneider Hello NG, I have learned a lot since my original posting of this question. I cannot say whether or not this is a "good" engineering design, but here is what I have found. On page 261 -1 of the Official Service Manual SAAB 900 (Bently Publications) it says: "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.) > 1) The SAAB appears to be engineered that way and > > 2) It is operating in the manner intended. Yay! > Any ideas ? or just live with it ? 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. SignatureGrunff >> 1) The SAAB appears to be engineered that way and >> [quoted text clipped - 9 lines] > temperature. Where you get one depends on where you are - in the UK, a > Unipart thermostat of the correct size would do the job. 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 > Hello , Hi! > 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? Original Equipment Manufacturer - so either Saab branded, or made by the manufacturer who originally made the part for Saab. As opposed to pattern parts. > Why would a non- SAAB thermostat be different than the original ? If > they claim computability shouldn't they operate the same way as well? 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. > How does one know ? That's a tricky one, and depends a lot on where you are and what pattern parts are available to you. > And would defeating this feature have a possible negative consequence? 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. > As it appears to be quite intentionally designed that way , it could be > of some necessity or importance? 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) > Going around it may have be risky with > unintended consequences - like over heating and serious engine damage. 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? SignatureGrunff >> Hello , > [quoted text clipped - 45 lines] > > Where do you live, and what are the seasonal highs/lows? I have north of New York City in the US. The temperature ranges from 0 F to 100 F . Does this help? > I have north of New York City in the US. The temperature ranges from 0 F > to 100 F . Does this help? Heh - well, you'd be fine in the winter, but you may overheat in the summer. If I were you, and it was really bothering me, I'd use a non-OEM tstat and see how it goes. At the first sign of the engine running too hot for too long, put the old one back in. SignatureGrunff >> I have north of New York City in the US. The temperature ranges from 0 >> F to 100 F . Does this help? [quoted text clipped - 3 lines] > tstat and see how it goes. At the first sign of the engine running too > hot for too long, put the old one back in. First I will wear gloves when I drive and expect to be in stop and go traffic. Second I will hand my glove and heavy sweater when she complains. Did I tell you that the heated seats don't work either so I have been catching it for that too. Happy New Year and Stay well. > First I will wear gloves when I drive > and expect to be in stop and go traffic. Second I will hand my glove and > heavy sweater when she complains. Good plan. > Did I tell you that the heated seats > don't work either so I have been catching it for that too. Well worth fixing imho. > Happy New Year and Stay well. And to you too. SignatureGrunff This clever feature is designed to heat up the passenger compartment first. (BMW, naturally, suggests you sacrifice your comfort and allow the engine to warm first :-) I think there is a consensus here. Saab intentionally designed the system to provide more heat flow to the heater core during warm up. However.... This particular car has a diverting valve that is not working correctly thereby freezing the passengers during periods of idle engine running. Must be really cold in the Winter there for the engineers to go to this extreme. The electric heated seats are silly enough but this....... You get the gold star beets The thermostat is likely blocking the flow to the heater core more than it should, due to its being out of calibration. This is only noticeable when low RPMs cause reduced circulation through the core. I am surprised that all of the "old time" Saab 900 owners don't know about the "summer" and "winter" thermostats for the C900s. As far as I know, from talking with the local technicians and other C900 owners in Northern Virginia, you can optimize the heating in the winter and cooling (AC) in the summer by switching thermostats. Allan's 900 appears to have the summer thermostat installed - the thermostat is set to keep the engine running cooler. > Hello, > [quoted text clipped - 11 lines] > > Allan Schneider > I am surprised that all of the "old time" Saab 900 owners don't know > about the "summer" and "winter" thermostats for the C900s. According to Saab those thermostats were meant only for realy hot countries, middle east & such. SignatureMH '72 97 '77 96 '78 95 '79 96 '91 900i |
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