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  #1  
Old 03-28-2009, 04:24 PM
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Location: chicago area, illinois
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heat works only at city speeds

so i took my car for a 240 mile trip yesterday and when im around town the heat works good but once i get on the hwy the heat blows cool, not super cold but not warm either and the temp gauge reads abt 80 so i was wondering if anyone had any idea what could be the issue... also my 2 middle vents on the dash wont blow any air out either, the vents open but no matter what i select on the selector panel the air only blows out of the defrost and the 2 side vents. any ideas would be greatly appreciated.

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Old 03-28-2009, 04:32 PM
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Do a search on monovalve cartridge.

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  #3  
Old 03-30-2009, 07:53 AM
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Lack of flow

The root of your problem is usually poor coolant flow through the heater core. This causes both problems- as the car goes faster there tends to be less heat, and because the air is not getting heated the climate control senses that condition and stays in heat mode to compensate which results in the center vents not opening.

Many times the monovalve simply needs to be replaced. There are a bunch of posts on this topic. In my case the heater core was not flowing well and it took a through flush of the cooling system to solve it once and for all. In addition to the symptoms you described I found that if I slowed down a bit on the highway and gunned it in third gear to get the engine up to about 4000 RPM it seemed to get the coolant flowing enough to start things working temporarily and I would get heat for five or ten minutes.
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Old 03-30-2009, 08:30 AM
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95% of the time poor heat on the highway is due to a bad monovalve diaphragm. Its an easy replacement and then you'll have heat back. Just do a search on it.
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Old 03-30-2009, 09:01 AM
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Not disagreeing with the monovalve suggestions, but you should also make sure your cooling system is up to snuff. 80 C is just a bit on the low side for coolant temperature. You might consider changing your thermostat if/when you open the cooling system. Also make sure your pressure cap is good and the system is holding pressure when hot.
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Old 03-30-2009, 09:32 AM
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A small tear in the monovalve diaphragm will still allow it to function at low speeds but at consistent highway speeds coolant is forced thru the tear from the higher pressures producing symptoms like yours. Could be something else but it only takes 2 minutes to check your monovalve.
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  #7  
Old 03-30-2009, 09:41 AM
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Look, pull your monovalve insert out and take a look at it. As stated, 95% of the time that causes these exact symptoms, and we hear about this all the time.
Other answers are valid, just makes poor sense to start with the more complicated stuff, especially when it isn't that common.
And as far as I know, 80 degrees C is fine for an engine temp. It isn't like these gauges can be trusted to be perfect to a single degree anyway.....
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Old 03-30-2009, 10:16 AM
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Quote:
Originally Posted by JimmyL View Post
Look, pull your monovalve insert out and take a look at it. As stated, 95% of the time that causes these exact symptoms, and we hear about this all the time.
Other answers are valid, just makes poor sense to start with the more complicated stuff, especially when it isn't that common.
And as far as I know, 80 degrees C is fine for an engine temp. It isn't like these gauges can be trusted to be perfect to a single degree anyway.....
Agree completely on the monovalve direction. I'd rather see 85C or so on the highway. 5 degrees may not sound like a lot, but 5 degrees C is close to 10 degrees F. 80C is only 176F. Personally, I'd rather be running at 185F.

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