The fourth generation Volkswagen Jetta, also called the MK4 or the Bora, is Volkswagen's family sedan.

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engine overheats and heater blows cool air

1999 jetta engine overheats. Thermostat & cooling fan control box replaced, but coolig fan not coming on. Also the a/c clutch not coming on & the heater blows cool air. Are overheat & heater related?

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If your cooling fan does not come on at all, disconnect the wiring harness and feed the power directly to the fan. this will bypass your temperature sensor. make sure that the fan is working. If your heater continues to blow cool air, even when your gauges are reading hot, check your thermostat again as well as your water pump.

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How does your radiator look? Was it replaced or flushed? It may be blocked. Also this could be a water pump problem if the thermostat is opening but no water is circulating through the block. Check every fuse not just one of them, because one fuse often controls more than one circuit. You can use a test light or VOM meter for the procedure. There is also a heater control valve on that vehicle that may not be working. In addition to that, there is also a vent door actuator that swings in one direction to allow the heater core to be exposed to the fan. If it is stuck in the other direction it will be wide open for the A/C. Just take it one step at a time starting with the radiator. As the car warms up at idle the upper hose should be warm after a period of minutes indicating that the Antifreeze is circulating through the pump/bock and thermostat. If it never gets warm, you found the source of over heating. Next with the car hot and running check to see if you have voltage at the fan. It could be the fan is defective. If no voltage is present look toward the newly replaced control module for the fan. If no voltage is present at that connector and the fuses are all good, you have a broken wire. Post your findings and I will try to help you further.

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Did this issue start after the thermostat was replaced, or was the thermostat replaced because it had this issue? If it started after the thermostat was replaced, you probably have an air pocket in the system that is preventing the coolant from circulating. This will cause it to overheat and will also keep the heater from blowing hot air.

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How do you get rid of an air pocket like this?

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Mayer, most new thermostats have a real small hole in them that allow's air to purge to prevent air locks.

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When you fill the cooling system, on most vehicles you want to run it with the cap off until the thermostat opens to make sure the system is completely full. One important step in this is to make sure that the heater is running and set to hot. If you do not do that, you have the chance of creating an air pocket because the coolant is not flowing through the heater core. Once the coolant is flowing properly, both the upper and lower radiator hoses will be hot, and the heater should be blowing hot air. Some vehicles have a bleed screw to help get air out of the system, but I don't think this vehicle does.

Disclaimer: Some vehicles have a pressurized cooling system (such as an '89 Jeep Cherokee). On a pressurized system, if you run it with the cap off, it will overheat and will not bleed the system properly.

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What if the lower hose dosent get warm

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