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I have a 1999 Dakota. I put a new water pump, heater core, 3 new thermostats, have flushed radiator numerous times and still have no heat. Also it builds up no pressure in radiator. You can take cap off after driving for 2 hours. I'm at a loss. Need heat. Thanks in advance.
 

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Well the no pressure sounds like the radiator pressure cap is not doing it's job. Does your vehicle have a cut off for the heater core in the system that possibly may not be working?
 

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Do a little more troubleshooting. Does the top radiator hose get hot? Do both heater hoses get hot? If so, your thermostat is working properly and the water pump is circulating coolant. The lack of pressure in the system may be a bad radiator cap. In warmer weather, this would eventually lead to your system overheating and blowing all the coolant out. If your hoses are getting hot, then you need to investigate whether the blend door in the HVAC system is working. Water circulates through the heater core all the time, but the blend door determines whether air is being routed through the heater core. I think your HVAC system may be controlled through vacuum lines. Check to see that you don't have any broken or leaking vacuum lines.
 

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I had similar prob when I just purchased 97 5.2 4x4. But after cramming to get stuff done before winter hit (Northern Wi) hadn't checked everything out. Seems previous owner put Mopar Perf. brain box in,which I knew he gave stock to me. But didn't tell me he removed thermostat. This was old trick to make them run richer.
 

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I have a 1999 Dakota. I put a new water pump, heater core, 3 new thermostats, have flushed radiator numerous times and still have no heat. Also it builds up no pressure in radiator. You can take cap off after driving for 2 hours. I'm at a loss. Need heat. Thanks in advance.
I realize this is an old thread, but would like to add one more item to check. I had rebuilt my 3.9 and put on a new water pump from Advance Auto. About a year later, 11 1/2 months to be exact I had to replace the timing cover gasket. When I pulled off the one year old water pump, I found that the cast aluminum inside, which forms the impeller housing, had eroded away. The truck had started to run a little warmer, and now I knew why. Got a free replacement, because it was less than a year old.

I have also replaced a water pump on a chevy that had the steel impeller vanes completely worn off!
 
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