I've been trying to get the heater working properly since I replaced the intake manifold gasket. I let the car idle with the heat (or lack of) on for about a half an hour with the cap off and didn't get any burps. I then hooked up a garden hose to the heater core and saw I had good flow in both directions. Connected the inlet back then connected a garden hose from the core return line to the radiator inlet and after a minute minute I had good flow of hot water and the heater blasting hot air. Shut off car and quickly connected the return line with minimal coolant loss. Started car and no heat. Is there something in the water pump that could cause it to put back pressure on the return line of the heater core?

 

It definitely sounds like you have an air bubble in the core that is not moving. Have you tried elevating the front of the truck as high as reasonably possible with the passenger (cap) side of the radiator slightly higher than the driver side?

 

Nope...never heard of that trick...I'll give that a show when I go back out. I was waiting for someone to tell me to check the blend door actuator which we know is good because I got heat on all settings when I ran the return hose into the radiator. It's about 90 out now so I don't think there's a rush on this job but it just bugs me.

 

I raised up the passenger side so that the tire was a couple of inches off the ground which should be about a total of 4 inches. The car was still hot and ran it with the radiator cap off for another 15-20 minutes...still no heat. I also squeezed the heater lines to see if that would get it going but that didn't work either. The next time my wife gets in the car she's going to whine about where all the gas went she put in yesterday. I've seen this before with weak water pumps but the car was running over an hour in the driveway without moving from 195 and I had good flow through my test hose so I think the pump is fine.

I have one trick up my sleeve yet..put my flush kit T on the inlet side and force water through the core while it's running. If I put a shutoff valve on the rig I can then disconnect the water from the T without having to disconnect the heater hoses. I'll just have a funny looking valve but I'll be all set for the next flush and fill in 3 years.

 

I repeated my previous test by running a hose from the output of the heater core back into the radiator and the hot air almost burned my hand coming out of the vents. Next thing I did was disconnect the return hose at the water pump and pumped coolant back through the core in reverse and it came out of the tube on the top of the water pump so that doesn't appear to be blocked in that direction.

The next thing I did was connect a water hose to the other end of the return line so I'm pushing water in the same direction the water normally flows and something appears to be blocking the flow back into the engine. If I have trouble getting 50 lbs of water pressure through I doubt if 15lbs of pump pressure would do it. For all of the above tests the engine was at full temp as in the gauge went up to 210 then back to 195 when the thermostat opened. This one has my pea sized brain stumped. Is there some type of control valve other than than thermostat? It's my assumption that water is constantly supposed to flow though the heater core and whether you get heat or not is dependent on the blend door.

Edit: 4:30pm - I performed one more test. With the engine off I connected a hose to the water pump at the heater return line and water started coming out of the heater core outlet so that line is not plugged but with the car running water does not want to go into the same place on the pump. Is there something inside the water pump that, when running, would prevent the return flow from the heater core? Some kind of bypass valve that's broken that's cutting off the return flow? I've never taken one apart to see how it works.

 

There shouldn't be anything that would prohibit the flow of coolant through the core. And yes, coolant should always be flowing through the core.

 

I'm having a similar problem to dobyken. The input to the heater core is HOT and the return is luke warm at best, probably just from engine temp. When I take the heater core return line off, and start the truck, coolant flows from the return line on the core. I can take that hose while the truck is not running and blow air into it and it again comes out the heater core return. It is almost like while running under 2k, there is not enough pressure to force coolant through the core?

While driving down the road if I race the engine I sometimes hear bubbles and it starts to get warm for a while but cools down quickly(it is -15 out). I realize there might be air in the system, but would a little air be the difference between a little heat and none?

I have replaced the heater core, thermostat and blower motor(<- pretty sure this froze up from how cold it has been here & no heat near it). I also got it professionaly flushed recently as a last resort and still no heat, return line still "warm".

After replaceing the heater core I made sure the blend door was hooked up, and I disconnected vaccum hose from it to see if it had pressure and it had plenty. If the blend door is stuck shut, would I still get a little heat when reving the engine?

I am completely at my wits end with this and have no clue where to go next. I think maybe I'll try putting in a new set of the HVAC controls, but don't really know what that would help? Anyway any help, ideas, solutions would be greatly appriciated.