Since tearing down my 2004 DII 4.6L engine to replace a slipped cam bearing, I've had an issue with the heater not puttiing out heat at idle. If I rev the engine to 2500rpms or more the heat begins to work, but once back to idle the heat is gone again. There's no waterfall sound and I'm pretty sure I've got the air out of the system. It seems like the water pressure of the pump just isn't sufficient to circulate the coolant through the heart core until the rpms are increased. Coolant is circulating through the expansion tank fine. Everything seems to be hooked up correctly and I'm not over heating. I just replaced the water pump since it was seeping out the hole but the issue still persist.

If anyone has run into this issue and has some advice it would be greatly appreciated.

 

Has to be air in the heater core/low coolant or your new water pump has a bad impeller. I'm betting on air in the heater core.

I'd unsnap the expansion tank and set it on the edge of the battery tray with the cap off then have someone idle it up to about 1500 rpms while you stand at the ready with antifreeze as air works it's way to the tank.

If that's not it I guess it could have a bad impeller but that's not likely. Or a blocked passage. That's really unlikely.

 

Just put a new impeller on so I'm sure that's not it. Does it matter which hose from the heater core is connected to the engine? Inlet and outlet can be crossed? I swapped them and no change occurred. Drove the vehicle up onto steep hill and lifted the expansion thank pulled the bleed fitting out of the tee ect. And still no luck. Guess I'll disconnect the Core and blow it out to make sure it not blocked.

 

I've never thought about it but it seems that it wouldn't matter which heater hose went where unless your core was blocked and they went on backwards ... I guess that could cause junk to dam u

 

I've never thought about it but it seems that it wouldn't matter which heater hose went where unless your core was blocked and they went on backwards ... I guess that could cause junk to dam up. But if it was blocked I'd think you would have no heat regardless of RPM.

 

Is your engine temp staying steady? I mean, read from a code reader.

 

Youre right. Its a pressurized system. If its blocked, no heat regardless. Possibly its blocked some place else or the pump not working properly only when you have high RPM its circulates the coolant a bit more.

 

I swap the hoses around that go into the heater core today and that helped but still have to get above idle to get good flow.

Cruising down the road temp is ~190f. When at idle it will go up to 210f but will drop down quick if I rev at 2k.

What seem strange to me is the top two hoses on Tstat and top half of Tstat are hot. Bottom half cold as though no circulation is happen in radiator. I would just expect the bottom hose to be much warmer.

If the top two hoses were switched on the Tstat would it make a difference?

I have the low temp Tstat install on the truck.

 

What do you mean by top 2 hoses switched on T-stat,? You can only instal it one way, the one that comes out of side of T-stat in an angle is return hose to supply the cold coolant from radiator back to engine, the one on top of T-stat is to give hot coolant to T-stat to activate it & make wax hot to open T-stat, I hope you didn't instal It other way, if you have that will explain your problem.

 

I would defo double, triple check you don't have hoses going to the wrong things.

I also recently learned/realized that the bottom radiator hose is supposed to be much cooler than the top. Means the radiator IS working properly.