I all for the design flaw theory, but the heaters in these trucks worked great when they were new. My 1998 Durango had fantastic heat when it was new and I'm the only owner.

Since I have run water from a garden hose through the heater core both ways and watched as it flowed easily and with high volume...I'm thinking there is a blockage somewhere in the block that's just enough to cause the flow to divert from the path through the core.

At the same time, I can feel the top of the heater core from inside the cabin and it's always burning hot to the touch. It's getting hot fluid but I don't think it's flowing through it. Since the core is 'capable' of that flow as tested by the garden hose...it has to be the engine.

 

I am noticing a common factor (or mostly common) with all the "lack of heat" threads. This may just be a a fluke but atleast 3/4 of the trucks having heat issues are running the 5.9 (360) engine. Now i don't know what the OP is running because he never told us. I run the 5.2 (318) and have always have had excellent heat in my truck. My neighbor running a 5.9 (360) is always griping about he can drive all day long and only having luke warm heat. He's spent alot of $$ replacing crap to try to get good heat and all have failed. so i just wonder if there might be some slight designed change that is causing flow issues?

 

OP - yes, 5.9

 

That makes me really wonder if its a 5.9 design issue.

 

But what type of design flaw in the 5.9L casting would allow for great heating capabilities when new, and diminished capabilities after thousands and thousands of miles.

I could put more faith in that if the heaters have always sucked, but I remember clearly how hot it used to heat the cabin up and now it'll barely defrost a windshield even after multiple flushes and two new heater cores.

 

For what it's worth I've got plenty of heat with my 99 5.2L, even during cold mornings in the Pittsburgh winter. By the time the temp gauge comes off the peg I've got heat. I had the original water pump up until last year, and I've noticed no difference.

 

That's what i was saying in my post above... i have never had any heating issues with my 5.2l... it does not make sense because the 5.2 and the 5.9 look all most twins except for there size.

 

Well, since the block is the same size for both of them, and on the 360 the cylinders take up more space in the block than they do in the 318, that means there is less space in the 360 block for cooling and oil passages. Maybe they had to change the cooling passages on the 360 which is causing the more restricted flow. Not sure if this is the case, but it seems like a reasonable guess.

 

Seems like... if 5.9 had a flow problem they would have it all the time - especially when old and towing - seems like they would run hot or at least generate heat or knocking complaints. But it's just heater output.

Anyone have any comparisons with thermostats with and without a steam / bleed hole? I think my OE had one, and my replacements have not, maybe generating air pockets that get then passed to core in use?

 

Well does any one know if heater output and input goes through the block somewhere? If so that has got to be where our issue is. Like there is a build up on some corner or bend. I wonder if someone took a bore scope and looked what one might see.

On a side note the 360 is a slightly different block than the 318 going way back. Even thought the heads are interchangeable. The 318 shared the same block as the 340 if I'm remembering correctly.