If the front cover isn't leaking, it isn't the problem.

Get the engine up to temp, and feel the heater hoses. The inlet side should be HOT, the outlet side should be a bit cooler. If they are the same temp, probably an air pocket in the heater core, if the outlet side is significantly colder, that's a flow problem of one variety or other. Blockage in the system somewhere, or failing water pump.

 

By front cover I mean winter cover over the grille / radiator.

I will check my inlet / outlet temps but I think I already went that route once before and checked them.

My whole point is what is there in this system that forces the water pump to circulate coolant through the core versus just sending it to the radiator or through the bypass back to WP? I don't see anything that makes it do that, which would seem to me to be a problem.

Steve

 

nort reallya problem, they built millions of vehicles that way, it's just part of the loop that the coolant has to follow. coolant flow schematic would be like many other makes/models before them.
the only ones I have seen with any kind of device that "forces" coolant thru a heater core are on vehicles with front and rear heat/AC where tehre is another heater core in the back..... and many with front and rear heat STILL don't have, just a bigger circulartion loop than those with front-only.

 

See the attached PDFs for the V6 and V8s (one from my 1988 FSM for the LAs, one from the 1996 FSM for the Magnums).

The reason it's in parallel with the radiator is so that on older LAs, we can shut the heat off completely to run A/C.

Also, if you go far enough back, the A blocks (which the LAs were derived from) were in cars that quite often used a valve there; making it a forced part of the cooling cycle would cause the interior to be hot all the time.

OTOH, my new heater core keeps my Dakota nice and warm. OTGH, it WAS up to 50F today, and the coldest it's gotten to so far this season is 21F.

RwP

 

I've seen a schematic similar to the 96, thats where the question about passive coolant circulation came from, there's nothing in the system that says coolant must follow the path through the core, even when the thermostat is closed. What's to prevent hot coolant from circulating through the bypass instead of the core?

So you're saying that hot coolant doesn't (or isn't supposed to) circulate through the core at all times? There's no valve that shuts it off on the 96, at least as far as I can tell, I only see that on the 1988 schematic. And from my recollection a great many cars have hot coolant circulating through the core all the time. All of my VW's do that and AFAIK GM products, at least the ones I've owned, had heater cores that were hot all the time and bypassed inside the case with doors that diverted air around the heater core instead of through it.

Anyway, I'm going to sew up a vinyl front radiator grille cover and see if that helps. I don't relish the idea of a core replacement, when I backflushed it last year it didn't have any problem flowing, can't imagine that there's an issue with it, but I suppose.

Steve

 

The cutoff valve was to keep hot coolant from fighting the A/C; with improved A/C systems, the cutoff wasn't needed.

The BYPASS hose is bypassing the THERMOSTAT, not the heater core. Follow the flow again.

Yes, more water flows through the radiator than through the heater core. This is new? Even on those where the core is always in the path, there's a bleed so that you don't get ALL the coolant through the heater core.

The heater core is in parallel to the radiator. Top side bypasses the thermostat, yes; thermostat feeds the radiator.

Bottom side is on the suction side of the pump, along with the lower radiator hose. So it'll be sucked through if it's all clear flowing. (That's the key note; the heating system sucks. Wait ... )

It's not passive at all.

It's also not REQUIRED to keep the motor from overheating; since it's in parallel to the radiator.

RwP

 

Ralph
Not to disagree but when you're talking about a parallel system with nothing that defines where flow will go (not talking about flow direction here), then it will always be the path of least resistance, which in this case I have to believe is the largest opening to bypass or radiator. Certainly the core is not the path of least resistance, in fact by its design it will be more restrictive than an open pipe or hose.

Following the flow is not the problem, I've seen these diagrams before, but regardless as to whether it's pressure or suction what guarantee do you have that the WP will pull coolant back from the core? That pipe return connection is open on the inside of the water pump, it's just another along with radiator return connection, there's nothing that says the wp has to pull from that return line.

Someone online proposed that the problem was a design flaw, which they resolved by taking the return line off the pipe at the WP, capping that opening and adding a 5/8" port to the top radiator hose. I'm not of the opinion that this is anything better but apparently it solved their no heat situation. Perhaps it's something to consider...

I will run my truck up to temp today and check the core hoses to see what's going on there.
Steve

 

Yes, it will pull some. Yes, the radiator will get more. There's nothing wrong with that.

If there's a problem it's either excessive restriction in the heater path, or a thermostat that opens too soon (since the bypass hose is 5/8" also, it means that the flow is 50/50 nominally between radiator and heater UNTIL the thermostat opens up.)

And I don't understand how putting the heater either side of the thermostat, but out of the suction path, would work. Now if they take the INTAKE fitting off, plug that off, and run THAT hose to the upper radiator hose, that might help some ... AFTER the thermostat opens up!

But not until, since now you're limited to the total flow through the bypass hose.

RwP

 

That's exactly the reason I questioned the effectiveness of the change, you're taking what was a cirulation path and closing it off because you're now putting it outside the closed thermostat.

At any rate, I drove mine up to temp today and the hose to the core is toasty warm, the hose coming from the core is also reasonably warm, so either I have faulty blend door cable attachment that isn't routing enough air through the heater core or I have a very bad seal on the door that's supposed to route air through the core. I bet it's a failed seal....

I was hoping that it wouldn't be this, but alas it seems to be the problem. I'll likely have to dig in to the repair sooner rather than later. I remember last winter it wasn't freezing in the cab but it never got warm enough to keep me happy. On max heat (temp slider) it would just spit out some warm air under the dash.

I suppose as a temp fix I could cover the outside air intake and force the recirc flap to shut off outside air but it might be cleaner just to go after the repair if I can find the time this next week. We're supposed to be in the 40's for most of the week I guess, but then late it's dropping back into the 30's and below.

Steve

 

Think I would replace the heater core while you were in there as well.