Is there a restrictor anywhere in the heater circuit?? That's about the only thing I can think of at this point.... aside from an air pocket. Gotta be a flow issue, or air pocket causing it. There isn't anything else left.

 

I think it’s a flow issue but not because of a restriction - and no it doesn’t have any restrictors or reducers, except for the restriction created by the sizes of the inlet [5/8] and outlet [3/4] fittings.

I forgot to mention it in my last post, but I also don’t think it’s air because last week I drained the system to inspect for crud/sludge just to be sure that wasn’t the issue. To refill it, I used our vac and fill setup - it pulls the system into vacuum and then draws coolant in from a bucket to fill the system as completely as possible. What little air may be left should be in the radiator and purged when it’s topped off.

I’ve also put a combined 8 hours of bleeding into it using various combinations of idling and revving to 2000 RPM for up to 5 minutes - with and without the bleeder funnel - as well as hard acceleration driving and nosing it up to about 45 degrees twice - all with the fan on high in floor mode which is Jeep’s recommended procedure.

I’m not saying it’s impossible for air to still be trapped, but the heater hoses being too hot to touch for more than a second really doesn’t support that.

The only isolating factor at this point is RPM, and since the belt is what transfers crank speed to the water pump, attempting to verify if the belt is at fault (even if I can’t fully reason how it’s possible without any indicators it’s slipping) seems like the next logical step.

 

Even if there is an air pocket, there will still be flow thru the heater core, and both hoses will indeed get hot. Just not enough of the heater core stays hot enough to keep the air passing thru it warm.

Another thought, does the motor have an under-drive pulley on it by any chance? That could substantially reduce flow at idle........

 

I’ll have to check on the under drive pulley tomorrow but I don’t think it does. This jeep is all stock as far as I can tell - at least it doesn’t have any of the typical mods like a K&N air intake, lift with giant tires, or off roading lights.

 

Any progress here?

 

I haven’t had much time on it since last posting.

I emailed 2 of my instructors from the automotive program (I’ve talked to them about miscellaneous diagnosis issues in the past) about it, listing the complete history from the work the previous shop did to everything I’ve done, and they’re surprised as well that it’s still not working properly.

One also suggested verifying the correct pulleys and the other said since everything else seems to be ruled out, it could still be an air pocket.

I did verify yesterday the pulleys are the correct ones, but otherwise I’ve had to focus on other repairs. I’m going to try to spend some more time on it next week.

 

If the heater hoses are easy to get to right close to the core, try the flush T in the return line right there. Try and bleed it there, engine running, and see if anything changes.

 

I’ve been mulling this over as my mental space has allowed, and an air pocket in the heater core still seems unlikely. The reason is because when the engine is being revved there’s no “gurgling” sound coming from the heater core. Thinking about past diagnosis where air ended up being the cause (which was almost always the result of low coolant level) I’ve always heard the movement of coolant in the heater core, and I haven’t heard that sound once with this jeep.

Additionally, I remembered that last summer, I pulled the head on another 2005 4.0 (with close mileage to this one) for a valve job, and the head gasket on that engine had some restricted coolant passages in it. When I reassembled that engine I had zero problems static filling the coolant and bleeding the heater core.

That’s not to say air couldn’t still be a problem, but considering the only isolating factor is engine speed, I think it’s worth it to drain it again, remove the thermostat housing, and try to inspect some of the coolant passages in the head gasket for restrictions with my bore scope. The system was flushed by the previous shop but that doesn’t mean all the contamination was removed, and if a stop leak product has been used in the past (which I don’t know if it has), that definitely could’ve clogged some of the passages.

 

Might not be a bad idea to do your own flush, with some heavy chemicals...... I know the consumer products available for such an occasion today, aren't nearly as effective as what was on the shelf a decade or so ago..... I've used CLR, or, just straight muriatic acid in the past......

 

I’ve considered that but the 2 times I’ve drained the system, there hasn’t been any significant contamination so I didn’t think it would actually help anything.

I had to reset myself on this due to overthinking it - my brain was getting into a “What if this what if that?” stage and really started getting carried away. I had to remind myself the only factor is engine speed (and by default, flow through the heater core) and to focus on the things that can be affected by it.

The coolant flows from the water pump, to the block, to the cylinder head, then the heater core/radiator, so the only thing that’s really between the pump and heater core is the head gasket. I remembered the details of the the other 4.0 I repaired. I also remembered another head gasket repair I’d done a few months later on a Chevy 3.4 and how the customer had tried using what I call “head gasket in a bottle” on it...




The “fix” ended up almost completely clogging the head gaskets.

So I’m wondering if there’s +/- 50% blockage in the head gasket - I can see how that could affect the speed of the coolant flow at idle without allowing an overheating condition (especially this time of year).

I’ll see what the bore scope finds next chance I get.