I’m troubleshooting a low/no heat problem on a 2005 wrangler with a 4.0 in it - as long as the RPM is above 1,000 it’s OK - at idle it’s not putting out heat at all.

Our customer brought it to us after a previous shop replaced several parts attempting to fix it. I don’t know what the reasoning/logic was behind these repairs, but here’s the chronology of them (over a few months time).

1) Heater core
2) Water pump & thermostat
3) AC compressor
4) Second heater core

I verified it wasn’t a temp blend door problem by carefully selecting a spot in the case to drill a hole and insert my inspection camera - the blend door is moving freely between the stops. The actuator is a bit noisy but it’s also moving fully stop to stop, regardless of fan speed.

I did find the inlet hose to the heater core was collapsing, so I replaced both heater hoses, but the no heat issue persists. Just to make sure it wasn’t air locked, at one point I drove the nose up a rock pile in our lot and ran the engine at 2,000 RPM - the heat was strong while doing so but still died off again once it returned to idle.

Both hoses are hot and I have no reason to think (at this point) it’s a flow problem.


Can anyone verify the direction of flow through the heater core?

The diagram I found (below) looks like the inlet (from thermostat) should be on the bottom, however, the hoses are sized so that the hose from the thermostat fits on the top of the heater core. There’s no indication that a reducer or coupling should be installed to adapt the hoses to hook up opposite what they are now.

Thank you.

 

Generally, low heat at low RPM is either low on coolant, possibly an air pocket in the heater core, or poor flow. Looking at the repairs done, about the only thing that really fits is low coolant/air pocket..... Is the core itself higher than the radiator?

 

Yes, the heater core is higher than the radiator - it’s the highest point in the system - which is why I drove the nose up the side of a rock pile. It was sitting at about a 45* angle while I was attempting to bleed it.

Additionally, after replacing the heater hoses, I used a bleeder kit. It’s a funnel that’s fixed to the radiator neck and filled about halfway full of coolant - as the engine runs, air bleeds out through the funnel and coolant is drawn back in. I left it to run that way for a 1/2 hour with the blower on high, then test drove it and then the rock pile after.

I forgot to mention I verified the pressure cap is good and there’s no indication of a head gasket leak.

I suspect an air pocket as well, but it just seems unlikely at this point, which is why I’m trying to verify the inlet/outlet connections. I haven’t had a chance to do it yet, but my boss suggested we install a length of clear hose to see what the flow actually looks like at idle.

 

The dodge full size trucks have the same problem. Getting all the air out of the heater core is a major pain in the keester. I have pics of my truck parked on a hill, when I was trying to bleed mine out as well. Significant pucker-factor, but, still ****-poor heat.

Might try putting a flush T on the outlet side of the heater core, as close to the core as you can get. Then, just start the engine, and unscrew the cap a bit to let any air bleed out. See if that works.

 

One reason I’m trying to verify the direction of flow through the core is because it doesn’t make sense for the inlet to be the top fitting, as that would easily allow for an air pocket to get trapped. If the inlet were on the bottom, then (in theory) the coolant would fill the heater core from the bottom up, pushing air out as it goes.

Thanks for the advice - I had to focus on other repairs so it’s still sitting there. I’m gonna splice in the clear hose on Monday and see what the flow actually looks like.

 

Let us know what ya find, and what works to actually fix it.

 

Update:

I spent some time really digging through the google and FINALLY found a coolant flow diagram that confirms the inlet for the heater core is supposed to be on the bottom.

The heater hose fittings at the heater core are different sizes, so I rigged up a temporary adapter to swap them. The results were encouraging as the heat was better than it has been since we started looking at it, but it’s still weak at idle, especially if the blower is on high.

This morning I spliced the hose to permanently adapt it from the larger to smaller size and have been letting it idle with a Lisle funnel kit to bleed while parked on a slight incline. It may take a test drive with the cap on to get it working right but we’ll see.

 

And the problem persists...

I’m starting to think the serpentine belt may be responsible. It’s near the need for replacement - the indicator mark on the tensioner is within range, so technically it should be OK, but it’s about 1/8” from the end of the range.

I’ve ruled out everything else, and I’m confident it’s not air locked. Considering the only factor that changes heat output is RPM, and the belt is the only thing driving the water pump, it doesn’t seem beyond reason that it may be responsible, even if it’s technically within operating range. At this point, the belt is the only thing that hasn’t been replaced.

I’ll give it a try on Monday.

 

Hhhmmm.... if the belt were slipping, I suspect you would definitely HEAR that.... Won't hurt to change it in any event. (I generally change the belt when I do a water pump anyway.....)

Maybe get some variety of flow indicator, and see what's happening?

 

And that’s the thing that’s prevented me from replacing it up to now - no noise, hot or cold, idle or 2000 RPM - and no visual signs that it’s slipping on the pulley, such as build up from the belt breaking down. The pulley doesn’t “look” new but I suppose that doesn’t mean it hasn’t been replaced by the first shop that worked on this.

The flow indicator is a good idea but I’m thinking an easier test might be to try and increase the tension manually with the engine running. If I remove the fan, it wouldn’t be too difficult to do so (safely) using a breaker bar as the tensioner has a 3/8” notch in the arm.

Thanks for sticking this out and helping me reason through this.