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Heat, only when moving 4.0L Jeep

I have a 2000 Jeep Grand Cherokee Laredo with a 4.0L. I have the common, lack of heat issue. I have done the following: Flushed the cooling system ( including the heater core), changed the thermostat, radiator, the blend doors and burped the system.
I have a little bit of heat while on the highway. But it fades and at times, disappears while in stop/go traffic. I have even heat on the hoses. There are NO leaks. Does anyone have any ideas?

How did you burp the cooling system?

Tester

I bought the funnel kit. My model doesn’t have a bleeder.

Okay. Try this.

Make sure the cooling system is full.

Start the engine and either take the vehicle for a drive or let it idle until it’s up to temperature.

While the engine is idling, loosen the upper radiator hose clamp and slip a small flat bladed screwdriver between the radiator hose and the neck on the radiator.

Allow the engine to idle until all the air is purged from the cooling system.

Remove the screwdriver and tighten the hose clamp.

Tester

While doing this, should I have the front end raised? Should I expect some coolant loss?

When the air is finished coming out…solid coolant will begin to come out of the hose…that is when you remove the screwdriver and tighten the clamp.

Also… Hot and Cold cycles should burp your system alone…but you have to have a good Rad cap and a functioning overflow canister with the hose properly applied inside the tank and top lid etc… About 3 cycles will burp most systems.

Thank you. Now, lets just say, solid coolant comes out immediately… Then what?

No reason to raise the front end as pressure is being used to purge the air from the cooling system and not gravity.

If there’s air in the cooling system, you’ll see it spit and sputter out of the hose. Then when there’s a steady stream of coolant the air has been purged.

You may want to place a catch pan below the radiator for that.

Then when the engine cools down, check the coolant level.

Tester

I have checked the hoses on the overflow canister. They’re fully functional. And I honestly cant tell if the cap is leaking. I have constant engine temp. So, I just assumed it was good.

That is not a measure of the rad cap… Its either good or it isnt…and if it isnt you will never get a good burp.

It could be as simple as a bad Rad Cap…

I will look into a new cap. Can’t hurt. Thank you.

Thank you, Tester. I will definitely try it.

Not sure if you are aware…but the Rad Cap and the overflow canister together work in unison to purge air from the system…and upon cooling replace that air with solid liquid coolant.

Think about it… Your engine and rad and subsystems may have some air in them…fine… Heat up the engine and coolant…the coolant expands and the air and coolant get pushed into the overflow…then upon cooling…the coolant shrinks which then siphons coolant out of the overflow.

This is why the overflow cap has a hose that goes to the bottom of the canister…this is so upon cooling the rad can suck solid coolant into the radiator… If any of these items aren’t working or set up correctly…you have no chance to be able to purge any air in the system. It makes perfect sense when you think about it…

The caveat is that you need to have as much coolant and as little air as possible when you begin the Hot Cold burping process…because if there is too much air…the expanding coolant will only blow up the rad hoses rather than make the rad cap purge into the overflow…

This system is notorious for this issue. From all I have read on it, it borders on a design flaw. Which is why the bleeder valve was installed after the design change in 2005. I know how the system works in general. It just happens to be my first Jeep. Every other Chrysler, Ford, Chevy, Oldsmobile and Honda I have owned, worked as is should have. I have plenty of coolant, the hoses are all functional. Google is a beautiful thing, sometime. But no one has offered the suggestions I have received from the others.

Yes some vehicles are prone to this condition due to whatever it may be… I do appreciate a burp nipple on vehicles. If yours does not have one and they were introduced later…you could always install one somewhere? Not too difficult really…

If you cannot…then your prior mention of getting the nose of vehicle in the air is on the right track… It will make the high point the highest point…and thus…help you burp it.

If there is any chance of animals drinking the spilled coolant, this is a must. It is poison and deadly if ingested. Dispose of it properly, please, and do the right thing.

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I worked at Jeep dealers for years and never had a problem refilling the cooling system on these vehicles. After the thermostat opens you can finish filling the cooling system, the bleeder just make this faster.

After driving the vehicle did you check the coolant level in the radiator? Air in the cooling system generally moves from the engine to the radiator after reaching operating temperature and increased engine speed.

I don’t recommend removing radiator hose clamps when the cooling system is under 17 PSI of pressure.

If the coolant never gets to its designed operating temperature this could be a result. Do you have a dash coolant temp gauge? If so, is it somewhere near the middle after the engine is warmed up, say after idling for 10 minutes? If you let it idle even longer does the radiator fan kick on? What’s the difference in the gauge readings between the two?

If the coolant temp is ok, after the engine is warmed up measure the temp at the top radiator hose vs the inlet to the heater core. This can be done pretty accurately with one of the infrared laser-guided temperature gadgets, just aimed at the hoses. The difference between those two readings can provide a clue.

You already know that air trapped in the cooling system will cause this. Raising the front end of the vehicle, say by driving it up on ramps, can only help w/the bleeding process, won’t hurt. You are turning the heater to full max heat when you do this, right? It can take quite a while to burp the air bubbles out, are you doing this for 1/2 or an hour or more? Do you hear any gurgling noises coming from the heater core area? If so and bleeding the normal way doesn’t help you could try to invent a bleeding procedure that focusses on the heater core, say by removing a heater core hose and pouring coolant into it, then reattaching.

If all that fails, you are looking at a water pump, hvac vent door actuator, or heater core problem I’d guess. If you could figure out a way to pump hot water through the heater core as an experiment, with the engine out of the circuit I mean, then you might be able to better isolate which of those it is.

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On a 19 year old car, It could very well be a partially plugged heater core.

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No offense, but I can send you numerous links to videos and Jeep forums on this exact issue. Perhaps your customers didn’t come back. I can also sent you photos of the numerous 99 to 04 Grand Cherokees, with the 4.0L, in the local junk yard because the average bill to correct this runs beyond the value of the vehicle. I mean no disrespect. I’m just telling you what I am finding and personally seeing.