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Heater failure problem

I have a 2002 Dodge ram 1500 quad cab. v8
Heater does not produce enough hot air, usually around 90 deg.
Within the last couple of years, it has had a new radiator, new heater core and new blend door.
I checked all the heater doors by creating a few small holes in the plenum when I replaced the blend door and they all appear to work fine.
I back washed the heater core and it worked really good for a couple days. Then back to little heat. I took to my mechanic and with the same results. Actually, I took it back to him later and he did the same thing again with the same results. Last summer I flushed the whole engine and used a cleaner on it. It did the same thing except the heater seems to be working slightly better.

I thought about replacing the heater core again but that doesn’t make any since unless it is partially plugged and isn’t being completely flushed out.

Any suggestions?

My two guesses would be a thermostat sticking open at times, or heater hoses collapsing internally.

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What is the appearance of the coolant coming out of the heater core when you back flush it? Is it clear or rusty?

I had the same problem with my 98 and 02 Dakota’s and the problem with both were that the heater core’s were plugged.

I don’t know if the whole dash has to be removed from the Ram’s as with the Dakota, but when I found the whole dash had to be removed…I balked. Stupid design if you ask me.

I think this may come from people adding non distilled water to the coolant, and the minerals in that water clogged the core.

I opted to flush mine on both trucks.
I pulled off both heater hoses, flushed a little clear water through to remove the coolant.
Then I attached clear hoses from the core to an electric drill pump. I filled the hoses first with “Lime away” then attached to the drill and ran the pump. I ran the pump one minute then let it sit for 5 minutes, pump one minute and let it sit for 5 minutes. I did this about 6-8 times, then reversed the hoses and did the same in the other direction.
I then flushed it all with a gallon of clear water. With the clear hoses you can see that things are flowing.

I know some will argue that “Lime away” is not intended for this, but I had no problems. Besides, I felt that if it damaged the core…it was or it’s last legs anyway and I’d only wasted the time flushing a core that had to be replaced anyway. Plus before I attempted this, I soaked a piece of aluminum foil in Limeaway and it didn’t even tarnish it in two hours.
A few cautions though. Use enough clear hose that you are working the drill on the outside of the fender, so any spills are on the ground and not on components under the hood. Also flush everything good where you did spill.

I also did an entire drain of the coolant to remove any impurities from the system, and refilled with coolant and distilled water.

Worked like a charm for me.


I pulled the thermostat and ran a test on the stove in hot water. When the
water reach the correct temperature it opened and closed when it cooled
back down.

I will give your idea of flushing it with lime away. That sounds like the
best idea yet and make since.

If you look on the bottles of LIME AWAY and CLR, it states:


Guess what heater cores are made of.


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After replacing a HVAC door or actuator motor the system should be calibrated with the DRBIII scan tool. If you can see the blend door(s) move fully from cold to hot this may not be the problem.

The 5.9L engine has a steel heater hose return tube at the water pump, if poor coolant or water was used this tube can rust inside and stop coolant flow.

The 4.7L engine has long aluminum heater tubes leading to and from the heater core, these are less susceptible to damage but should be inspected.

By testing the temperature of the heater hoses going into and out of the heater core you can judge if there is a flow problem. If there is believed to be a coolant flow problem remove the coolant return line from the water pump, connect the line to a hose and into a bucket. With the radiator cap off start the engine, there should be a strong flow of coolant, a gallon in 15 to 20 seconds.

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Good advice above. If all that doesn’t work, the two times I’ve had this problem, the cause was some air remaining in the cooling system. Air bubbles cause more problems with the heater core than other places in the cooling system b/c the heater core, being under the dash, is usually the lowest point. You know how hard it is to push a basketball under water? Same thing w/air bubbles. The buoyancy force of an air bubble in either of the two hoses going to the heater core can partially block the flow of coolant, yielding you less heat. It may be that when you back-flushed the heater core that one time, you cleaned out some of the air inadvertently, and that’s the reason the heat situation improved.

That is a possibility but my truck has bleed valve on the return hose to
the radiator which I bleed several times after the back wash. I suppose
that the heater could still have an air bubble.

First back wash it was pretty brown but turned clear pretty quickly.

" First back wash it was pretty brown but turned clear pretty quickly."

That’s the reply I was waiting for. You probably have a lot of rust sitting in your cast iron block. I once bought an old Chevy Nova that had the same problem. I had to repeatedly back flush the heater core and finally had to replace it but the problem persisted. The previous owner had badly neglected the cooling system.

You may need a professional chemical flush. The chemical flushes that the consumer can buy don’t work very well.

Your truck must have the 4.7L engine, it has a (bleed) plug in the water outlet. It isn’t necessary to remove that plug when refilling the system, I think it is there for fast filling the system on the assembly line. Make sure the reservoir is full and the air will purge by itself.

It is hard to believe you have so much iron debris in your cooling system to block a heater core in a few days, does the coolant look polluted? Has it been changed with each cooling system repair?

Yes, it does have a 4.7 V8. Not sure if the mechanic used chemicals but he
did flush it twice. I used the best chemicals that Autowise had available.
I flushed it twice myself also. I did check the heater core hoses both
inlet and discharge. Was 130.5 deg in and 120 deg. out. Which only produces
about 85 deg. on the heater core

Only 130 degrees at the inlet to the heater core? That seems pretty low to me. There should be hot coolant near to the same temperature as the engine coolant (which should be in the 180-210 degree range) coursing through both those hoses. Are you certain there’s a good flow of coolant through it? If not much flow, that would explain why those temperatures you measure is so low. What if you removed the outflow hose and got a visual on the amount of coolant making it through the heater core? Seems it would be fairly easy thing to do, and might provide a clue what’s going on.

I understand your concern @Tester about using “lime away” on aluminum, and am well aware that heater cores are aluminum.

I tested it well…for much longer than I would have the chemical in the heater core.
As I stated it did nothing to the foil and a thin strip of aluminum that I soaked.

I presume that they put that warning on the bottle to keep you from using it on a highly polished fixture
and ruining the finish.

I have done this same type of flush to at least a half dozen heater cores …other than mine… and have had no problems.Never had a car come back and all were owners that still frequint my shop. So I’m sure they would have complained to me if it had done damage.


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You have to understand that if do something to your own vehicle that may cause expensive damage, that’s one thing. And you end up being the one to replace the part you just destroyed from not following the warnings.

But to tell someone you don’t even know to follow your suggestion where the product clearly states not to use it in that situation, are you willing to pay for the expensive repairs that may result from that suggestion?

I didn’t think so.


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It would seem that Yosemite has used good judgement

He knows the repairs are successful, because he’s seen those cars time and time again.

This is like the discussion with the pinch bolts being reused. Sometimes a savvy mechanic can get away with doing something that is technically wrong, but the problem is resolved, and the customers are happy

“I presume that they put that warning on the bottle to keep you from using it on a highly polished fixture
and ruining the finish.”

I agree that is the most likely explanation for the warning. Those are my non-scientific thoughts, FWIW

I appreciates everyone’s concerns.If I do decide to try it. I will let you
all know. I have a couple other things to try first.

Thanks to all of you guys,

A local mechanic tells me he pours straight CLR into clogged heater cores and after and hour or so flushes it out both ways with a hose. He’s done dozens of customers’ cars with good results and never a come-back.

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