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post #1 of (permalink) Old 01-13-2007, 08:52 AM Thread Starter
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98 TJ Heater PRoblem

I have a 98 TJ 4 cylinder. The problem I have is the heater gets "warm" but does not get hot. I replaced the thermostat thinking it would help but it didn't. I have been thinkingf replacing the heater core thinking it may not be flowing very well. Has anybody experienced this problem or have any suggestions??

Thanks in advance!
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post #2 of (permalink) Old 01-13-2007, 09:11 AM
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Re: 98 TJ Heater PRoblem

There could be many causes. They've got to be eliminated one, by one. So, as a first step, look at the Factory Service Manual, Section 24, page 24-14.

[ QUOTE ]
MAXIMUM HEATER OUTPUT

Engine coolant is delivered to the heater core through two heater hoses. With the engine idling at normal operating temperature, set the temperature control knob in the full hot position, the mode control switch knob in the floor heat position, and the blower motor switch knob in the highest speed position. Using a test thermometer, check the temperature of the air being discharged at the heater-A/C housing floor outlets. Compare the test thermometer reading to the Temperature Reference chart.

If the floor outlet air temperature is too low, refer to Group 7 - Cooling System to check the engine coolant temperature specifications. Both of the heater hoses should be hot to the touch. The coolant return heater hose should be slightly cooler than the coolant supply heater hose. If the return hose is much cooler than the supply hose, locate and repair the engine coolant flow obstruction in the cooling system. Refer to Group 7 - Cooling System for the procedures.

OBSTRUCTED COOLANT FLOW

Possible locations or causes of obstructed coolant flow:
<ul type="square">[*]Pinched or kinked heater hoses.[*]Improper heater hose routing.[*]Plugged heater hoses or supply and return ports at the cooling system connections.[*]A plugged heater core.[/list]If proper coolant flow through the cooling system is verified, and heater outlet air temperature is still low, a mechanical problem may exist.

MECHANICAL PROBLEMS

Possible locations or causes of insufficient heat:<ul type="square">[*]An obstructed cowl air intake.[*]Obstructed heater system outlets.[*]A blend-air door not functioning properly.[/list]
[/ QUOTE ]

As you can see, this can be a combination of electrical, vacuum and mechanical problem. Where would you like to start? I'd start by feeling the two heater hoses in the engine bay to see if they're both hot. That tells you that there is coolant flowing into the core, and back out.
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post #3 of (permalink) Old 01-13-2007, 10:14 AM
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Re: 98 TJ Heater PRoblem

Yes, definately start there.

Does the air conditioner get cold OK? It may not as well.

My '03 Rubi had a problem with the blend door not moving all the way. It wouldn't get cold, or hot - just "in between."
The blend door mixes the air - hot and cold - but somewhere in the wiring the ground was bad. The door moved a little, but not enough to isolate the two temps of air.

It looked like a nightmare to tear it all apart looking for a bad ground connection - so we just ran a new ground to that point. No negatives to that -- Whatever else was on that side of the bad connection would work better now anyway.

As I recall, you should be able to see the blend door lever operate from under the glovebox.
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post #4 of (permalink) Old 01-14-2007, 08:10 AM Thread Starter
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Re: 98 TJ Heater PRoblem

Well, now that you mentioned it the AC did stop cooling this past summer. I will take a peek and see if the air blend door is working properly. I'll also check the flow through the heater core. When I replaced the thermostat there the coolant looked good in the radiator but when I drained it, it looked VERY dirty which is why I was suspecting a plugged heater core.

Thanks for all the help!!
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post #5 of (permalink) Old 01-15-2007, 06:59 PM
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Re: 98 TJ Heater PRoblem

Is the jeep running at a normal temp? 1 time (not at band camp) our ambulance defroster wouldn't keep up with its job. looked at the engine temp and it was cold...pulled over and snaped the grill covers in place and temp went back to normal, and the defroster started to work again. go figure.
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post #6 of (permalink) Old 01-16-2007, 01:24 PM Thread Starter
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Re: 98 TJ Heater PRoblem

It is running at normal temps. Before I replaced the thermostat is was running a bit cooler. I figured the previous owner may have installed a cooler unit. I am going to check the air blend door and see if it is working properly this weekend.
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post #7 of (permalink) Old 01-21-2007, 01:56 PM Thread Starter
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Re: 98 TJ Heater PRoblem

I've check the air blend door and it appears to be working fine. I checked the hoses going into heater core. The upper hose seems to be about twice as hot as the lower hose.

How difficult is it to replace the heater core? Does the entire dash have to come out to replace it???
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post #8 of (permalink) Old 01-21-2007, 02:17 PM
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Re: 98 TJ Heater PRoblem

Next step - disconnect the colder hose - hold it high up to prevent much coolant loss.
Start the engine - you should get a big stream - flood.
If not, just a dribble, core is plugged. Or valve is restricting, hose is kinked etc


If it's a gusher, core's ok, the heated air is escaping or being diluted somewhere else.
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post #9 of (permalink) Old 01-21-2007, 04:07 PM
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Re: 98 TJ Heater PRoblem

That's a good test, although the big difference in hose temperatures suggests that the core is working fine, giving off heat and cooling the return water. I'd bet a cold beer that it's a door problem.
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post #10 of (permalink) Old 01-21-2007, 05:18 PM
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Re: 98 TJ Heater PRoblem

I'm not sure if the following will be much help since my experience is with the 4.2/4.0...not real sure about the 2.5.

The above is a good test. I also use a water hose to flush and confirm that the heater core is clean and flowing....disconnect the input (at the engine) and then plug int the water hose and turn on the water....you will be able to tell quickly if it's flowing right.....the usual key is if you get a face full of water due to the back pressure....if it's kind of blocked you will see dirt coming out....if so, reverse the input and back flush...this will help to push junk out of the passages...especially the stuff thats too big to fit through the passages.

A problem I had on my 4.2 some time ago was a slightly blown head gasket. I think the leak was in just the right place so that it created enough pressure so as to prevent the water flow through the intake manifold (which on the 4.2 is the input source for the heater). As a result, little to no flow through the heater core.

Once you have confirmed that you don't have a restriction in the heater core, then do the test mentioned earlier...unplug the return side and see how much flows out.

This is one of those posts that requires you to post the findings when you figure out what is wrong....failure to do so could result in a constant barrage of harassment from us until you post the results.....don't let us down.....
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