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Old 02-17-2014, 04:29 PM   #1
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Odd heater issue

97 TJ, 4.0, 3sp auto, no air
I have an odd heater issue. My heater heats just fine but I can only get air through the defroster vents OR the floor ducts; the heater control does noting regardless of the setting I put it on. When I turn the heat on it starts with defroster, for awhile, at some point I hear a loud Thump and my heated air is now coming through the botton ducts (the faster I run the motor the sooner this will happen). Turning everything off and starting again will just repeat the cycle.
I have removed and (visually) checked the heater controls and all seems okay and I am not sure where to go from here.

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Old 02-17-2014, 04:46 PM   #2
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You've got a vacuum line leak, look under the hood and follow the line on this diagram.



common problem areas are the fitting on the intake, #5,




or lay under the Jeep behind the right front tire, look up along the frame for the line that connects to the #8, it often gets cracks in it and will cause intermittent problems.


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Old 02-17-2014, 06:58 PM   #3
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But if it's changing itself you'd think the problem would be under the dash, blend door perhaps?
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Old 02-17-2014, 07:00 PM   #4
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Is it always on defrost from a cold start? If yes then vacuum leak.
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Old 02-17-2014, 07:03 PM   #5
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Quote:
Originally Posted by Junky_One View Post
But if it's changing itself you'd think the problem would be under the dash, blend door perhaps?
Since it randomly changes itself, I'm leaning to it being a crack in the line at #8, not fully broken, just cracked.

The canister at number 8 is the vacuum reserve canister, its job is to maintain vacuum pressures to the hvac, cruise control while the engine load fluctuates, when there is a leak in the system it will cause erratic behaviors.

The faster the op runs the motor, the quicker they run out of vacuum pressure.
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Old 02-17-2014, 07:05 PM   #6
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But it's response to changing temperature within the system is what's odd. He says the faster the blower runs the sooner the thump comes.
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Old 02-17-2014, 07:13 PM   #7
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I see what you are saying, I read the first post and assumed when they said "the faster I run the motor the sooner this will happen" they were talking about the faster the idle in the engine, like driving under load.

If the thump comes from a faster running blower motor, then yeah you're right and should look into the blend door. blend door actuators.

My mistake.
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Old 02-17-2014, 07:23 PM   #8
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No you had spot on advice I just couldn't understand a faster running engine "motor" creating more vacuum to make it switch.
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Old 02-17-2014, 07:27 PM   #9
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Going off what I was trying to say, the faster the engine runs, it takes vacuum away, from the vacuum canister, that combined with a crack in the line it'll make the vent position move.

Mine was cracked last summer, and when I'd go up hill or give it some throttle from a stop I could feel my air move around until I let off. I fixed the crack and everything was fine.
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Old 02-17-2014, 07:42 PM   #10
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Yea I get you there, I'm not sure what order the blend door travels but I know for a fact it starts on defrost everytime it starts from 0 vacuum. Defrost is default and if it moves from there then it's got some straight up dyson action going on.
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Old 02-19-2014, 08:34 AM   #11
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Thanks for the info. To clarify the faster I run the heater motor the faster it "thumps" and changes. I will check the vacuum line especially the point indicated but in later posts it sounds like it may be the blend door actuators; if so, how do I check.

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