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Unread 10-22-2014, 09:57 PM   #1
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1994 ZJ 
Join Date: Oct 2014
Location: Louisa, Kentucky
Posts: 1
Zj heater help please

I am new to this site. But I really need some advice on my heater. for a while now it has been giving me problems. First off if you let the Jeep warm up then turn the heater on the temp gauge falls half way back down. Heater is still warm though. When you switch from vent to defrost it is all cold air on defrost. switch back to vent or floor and its warm air. Once you start driving the vents then turn to cold air as well, especially on really cold mornings and nights. I have flushed and back flushed heater core numerous times only to get clear water each time. blend doors work great cause I can switch from cold to heat. Also if I leave my fan speed knob on low I get hot air from vents. The very confusing part is this however, if I reach 55 mph or anything above 2,000 rpm the vents will lose all air coming from them. once I get back under 55 mph or 2,000 rpm I get air flow again. Has anyone had problems like this before? Jeep is a 94 zj with 4.0

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Unread 10-23-2014, 12:42 AM   #2
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1996 ZJ 
Join Date: Jan 2012
Location: Central, Ohio
Posts: 3,843
It sounds like you have a vacuum leak with your front vents cutting off when reaching higher speeds or maybe when climbing a hill. Most often it will default to defrost with lack of vacuum to the HVAC switch which does the routing. This is for a manual heating/AC system and not for ATC (automatic temp control).

Not sure if the vacuum line routing is exactly the same but usually it's the line going through the firewall next to one of the AC evaporator tubes from the "T" to the HVAC switch or the tube under the battery tray has rotted/come off. The bottom of the battery box is the vacuum reservoir for storing vacuum when the engine isn't drawing much like at your higher speeds/under a load.

This vacuum line on my 96 4.0 is as follows. It connects to the far driver's side of the top of the intake manifold then turns towards the passenger side while traveling along the top of the firewall. Then it T's off as already mentioned, then makes a 90 degree turn along the fender towards the front, T's off to the vehicle speed module (cruise control) then travels along the fender until it attaches with a hose under the battery box. Check the whole line for loose connections, cracks, or broken plastic line.

Here's a photo where it T's through the firewall to the switch and welcome to the forum! We'll see what this does then work on the heat if it doesn't solve it. Good luck!

PS. strange your temp gauge would drop half way down once you turn the heat on. Sure you don't still have air trapped in the cooling system and can you hear any gurgling in the heater core? If you grab both your heater hoses outside the firewall going into the core are they both warm/hot the same? What's your normal operating temps?

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 93 4.0 XJ (spare), 96 4.0 XJ (son's)

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Unread 10-25-2014, 03:17 PM   #3
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Join Date: Nov 2011
Posts: 409
check the connection at the T where it hits the manifold for debris internally. friend's XJ had that issue. also check the vacuum line to the cruise control servo, and going to the vacuum res under the battery.
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