Any side effects on heater core bypass with AC?


Bird Dog

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Jan 8, 2017
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Lexington, SC, United States
It is time to replace the heater core on my TJ. I don’t have the time right now to get to it since it is my DD so I am going to bypass the core to keep the coolant out of the floorboards for now until the weather cools off In the Fall. My general question is, are there any side effects of bypassing the core with the AC? I rarely use the AC but it is nice to have when I need it. I know I have to evacuate the system to change the core but are there any problems running the AC without a functional heater core? Thanks.
 

Steel City 06

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Mar 12, 2019
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Pittsburgh, PA
It should actually improve the performance of your A/C a small amount. A lot of modern cars have an automatic core bypass for when you max out the A/C.

Old Air Products makes a decent electronic bypass kit to improve AC performance, though it wouldn’t work to stop a leaking heater core.

The cheapest and easiest way is to use a double ended barbed fitting (you might need a size adapter) and merely plug the two heater hoses into each other end to end. Secure temporarily with a couple of worm drive clamps.


Be sure to set it up as an actual bypass that allows coolant to flow through the hoses to and from the water pump.

Do not deadhead the flow by plugging the hoses or installing shut-off valves. The TJ uses the heater core as the recirculation line, and by cutting it off you stop all forced coolant flow in the block until the thermostat opens.

If you do have to deadhead the flow in the tubes for whatever reason, drill a good size hole in the thermostat (I would do at least 5/16”) to use the radiator as the bypass to make sure there is some bypass flow. This is not preferred as it will take the engine longer to warm up, and performance, fuel economy, and emissions will suffer.

But deadhead the flow with no bypass and you risk boiling coolant in the block and creating huge expansion stresses as localized heating is not smoothed out by coolant flow before the thermostat opens.
 
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Viking Jeeper

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Jan 10, 2019
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Jacksonville FL
My heater core is bypassed and it has only come up once but with the core bypassed you cannot regulate cool air with the A/C running. It is always full cold even if you set the temp dial to full hot. Makes perfect sense, I just never considered it.
 

Jeffseven777

Man Bear Pig
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May 25, 2021
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Ohio
Yea doing the heater core is about a 4-6 hour task + or - how much drinking you do. i did mine did all my light bulbs and stuff at the same time radio stuff. just make it worth ur time.
 
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Bird Dog

Bird Dog

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Jan 8, 2017
Messages
135
Location
Lexington, SC, United States
Thanks for all the great responses. The heat coming from the floorboard actually does better than the heater itself. I thought about making the bypass permanent but the big problem comes from defrosting the windshield with no heat source. We don’t have it like Floridians but there are a few spells of cold here in SC. I can still probably put off the replacement until December!