Heater core bypass—hoses as short as possible?

J

JohnJ78

TJ Enthusiast
Joined
Dec 26, 2020
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Australia
I put new heater hoses all the way to the firewall where the heater core connects, with the intention of connecting up the heater core.... but then I decided to just put a U peice in and bypass the core entirely in case it leaks. Not like I'll ever use it where I live.

But now I'm thinking, maybe I should just put the U piece as far down towards the thermostat housing as possible. Having those long heater hoses draped over my engine is surely adding to my heat problem.

Anyone else done this?
 
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Blocking it off puts extra pressure on the pump before thermostat opens. Not a lot, but on an old pump it could (could) cause pump to start weeping.
Dosen't matter where you loop the heater hoses at, just do it so they aren't laying on the hottest parts of the engine.👍
 
You need the bypass flow somewhere. In the TJ there is no separate bypass circuit, only the heater core.

The bypass flow allows the coolant to circulate in the block and prevent hotspots and cold spots from forming in the engine. Without it, you will get very uneven block heating, resulting in localized overheating before the thermostat ever opens and possibly significant engine damage.

The only time you could safely deadhead the heater core flow is after the thermostat has significantly opened. But before that you risk creating hotspots.

If you had no other choice but to block the flow, you could drill a significantly sized hole in the thermostat to create bypass flow. This may result in overcooling, longer warm up times, poorer emissions and fuel economy, and moderately increased engine wear, so I wouldn't recommend it.

A shorter bypass directly from the thermostat housing to the water pump should be fine. It may result in increased bypass flow from the reduced resistance, but it likely won't be a problem except in the worst use case scenarios. In that case, you would want to add a flow restriction (not a blockage) to reduce bypass flow.

If you're frequently overheating, you could set up the heater hose to dump into the radiator intake tube using something like this to ensure that 100% of the water is cooled prior to reuse, however this will also result in longer warm up times and possibly overcooling.
 
In all my years of being a technician, I have never heard this.
I've built street rods, race cars all without heater cores, capped at the water pump and have never had an issue.
Well, guess you learn something new everyday.
 
Some cars actually have a separate bypass from the heater core. Those can be dead headed without issues.

A lot of luxury cars also use a bypass valve that bypasses the core during high A/C use to maximize cooling. These valves can be easily retrofitted to a TJ, though they are not designed to fully isolate the heater core, but instead just redirect the flow.
 
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