Heater core bypass

The heater core is leaking on the ZJ. Its summer here and I don't feel like taking out the entire dash right now. Any issues with bypassing it until I feel like dealing with it?

I'm pretty sure the first heater core bypass happened the week after the first heater core. 

Cap the lines, otherwise you are short circuiting the radiator, with potentially less cooling.

mlwebb said:

Cap the lines, otherwise you are short circuiting the radiator, with potentially less cooling.

This can happen depending on the layout of the engine's cooling system - I don't know if that's the case with this engine model or not, but it's something to be aware of, although it's not universally true.

Gameboy is right.  It depends on the engine.  Engines like small block chevys or Cadillac 472/425/500 have an internal bypass so you can cap the lines.  Engines like early Pontiacs, the bypass IS the heater circuit, so if you cap the lines, you get no flow through the engine until the thermostat opens.

I can't think of any situation where the bypassing (hooking the in and out hoses together instead of through the core) is a bad thing.  The only "short circuiting" of the radiator is that you remove a very small restriction through the core.  The amount of flow increase you allow to bypass the radiator is negligible.  Suffice it to say, if you suddenly have an overheating problem after bypassing the core, something was about to fail anyway.

The core in the LeMans started leaking, so I just pulled the hoses off the core and stuck the supply line onto the return nipple on the head.

There are engines were hooking the two hoses together is problematic (Nissan L6's for one). Folks normally use heat when its cold, so a heating circuit is normally closed at max cooling loads. So yes, check your particular engine cooling diagram.

On a V8 ZJ, you can either bypass it or plug the heater circuit, either is fine.  There's no flow control valve in the heater lines and there's another bypass circuit, so the heater circuit isn't required at all.  

As a heads up, if you've got a helper for a few parts of the job, you can do the heater core and A/C evap in a day (figure 10-ish hours, give or take a bit).  If you're just doing the heater core, you can probably do it in 8 hours.  

I did the core/evap/blower in the XJ last fall in about 6 hours. 

The core was bypassed most of last summer with no issues. 

I'm kind of leaning toward planning a full weekend and doing the core, the evap, and receiver/drier too.  I've got a slow leak somewhere in the AC as well, so while I've got the core bypassed I'll drop some UV dye in the AC and maybe I'll luck out that the leak will be in that part of the system.

I think I'll just run a heater hose from the steel tube on the driver's side to the tube joint on the psg valve cover and call it good for now.

Iono, man. Sure, you don't need it now. But when it gets cold, you're going to kick yourself, for not fixing it, when it was warm, and you're freezing your nuts off in November, and have to tear it down, then. 

Food for thought... 

Well, I'm in Georgia, so my warm season is longer than most. And you may be right but I'm just trying to put it of for six or eight weeks. The leak is getting bad enough that I need to do something about it losing coolant NOW, but I don't have a weekend to spend trhowing parts at it. 

Besides, I have a Mustang to drive if it's that cold.

EDIT: Oh, and I have a heated shop. So I wouldn't be fixing it in the cold anyway.

Toyman01 said:

I did the core/evap/blower in the XJ last fall in about 6 hours. 

The core was bypassed most of last summer with no issues. 

Yeah, but you're the same guy who swapped a 7.3L diesel in about 6 hours, so you don't count!  Haha!!