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bypass the heater control valve?

ehall

ehall

NAXJA Member
NAXJA Member
Location
Southern Maryland
One of the things I'm considering for simplification is to eliminate the heater control valve, so that coolant can flow from the thermostat housing through the heater core and back to the water pump unimpeded. In my limited research I see that some of the later models had the the control valve eliminated by design, but the rest of the HVAC system is also quite different for those years as well. I assume that somebody here has already tried to take the hoses from a later model and put them on a pre-97... Do the hoses work okay? Any problems come up with running coolant through the core all the time? Any gotcha issues?
 
heater hose is cheap. you can buy it by the foot and simply replace the whole thing. or you can get a couple of unions from any parts store and just put them inline where the valve was. you can do it all for under $10
 
X2^^

I've not tried the newer hoses, but I definitely removed one and replaced it with a union and a flushing tee in the parking lot of the store for someone that had it break.
 
have either of you done that? the stock plumbing uses different sized fittings, so regular heater hose won't work work with the stock setup since one end of the hose is always either too large or too small. is this still a problem after eliminating the control valve?
 
I wonder this myself. My 00 has no heater control valve whereas my 90,94 and both of my 96's do. I would love to do away with the heater control valves which are a constant leaking mess. I would rather not do a bunch of plumbing to achieve this. Looking at the parts schematics of all the different year models the only difference is the heater core itself so there may be a different design to the heater core.
 
it's stretching my memory, but IIRC the connections at the heater control valve are all 5/8th hose. I seem to recall using a union as opposed to running all new hose due to the need for the hose to "neck down" (closer to the engine) and we didn't have the exact size on hand.
 
The outlet on the T-stat housing is larger than the heater core inlets. I use the GATES heater hose which is molded and is cut to length. It would reach past the heater control valve without issue.
 
My biggest question is does there need to be a bypass before the heater core? I guess there could always be coolant flowing in the heater core but cabin temps would be hotter in the summertime, AC would be working OT.

I noticed the YJ heater control valve has a different design than the XJ unit. It may be worth looking into.
 
as best I can tell, the temp blend door is going to stop any heated air from reaching the evaporator or the cabin when the setting is on "cold" anyway. I know that I've "done away" with the control valve on other brand vehicles without any adverse affect, so I'd say all should be fine.

With a good seal on the blend door and proper adjustment (so it seals well) I can't see any need for the control valve.
 
On my '92, I did away with the valve after it broke. I studied the broken-open valve, so that when I hooked the hoses directly to the core, the coolant flowed the same direction. The A/C hadn't worked for a few years, so I wasn't worried about the evap coil working too hard. And, I haven't noticed any excessive heat during the warmer months. The blend door does a good job of keeping the core isolated when the HVAC slider is on cold. I don't know if the '97+ HVAC is designed differently, maybe eliminating this troublesome piece was done by the beancounters.
I think the two hose sizes are 5/8" and 11/16", although I used the 5/8", and it worked fine. No one around here wants to stock the 11/16".
 
Check out 5-90's site, he's got a page on replacing the whole shebang with copper house plumbing - I intend to do this at some point but it's been on the back burner for a while. Just use 1/2" trade size copper pipe, it fits very nicely inside the stock heater core hoses. When my heater control valve on my XJ (daily driven! had no way to hit the JY) broke in half in my driveway, I grabbed about a foot of copper pipe, two 90 degree elbows, and soldered up a loopback for it that just fed the coolant back into the engine so I could drive while I waited for a new valve to become available. When this one breaks I'm going to just eliminate it entirely.
 
Correct. I got tired of blowing something like a C-note on moulded heater hoses, and whomped up the copper job in a half-hour (using material on hand.)

1/2" copper tube fits neatly inside 5/8" ID heater hose. For the two spots that "bell out" to 3/4" hose, simply solder on a straight union to increase the OD of the tube - problem solved.

I haven't refined the measurements yet, but after eliminating the heater control valve (which don't do much useful but make parts makers money, and adding a hose bibb fitting (for effective flushing) and a backflush valve (to force water to flow through the system backwards when closed and hose-fed,) I can honestly say I've been happy with the results.
 
i just took a bunch of brass fittings and teflon tape and made a piece that looks just like the vavle assembly minus the valve. everything works fine and the oem hoses will fit
 
I just used about 5 feet of 3/4" heater hoses and it worked fine. I know the hoses should be 5/8 but the stores dont carry them so I got the 3/4" and cranked down the clamps and it seals fine.
 
heyhar said:
On my '92, I did away with the valve after it broke. I studied the broken-open valve, so that when I hooked the hoses directly to the core, the coolant flowed the same direction. The A/C hadn't worked for a few years, so I wasn't worried about the evap coil working too hard. And, I haven't noticed any excessive heat during the warmer months. The blend door does a good job of keeping the core isolated when the HVAC slider is on cold.
Click to expand...
The diagrams in the FSM is kind of interesting. According to what is shown there, the heater core is not isolated but is instead kind of halfway exposed--when set to HOT the blend door forces the air to pass through the back of it and when set to COLD the air just passes in front of it. On the other hand, air is always forced to pass through the evaporator, and there is no bypass for it. So to the extent that the diagrams are accurate then the heater valve is important in that the heater core will always increase the air temperature just by the fact that air passes over it (but not through it). It will definitely make it harder for the A/C to do it's job which is unfortunate given that the compressor is on the small side.
 
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