Found during valve cover gasket install

[cruiser54]

Nah. I live in Arizona and bypass the heater control valves all the time. 

The blend air door is what switches the heat in reality.

[AZJeff]

4 hours ago, saveevryjp1998 said:

 

It's there so hot coolant isn't running constantly through your heater core and to have proper working heat. That vacuum line opens the flow of coolant through to the core. Without it, no heat. With it constantly flowing bypassed with straight hoses, your heater core is hot in summer fighting your A.C. Pick your loss but it's there for a reason. They don't just put parts on because they don't have a function. Plug the tube, and you'll always have air trapped inside a chronic never ending closed coolant system. It works ya to get you home but it will catch up to you eventually. The bypass the bypass idea is just another bad idea which helps give the renix closed system a bad name. They fail because of dumb stuff like this on top of people not understanding the function. Think of it like you house plumbing. Block a vent on your crapper and see how well the turds continue to flow. I always fix that part, it's cheap, and easy. I also know my coolant system is properly working and my heat that way. It's also a good way to rot out a heater core leaving uncirculated coolant 24/7 year round to then eventually leak or rot out a passenger floor pan if you bypass core entirely. Guys do this multiple ways to save essentially a free junkyard part. Look for a metal one pre 90. They rarely go bad and are easy to test by simply opening the valve manually and plugging the tiny hole with your finger. If it holds open, the diaphragm is good and holds pressure. It's that simple. I'd fix the vacuum line, plug it on the bypass, and you're likely going to have full working heat as well as an actual defrost. It's common for the renix vacuum lines to dry rot and fall off the bypass. Once you fix the line, make sure you get up to operating temp, open the heater valve to high, and run it a bit. Chances are you'll need to purge the air in the system and refill some coolant if anything coolant wise in the system has been changed since the last time the heat was working properly.

I am a retired mechanical engineer, and I fully understand WHY the valve was put in.  In fact, such valves were not uncommon on other brands/models at one time.

 

However….auto manufacturers deleted that feature after they analyzed and determined that AC performance was not impacted significantly by letting the heater core receive a constant flow of engine coolant at all times.   The driving reason for doing this, of course, was manufacturing part cost reduction and assembly simplification.   

 

It has been demonstrated by multiple XJ and MJ owners that deleting this valve does not seriously alter the performance of a properly functioning AC system.   (it should be noted that deleting the valve does NOTHING to reduce heater performance whatsoever.)
 

in other words, taking the valve out simplifies the heater hose routing, and has minimal, if any, noticeable effect on AC performance.  That makes it a win-win for most owners.

[cruiser54]

This^^^^  And you live in a way hotter place in Arizona than I do.....

[NC Tom]

8 hours ago, AZJeff said:

 

Thanks for the clarification. Sounds like the way to go.

[Armyvet25]

15 hours ago, saveevryjp1998 said:

I see people go through more expense and effort usually than to simply fix a broken vacuum line and purge the system for next to nothing potentially saving major headache replacing a core down the road. Depending on where you live and what you do with your Jeep, not having properly working HVAC could potentially be. You might reconsider that comment if you live anywhere there is a serious winter season and it's your daily.

Confused about what statement I should reconsider, like you I believe the OEM system is the way to go, I'm actually doing preventive maintenance with a flush and reinstalling an appropriate style replacement. Unless you think the world will end and my beer will sour

[Armyvet25]

looky what I just got in the mail

 

[cruiser54]

send it back. 

[Armyvet25]

Just now, cruiser54 said:

send it back. 

nope lol

[Ωhm]

88 FSM

FYI Check out Water Valve truth table.

 

heater system.pdf

[cruiser54]

That heater valve, likely made offshore, will work great. Until it explodes unexpectedly one day, emptying the cooling system in seconds. 

 

Or maybe you'll be lucky like me and bump the HCV with your elbow removing the oil filter from a totally warmed up XJ. 

Have it explode and shower yourself in the face and upper body with boiling hot coolant. Something to look forward to down the road I guess. 

[Armyvet25]

2 hours ago, cruiser54 said:

That heater valve, likely made offshore, will work great. Until it explodes unexpectedly one day, emptying the cooling system in seconds. 

 

Or maybe you'll be lucky like me and bump the HCV with your elbow removing the oil filter from a totally warmed up XJ. 

Have it explode and shower yourself in the face and upper body with boiling hot coolant. Something to look forward to down the road I guess. 

Edited February 25, 2023 by Armyvet25
Decorum

[gogmorgo]

It’s actually totally normal for the a/c and heater to “fight” each other. The a/c comes on in defrost to pull help pull moisture out of the air. The water condenses on the cold evaporator instead of blasting humidity at your windsheild. 
Minuit did a very well done comparison between a deleted heater valve and one still in the system and found there was a noticeable increase in vent temperature with the valve deleted while running the a/c. 
On the flip side, if you never run the heat and the valve is still in the system, the closed valve is very likely to lead to an accumulation of corrosion or otherwise that could block the heater core or stick the valve closed, meaning you won’t have heat when you need it. 
 

There are advantages and disadvantages to both having it and getting rid of it. Depending on what you prioritize, based on the climate you live it or whatever personal reasons. There’s really no sense fighting over it. 

[Armyvet25]

1 minute ago, gogmorgo said:

It’s actually totally normal for the a/c and heater to “fight” each other. The a/c comes on in defrost to pull help pull moisture out of the air. The water condenses on the cold evaporator instead of blasting humidity at your windsheild. 
Minuit did a very well done comparison between a deleted heater valve and one still in the system and found there was a noticeable increase in vent temperature with the valve deleted while running the a/c. 
On the flip side, if you never run the heat and the valve is still in the system, the closed valve is very likely to lead to an accumulation of corrosion or otherwise that could block the heater core or stick the valve closed, meaning you won’t have heat when you need it. 
 

There are advantages and disadvantages to both having it and getting rid of it. Depending on what you prioritize, based on the climate you live it or whatever personal reasons. There’s really no sense fighting over it. 

Well spoken

[NC Tom]

Okay, so I plugged the line the and valve. We'll see how it goes.

[NC Tom]

On 2/21/2023 at 6:32 PM, saveevryjp1998 said:

They fail because of dumb stuff like this on top of people not understanding the function.

Okay, now I'm cornfused and I definitely do not understand the function.  However, @cruiser54 seems to have been through this many, many times with no issues and real-world experience holds a lot of weight with me. I do understand that typically, parts aren't added just for looks. But, it is possible that what was thought to be needed in 1988 turns out to be incorrect.

Just playing devil's advocate, not picking sides.

[gogmorgo]

4 hours ago, NC Tom said:

Okay, now I'm cornfused and I definitely do not understand the function.  However, @cruiser54 seems to have been through this many, many times with no issues and real-world experience holds a lot of weight with me. I do understand that typically, parts aren't added just for looks. But, it is possible that what was thought to be needed in 1988 turns out to be incorrect.

Just playing devil's advocate, not picking sides.

 

It definitely serves a function. It reduces heat in the cab. This has been demonstrated very thoroughly. Whether that function is important or even useful is going to be a matter of personal opinion.

To me personally, having a vacuum leak prevent me from getting heat into the cab when it's -40° is a significantly more important concern than keeping the cab as cool as possible at 100°F, simply because we spend a lot more of the year below 0°F than we do above 80°F up here, and in general preventing coolant from stagnating anywhere in the system is important for heater performance. But for someone with a climate where 100°F isn't even considered extreme, keeping the hot coolant out of the cab makes a ton of sense, especially with an a/c delete truck. Most of us are somewhere in between, which is where it comes down to personal preference whether risking heater performance is more important than compromising a/c.

Although I'll point out that up here, the requirement for cold-weather performance means you don't get away with dumping straight water in, and corrosion inhibitors in antifreeze go a long way towards maintaining cooling system health and preventing your heater control valve from seizing or leaking. When I pulled my ZJ's original water pump out at ~289,000km, the inside of the block was still shiny bare cast iron. Vacuum leak leading to a non-operational valve is a far more likely occurrence. And another testament to that, an XJ I had a while back, the previous owner had it for about 15 years and said the heat never worked worth a damn since they'd bought it. It sat five years before I got it. I found and fixed a vacuum leak, and got full function of the heater valve back after it hadn't been opened for 15 years, more than adequate heat even at -45°C. I hadn't even put new fluids in it because it was just a parts rig. Look after your cooling system and it'll look after you.