Does It Matter? Heater hoses

No Heater Control Valve

Does it matter which heater hose goes from/to the t-stat/water pump...top or bottom. Which way does the flow go? t-stat to heater to water pump OR water pump to heater core to t-stat?

Not to me. But then again, my heat works and my Jeep isn't overheating. Just kidding!!

Generally, a heater core has a larger inlet than outlet but it's not critical. Did you pull your CTS and run water through the water pump opening when you had the pump off?

Does it matter? not really. But the hose sizes being different size, it is difficult to get them switched. adapters are available to switch them.
The flow of water is from the t-stat to the small heater core port (5/8") and from the larger core port (3/4") back to the water pump.

Switching the hoses can be effective if there is loose debris blocking the core.

Thank You.

ParadiseXJ said:

No Heater Control Valve

Does it matter which heater hose goes from/to the t-stat/water pump...top or bottom. Which way does the flow go? t-stat to heater to water pump OR water pump to heater core to t-stat?

Click to expand...

I doubt it matters which way coolant flows through the heater core - but I keep it OEM anyhow, just in case (it's not critical, since it's not relied upon for rejection of operating heat, and I doubt the HVAC system really cares - as long as it gets heat.)

cruiser54 said:

Not to me. But then again, my heat works and my Jeep isn't overheating. Just kidding!!

Generally, a heater core has a larger inlet than outlet but it's not critical. Did you pull your CTS and run water through the water pump opening when you had the pump off?

Click to expand...

My heat works...boy does it work. But as you know I'm having the overheat problem. Somehow I refuse to believe that a 1988 Jeep can be so frikkin' complicated, so I'm still going the "simple" route. Yes, I did pull the CTS and also the block drain plug, nuttin'! No goop, rust or anything but clear water came out. I flushed it thoroughly!!

5-90 said:

I doubt it matters which way coolant flows through the heater core - but I keep it OEM anyhow, just in case (it's not critical, since it's not relied upon for rejection of operating heat, and I doubt the HVAC system really cares - as long as it gets heat.)

Click to expand...

It was just a thought, since I've gone through, replaced and/or repaired the entire system EXCEPT the radiator, thus far. When I pulled the WP I noticed a bit of corrosion on the tips of the vanes and on the little deflector inside the pump in the "return" hole, just south of the hose/pipe.

A quick overview...

In order to promote coolant flow through the core, the design Engineers selected hoses of differing diameters. The smaller line feeds the core from the thermostat housing and the larger return to the water pump intake. This source is a thermostat bypass so that flow is always available.

How it works is that when you feed a finite volume of a working fluid into a larger space the pressure drops. It is the pressure drop that promotes the flow. Not all vehicles employ this strategy but, the Jeep does.

So, to answer the OP's question, yes it matters as the input/output connections at the core are not the same diameter.

If memory serves, the core input line is 5/8" and the return is 11/16". Not a big difference, but enough to cause the pressure drop required.

It is all about pressure differentials. Simple physics.

The intake side of the pump is at a lower operating pressure than the output. I realize that is an obvious thing to say, but some folk are just starting out learning how things work.

IMO, it is the core being located above the engine that mandated this arrangement.

Maybe someone else with a stock 97+ hose setup could look at theirs as well, as I'm curious about that explanation. On my 97, the larger hose connects the thermostat housing and the core, and the smaller hose is the return to the water pump. It was setup this way when I bought it in '02, and the Gates hoses I have now fit exactly the same as the Mopar hoses they replaced.

A couple of older pics to illustrate; my HDD bricked and I lost everything recent.
http://s266.photobucket.com/user/AHhub/media/forum stuff/DSCF5589.jpg.html

http://s266.photobucket.com/user/AHhub/media/forum stuff/DSCF5590.jpg.html

I know we touched on this in another thread, and there was an odd illustration in the FSM regarding the hose layout, but my hoses only went on one way, and that's as shown. Thermostat housing to upper core fitting, lower core fitting to water pump. I'm under the impression the water flows from thermostat housing to core to water pump. Is it actually the other way around?

hubs97xj said:

Maybe someone else with a stock 97+ hose setup could look at theirs as well, as I'm curious about that explanation. On my 97, the larger hose connects the thermostat housing and the core, and the smaller hose is the return to the water pump. It was setup this way when I bought it in '02, and the Gates hoses I have now fit exactly the same as the Mopar hoses they replaced.

A couple of older pics to illustrate; my HDD bricked and I lost everything recent.
http://s266.photobucket.com/user/AHhub/media/forum stuff/DSCF5589.jpg.html

http://s266.photobucket.com/user/AHhub/media/forum stuff/DSCF5590.jpg.html

I know we touched on this in another thread, and there was an odd illustration in the FSM regarding the hose layout, but my hoses only went on one way, and that's as shown. Thermostat housing to upper core fitting, lower core fitting to water pump. I'm under the impression the water flows from thermostat housing to core to water pump. Is it actually the other way around?

Click to expand...

Output flow from the engine is always from the thermostat housing on the 4.0.

inlet on the heater core varies from application to application, typically, the larger port is the out-flow, but not in every case, and, it seems, those years without the heater flow valve, the larger port is the inlet.

The heater core doesn't care which way the water is flowing, it still transfers heat to the air flowing though it.

Hey ParadiseXJ, without re-hashing everything you have done in upgrading your system, there is the possibility that your system is too efficient--by that I mean it is circulating coolant too fast for proper heat exchange in the radiator.

Read this article at Gojeep: http://jeep-xj.info/HowtoRadiatorRestrictor.htm

OK...old thread, but I'm sure still relevant to many. I am posting my observations. I updated the cooling system in my '89 XJ Waggy to open system. Changed out the therm housing so I can have a switch for the aux fan. The aforementioned observations in previous posts are correct. The coolant flows from the therm housing to the heater core (3/8"on late model), going thru the flow valve and to the lower (on the 89, 5/16") connector. Return line is 3/8" fitting out of the top. I watched some YouTube stuff where a kid upgraded his and used the two preformed hoses from a late system. The stock system does put the hose coming out of the therm housing to the lower fitting on the core, the upper core fitting connecting to the one behind the AC compressor. Ideally I would assume you would hook it up like factory...HOWEVER, I decided to do away with the flow valve and let the coolant flow continually thru the heater core. Hooking it up like factory presents a problem....you hook the 3/8" hose coming off the therm housing to the 5/16" fitting on the heater core. This requires two adapters, one reducing the hose coming out of the therm housing from 3/8" to 5/16" and another reducing the hose out of the top heater core fitting from 3/8' to 5/16". You can use the two hoses you bought, but you will have to cut both of them and use pieces of each.

At this point, any argument about retaining the size of the hoses for maximum flow becomes a moot point because the hose will only flow to the extent of its smallest point. I figured I'd hook up the heater core backward because it allows the same hose sizes to be connected to the heater core without splicing. So, I did it....and it works fine. Not only that, it looks a hell of a lot better than two spliced hoses on the side of the valve cover. I'm about convinced that vacuum valve is an invitation for a coolant flow problem when its shut off any period of time. The heat flap inside seems to be doing a good job turning the heat off and on, and if it gets too hot in the summer you can always add a manual valve on the them housing to heater core hose like we do on big trucks to stop the flow.

I also am in the process of modifying the 89 overflow tank and I believe it will work in the stock position once I've disabled the pressure cap and added a 1/4" nipple on the bottom to accept the hose from the radiator.If anyone is interested, I will post some pics when I complete the overflow tank.

Isn't it 5/8 and 3/4 inch hoses????????

cruiser54 said:

Isn't it 5/8 and 3/4 inch hoses????????

Click to expand...

That is what the heater hoses are, yes.

Johnnie Walker said:

That is what the heater hoses are, yes.

Click to expand...

Yep. Look at the numbers in his write-up though.............