Overheating solved

[ehall]

Can anyone point me to a diagram of how coolant flows through the 1992 XJ cooling system? This system has the unpressurized overflow tank unlike earlier systems. I'd like to understand what drives water through the cabin heater core and how the heater control valve works.

This is for your 92

The water pump pulls cold coolant from the radiator and pushes it through the block and head. From there it hits the thermostat housing, which has one hole that returns to the radiator through the thermostat, and another hole for the heater core circuit. Since the coolant has the pump pushing it (and fluids don't compress) the coolant goes through the path of least resistance (or both paths if they have similar resistance). Coolant that goes through the heater hose hits the valve, which either returns the coolant to the pump or directs the coolant through the heater core before returning to the pump (the valve is only open when the cabin temperature slider is set to HEAT and the blower is on). Note that coolant in the heater circuit does not pass through the radiator, instead the hot coolant simply recirculates through the motor. The design theory is that this helps to bring the engine to operating temperature quicker and the thermostat will open whenever the coolant gets too hot. The theory posited in this thread is that closing the circuit forces coolant to circulate through the radiator all the time which results in cooler engine performance.

 

[trippled]

Isn't there something about how some amount of flow thru the head is good? Basically dead uncirculated coolant sitting there would require all the coolant reaching the thermostat to be warmed up enough to make the stat open. Also hot spots, etc like ehall posted earlier. I'm not sure but it seems like no circulation would lead to pump cavitation too. I would like to see a restrictor tested over just closing it off completely if anyone wants to test?

 

[Patrick S.]

Well I tested clamping the heater hose off and at least in the case of my 93 XJ it had no effect. I have a high flow water pump, stock thermostat and I used a dremel on my thermostat housing to slightly increase the flow.

I don't overheat but these XJ do run hot. I never run past the middle of the temp gauge except in up hill driving and that was exactly the same (in the case of the heater hose clamping). On really steep trails I can get just short of the 3/4 mark but it immediately comes down after a few minutes idling so that's fairly normal I believe. How do you all compare? Would be interesting to know.

On the 93 the temp gauge sender is in the rear of the block so it's measuring the hottest point. Is this the same for all the XJ's? Maybe that has something to do with the results.

I also realized that having the valve or not shouldn't have any important effect on the engine temp as fluid still will flow to and from the same place except in one case it will go thru the heater core.

Maybe you have a clogged radiator, restricted thermostat or debris somewhere? Restricting that heater hose might be forcing water thru the radiator as it should. Just something to check.

On a side note now regarding air flow and not water flow looking at the front of my condenser I just don't have enough space for pusher fan. (Only have 1.5 inches) The condenser core is pretty thick and dense (Might not be OEM). Was reading up on that and some people mention some condensers designs can really restrict air flow.

 

[lawsoncl]

Isn't there something about how some amount of flow thru the head is good? Basically dead uncirculated coolant sitting there would require all the coolant reaching the thermostat to be warmed up enough to make the stat open. Also hot spots, etc like ehall posted earlier. I'm not sure but it seems like no circulation would lead to pump cavitation too. I would like to see a restrictor tested over just closing it off completely if anyone wants to test?

Exactly. If you pinch off the heater circuit entirely, when the thermostat is closed there is no flow at all through the engine.

 

[mhead]

Great! Thanks for the drawing.

Looking at the drawing and my XJ I can see the cooling circulation.

Coolant travels from radiator lower into the pump where it is pressed into the engine block. It flows upward into the head, through the thermostat and returns to radiator upper.

The heater core (HC) is connected to radiator upper at the thermostat housing and to radiator lower at the water pump. This connection places it in PARALLEL with the radiator.

I'm not sure what the heater control valve does. I don't have one to tear down (since mine recently burst - another story). I suppose this valve could close off flow in both directions or shunt flow across the HC. I'm guessing a shunt since simply cutting off flow could be done with a two port (and not the existing 4 port) valve.

Since the HC is in parallel with the radiator coolant could travel this HC path and perhaps not loose as much heat as would have occurred if this coolant had flowed through the radiator.

My heat problems occur at the edge of the design envelope. I see problems in hot air when trying to use all available horsepower from the engine. In cool air or on level ground my temperature runs steady just left of center.

So I might notice a 10% extra cooling boost from a closed HC.

I'm done with plastic heater control valves. I'll be adding quality valving. When this is done I can try open/closed and see if there is a correlation in engine temperature under similar hot climbs.

 

[techno1154]

The heater valve is designed to route the coolant back to the water pump when it is closed. When opened, the coolant will go through the heater core before returning to the water pump. The valve allow the coolant to keep moving whether or not heat is required in the cabin.

There was an early GM vehicle with the same/similar valve but made from copper that some have used to replace the crack prone unit on the XJ.

 

[mhead]

Thanks for the info Pat! Ok the heater control valve shunts the heater core. Since the core Parallels the radiator it also shunts the radiator which is consistent with the idea in this thread of improved cooling resulting from closing off the heater flow.

My plan is to halt coolant flow through the core via valves I will replace the heater control valve with. I do not plan to implement a shunt flow. However those who designed this heater system took pains to build a complex heater control valve deliberately to shunt the heater core. They took this more expensive approach for a reason and I would like to know what I'm going to mess up with my plan.

Anyone have a guess as to why the heater control valve shunts the core rather than simply cutting off flow?

Mike

 

[lawsoncl]

My assumption is that the shunt is needed so you have some flow through the block while it's warming up. Otherwise the head is heating up while the thermostat still sees cool water.

 

[mhead]

If I am correct in my analysis of the system, the shunt/heater core is connected in parallel with the radiator.

The thermostat halts flow around the cooling circuit

-radiator/pump/engine/thermostat/radiatort-

and so the thermostat will halt flow through the heater core/shunt as well.

Seems I recall thermostats have a small hole so that they do not completely block flow in the cooling circuit. I think this feature is implemented to allow engine heat to reach the thermostat via a small cooling circuit flow.

So I believe that the heater/shunt is not involved in warm up. Maybe I've got my cooling circuit wrong somehow?


Mike

 

[lawsoncl]

You are correct that the heater circuit is in parallel with the radiator, however the thermostat only closes off the radiator circuit. The small hole in some (not all) thermostats is to help bleed air out of the system.

Here's a comparison of regular versus high-flow thermostat housings. It looks like the passage for the heater circuit is larger as well? Perhaps that is affecting the ratio of flows between the rad and heater.

 

[trippled]

The heater control valve bypasses the heater core when ac is set to cold. Gives you slightly better ac performance. That's it.

 

[mhead]

Wow thanks for pictures under the thermostat housing.

So my model of the cooling system with the heater core shunting just the radiator isn't correct. I'll have to revise it, assuming coolant in the head is free to reach the thermostat as well as the port for heater.

Things are getting complex so I've placed a sketch of what the revised model is.

Revised model:

Question: is there any unusual flow in the head? Is fluid free to reach the heater port equally as well as the thermostat?

Mike

 

[mhead]

Well for some reason doesn't display my sketch. Here's a link

https://drive.google.com/file/d/1hDUs6wurvpKMhkOycL9Dlpy8Oi0iB5T9/view?usp=sharing

maybe this will work...

 

[ehall]

Yeah the heater circuit is always open (regardless of the thermostat or heater control valve), allowing coolant to constantly move through the motor even when the thermostat is closed. Otherwise the coolant in the head would boil before the thermostat opened.

 

[ehall]

Question: is there any unusual flow in the head?

There are some weird passageways in the head, if you ever look at one you will wonder how all the air ever gets out. But nothing weird in terms of direction.

Is fluid free to reach the heater port equally as well as the thermostat?

The thermostat and the port for the heater circuit are right next to each other in the housing. The pump is actively pushing fluid. When the thermostat is closed the least resistance is through the heater circuit into the valve and heater core. When the thermostat is open that path has a little less resistance due to the size of the radiator return hose (although resistance will quickly build up, at which point the path through the heater circuit is equal resistance).

Just as a sidebar I replaced the convoluted 4-port heater valve with an H-shaped tee and a 2-port valve on the heater circuit. When the valve is closed the fluid backs up and the path of least resistance is through the H-shaped tee upstream. When the valve is open fluid can flow straight through the hose and heater core, which has less resistance than taking the 180 turn through the H.

 

[NXJ4CB]

Hello,
I haven't forgotten this thread. I appreciate all the input everyone has been giving. I've been tracking my temps this last week and found that it will run in the 220s unless I keep the heater bypassed. It ranges under load but tries to maintain 180 to 190 mostly.
Also, I read the temps at the radiator tanks and they are uneven. I believe there is a restriction in my radiator or its too restrictive by design. I've had two of these CSF three rows with the same results, and assumed they were both good.
I'm going to replace the bypass radiator and leave the valve in. My current theory in my case is still the same. Only a different a different radiator and more tests will really confirm that flow theory.
An added benefit of the heater bypass is my AC is more efficient now. Maybe my blend door leaks too but, that is for another time.
I'll keep reporting as I have data to provide.

Thanks,
NXJ4CB

 

[mhead]

Hi NXJ4CB,

Your "Also, I read the temps at the radiator tanks and they are uneven" - can you give a little more description of what this means? Where on tanks? Are these the side tanks that run vertically when radiator is in vehicle? Horizontal tank at the top? Bottom? How uneven? How do you read temperature at spots on these tanks? Interesting to explore heat distribution within the radiator.

Thanks!

 

[mhead]

And with respect to the heater core and heater control valve - I guess as someone mentioned previously that the weird heater control valve shunting feature is there to allow limited circulation during engine warm up and at other times when the thermostat valve is closed due to cold temps. Whoever designed this feature knew something I do not (easy thing...) about engine heat. So I have now come to believe that my plan of simply closing off flow in the heater core and eliminating the heater control valve may result in engine hot spots when the thermostat is close. Probably a bad idea.

But... the HC and HCV do shunt the radiator to the degree that coolant flows through them and not through the radiator. So ehall's shunt in the picture above is kinda scary since it is connected through hose length of only about 1/2 stock vehicle length and therefore shunts perhaps 2X the amount of the normal stock HC/HCV arrangement.

I'm going to have to re-think my HCV elimination plan to be more like ehall's.

Mike

 

[techno1154]

Just as a sidebar I replaced the convoluted 4-port heater valve with an H-shaped tee and a 2-port valve on the heater circuit. When the valve is closed the fluid backs up and the path of least resistance is through the H-shaped tee upstream. When the valve is open fluid can flow straight through the hose and heater core, which has less resistance than taking the 180 turn through the H.

Interesting valve system. I like it although most/all seem to be plastic which I am always a little leery of,... breaking at the most in-opportune time.


The simple way to eliminate coolant going through the heater core while at the same time maintaining the flow when the thermostat is closed is to use a copper/brass "U" in the line out of the thermostat housing and the line going back to the water pump. Restrictors of plastic or metal could also be inserted into that line at the time.

On my XJ, that 4 way valve do not completely stop the coolant going into the heater core. Both hoses from the core and the valve exhibit a substantial temperature rise with the valve closed. I never gave it a second thought but simply accepting that there is a bit of internal leaks inside the valve. The valve do need vacuum to close it and keep it closed. I do like Ehall's method. Being from the Caribbean, I really hate the cold.


Many years ago, I had one of those 3 core radiators. My XJ ran hot with it. A scan revealed the radiator had different temperatures in unexplained places. I kept it 2 weeks then tossed it,...TWO weeks brand new.

 

[2X_01_Jeeps]

My '01 doesn't have any heater control valve at all (unlike the '84 I used to have).


I plumbed in a ball valve in the heater circuit and tapped across it to supply flow thru a coolant filter I've put in. The valve I put in is a brass ball valve (better at ON/OFF than common faucet type valves) I purchased at ACE hardware. I used plastic coolant T fittings from Carquest. (for use in coolant and high temps)



Using parts obtained from ACE and T fittings from NAPA one could fashion a bypass that you can control manually when you won't be needing heat but which will still allow circulation as if the heater core is present.



Just an idea.....