1999 XJ Cooling Modifications

Does anybody have any scientific based information with regard to a 180 degree thermostat? I have read all sorts of "answers" as to whether it stays in open loop or goes into closed loop. I would welcome comments on a high flow water pump, thermostat housing and thermostat as well. Thanks in advance.

IMHO a properly maintained open cooling system with a stock 195* thermostat will run at 210-220* all day and and all night. There should be no need for cooling system modifications unless you are converting a closed cooling system to an open system. I see no harm with installing a 3 row radiator, but I also see no need for it. Hood vents do keep the coolant temps 5-10* lower on pavement or on the 4x4 trails.

provided your cooling system works good.. I would even touch it, but if and when parts start to fail then switch them out for better. My water pump died a 2 years ago and replaced it with a flowcooler, next is my thermostate

This might help with your question: the fuel injection system has a built in "choke" circuit, that requires the engine to get to full operating temperature to turn off, if the tstat runs it too cold then it will continually run too rich. If you need this to get your temps back into range you are only using a weak band-aid to cover a bigger system problem.
Hope that helps

A 180 t-stat is fine. You might notice your heater is a little cooler though, but that's the biggest difference you'll find. As far as high flow stuff, I would say no - stay away from it. IMO the stock system already flows too much. This is because there is an ideal rate of flow at which engine coolant can absorb and transfer the most heat. In other words, faster isn't better. In fact, there are some on this forum that've had good luck with adding a restrictor plate, partly in order to reduce the coolant flow for better heat transfer.

rokhed said:

This might help with your question: the fuel injection system has a built in "choke" circuit, that requires the engine to get to full operating temperature to turn off, if the tstat runs it too cold then it will continually run too rich.

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Nope. I dunno what "choke system" you're talking about but the ECM leaves open loop long before 180 degree coolant temp. Failing to "warm up" because of a cooler t-stat has got to be one of the most prevalent automotive myths out there.

And if you actually stop and think about it, it makes sense. Because you'll realize that engine temperature and coolant temperature are two very different things. The engine (especially the combustion chambers) get much hotter, much faster than coolant temperature. Also, heated o2 sensors were developed for just such problem. They're designed to heat up to operating temp faster in order for the ECM to leave open loop faster and therefore be more fuel efficient and reduce emissions.

I would even venture to say that spark plug heat range has more to do with combustion chamber temps than coolant operating temperature does.

Tim_MN said:

IMHO a properly maintained open cooling system with a stock 195* thermostat will run at 210-220* all day and and all night. There should be no need for cooling system modifications unless you are converting a closed cooling system to an open system. I see no harm with installing a 3 row radiator, but I also see no need for it. Hood vents do keep the coolant temps 5-10* lower on pavement or on the 4x4 trails.

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I agree completely.....with evrything he said.

good answer.

BOB

Everything else I've ever built I'm crammed the biggest radiator I could into. I was broke when I had to replace the one on my XJ and just grabbed a $100 base model from AutoZone. If I'm moving at all, I'm sitting at 195°. I'd buy another one any day, although I won't have to as it has a lifetime warranty.

I feel like everyone always tried to fix a cooling problem with some "better" part, but it seems they really just don't know how, or don't care to spend the money to replace or fix the underlying problem. If you're system is working properly, like Tim said, it should run perfectly and within temps under all (normal) conditions. Of course this doesn't include jeepspeed blasting through sand, towing through the sierra's, etc. Start with a decent radiator. Get a new water pump, get the correct thermostat, make sure your e-fan is working, no clogs, correct coolant level and mixture, working rad cap, and you should be golden.

My last jeep had the cleanest, and best working cooling system I've ever experienced. Never once did it go above 220, and all parts worked immaculately the entire life of the vehicle.

My current jeep has the WORST cooling system I've experienced. Mud-sludge coolant, clogged passages, non-working e-fan, clogged heater core. And of course, it heats up real quick. I have come to grips with the fact that to rid the problem, EVERY bit of the engine that sees and touches coolant must be replaced...

Now we're back to the original post. Why are you putting in a 180 degree thermostat? There's really no need.

Milford Cubicle II said:

Nope. I dunno what "choke system" you're talking about but the ECM leaves open loop long before 180 degree coolant temp. Failing to "warm up" because of a cooler t-stat has got to be one of the most prevalent automotive myths out there.

And if you actually stop and think about it, it makes sense. Because you'll realize that engine temperature and coolant temperature are two very different things. The engine (especially the combustion chambers) get much hotter, much faster than coolant temperature. Also, heated o2 sensors were developed for just such problem. They're designed to heat up to operating temp faster in order for the ECM to leave open loop faster and therefore be more fuel efficient and reduce emissions.

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Every time I see someone say that the engine in the XJ would not get to open loop if a 180 degrees thermostat is used I die a little inside. I do not refute that because I do not have my shop manual with me and I want to quote the exact words wtitten in the manual.

According to the 1996 shop manual, the PCM/ECM depends on the heated oxygen sensor to determine when to change between open and closed loop not the water temperature. As a matter of fact, I am sure I read it some where in the manual that the change over could take place before the water temperature reaches 100 degrees. Those of you who have a real time OBD2 scanner, set it read when the engine changer between open and closed loop and see for yourself. Do not take my word for it.

My Scanguage that is connected directly to the OBD2 port confirms what I just said.

Next time I work on the cooling system in my XJ, I will toss the genuine OEM thermostat in the garbage and insert one that is 180 degrees. The 210 degrees that the 4.0 run at is a little too close to ever heating for my comfort. The OEM cooling system in the XJ is marginal when all systems are at their best. I would like to see a bit more buffer space (temperature).

210* is the normal 4.0L operating temperature. A 180* thermostat doesn't change that. 195* thermostat is what the FSM calls for an nobody has ever given any logical reasons or solid facts that support using a 180* thermostat.

IMHO, properly maintained and routinely serviced, the XJ cooling system works well in most cases and needs no modifications. At 11 or more years of age, you should expect that the original cooling components may need replacement to achieve maximum cooling efficiency.

Lowering the under hood temps is one good reason just for starters. Also, XJ's are notorious for extremely hot intake air temps which kill power, I noticed a slight improvement with that with a cooler t-stat. Other than that it's just personal preference. I don't like running at 210 no matter what people say the "normal operating temp" is. I much prefer to stay at a nice and cool ~195.

Are there any logical reasons or solid facts that support not running a cooler t-stat?

techno1154 said:

Every time I see someone say that the engine in the XJ would not get to open loop if a 180 degrees thermostat is used I die a little inside. I do not refute that because I do not have my shop manual with me and I want to quote the exact words wtitten in the manual.

According to the 1996 shop manual, the PCM/ECM depends on the heated oxygen sensor to determine when to change between open and closed loop not the water temperature. As a matter of fact, I am sure I read it some where in the manual that the change over could take place before the water temperature reaches 100 degrees. Those of you who have a real time OBD2 scanner, set it read when the engine changer between open and closed loop and see for yourself. Do not take my word for it.

My Scanguage that is connected directly to the OBD2 port confirms what I just said.

Next time I work on the cooling system in my XJ, I will toss the genuine OEM thermostat in the garbage and insert one that is 180 degrees. The 210 degrees that the 4.0 run at is a little too close to ever heating for my comfort. The OEM cooling system in the XJ is marginal when all systems are at their best. I would like to see a bit more buffer space (temperature).

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Exactly, we're on the same page. And according to my scan gauge, open loop goes away well under 100 degrees coolant temp. Also, I'm currently running a 180 t-stat and my XJ operates at right around 195.

Milford Cubicle II said:

.......Also, I'm currently running a 180 t-stat and my XJ operates at right around 195.

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Could you please explain why running a 180* t-stat makes your Jeep run at 195*?

Mud Butt sometimes your sarcasm is a little much for me lol, in this case I can't tell. If you're serious - it's cuz the 180 rating is when the thermostat starts to open. It's always fully open (about) 15 degrees later than the rating. So your 195 rated t-stat will be totally open at 210.

Edit: I'm an idiot that can't read. Now I would also like to know the answer to this question?

Original_MudButt said:

Could you please explain why running a 180* t-stat makes your Jeep run at 195*?

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For most of the thermostats out there, the number stamped on the thermostat is where it begins to open. They are fully opened at several degrees higher. What that means is, a 180 degress thermostat may be fully opened at 195 degrees. Have you noticed that everyone ( well almost everyone) who are using a 195 thermostat boast the temperature of their XJ's sits perfectly at the 210 degree mark?

Ever checked the temp with a ScanGauge or an IR thermometer when it's showing 210 on the dash? At least on my junk, with an OE CTS that tests within spec, it's not really up to 210 when it's reading 210.

I saw a decent drop in average coolant temp just by replacing all the hoses and installing a new OEM t-stat. I thought the old t-stat was lagging a bit so I just threw in a new one. It basically runs the same way it did 130k miles ago.

hubs97xj said:

Ever checked the temp with a ScanGauge or an IR thermometer when it's showing 210 on the dash? At least on my junk, with an OE CTS that tests within spec, it's not really up to 210 when it's reading 210.

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I do not have an IR Thermometor so I cannot speak along that line. I do have an Auto Zone mechanical temperature guage that I sometimes throw in the system to verify things when I am in doubt. Considering that none of the XJ electronics are what I will refer to as being calibrated, when the guages all read within 5 degrees of each other it is perfect in my mind. When the 4.0 is properly warmed up, all three guages; dash guage, manual guage and the Scan Guage read within 3 degrees of each other.
When the ambient temperature is 70 or below, the XJ run just below the 210 mark on the dash guage. Above 70 and it runs just above the 210 mark. I would love to see the temperature remain at least 10 degrees cooler. The electric fans come on at about 212 degrees triggered by the thermostat switch I have installed in the system. The ECM activates them at 230 +/- degrees.

dj2790 said:

Does anybody have any scientific based information with regard to a 180 degree thermostat? I have read all sorts of "answers" as to whether it stays in open loop or goes into closed loop. I would welcome comments on a high flow water pump, thermostat housing and thermostat as well. Thanks in advance.

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Its probably worth asking why - you're after the info ...

Are you looking for a better result from your cooling system .... or do you just want info on the components ???

DanMan2k06 said:

Mud Butt sometimes your sarcasm is a little much for me lol, in this case I can't tell. If you're serious - it's cuz the 180 rating is when the thermostat starts to open. It's always fully open (about) 15 degrees later than the rating. So your 195 rated t-stat will be totally open at 210.

Edit: I'm an idiot that can't read. Now I would also like to know the answer to this question?

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No sarcasm... I'm just interested in learning.... I really just can't wrap my head around it.

I understand that thermostats begin to open at the temperature they're rated at. But I don't understand how they affect the temperature the engine runs at. I would think that temps are a function of cooling capacity and efficiency.

Once the t-stat is open, it's open.... Unless the engine begins to run cooler than whatever the t-stat is rated. Then the t-stat will begin to close again, and that's unlikely unless you are driving somewhere where it's really cold.

I don't know know if any of that makes any sense. I'm still a little fuzzy on the whole debate.

all of the jeeps i have owned have run below 210 on all stock parts only time it goes up or above the 210 mark is when the ac is on and its 99+ outside. or wheeling. other then that there is a component that is not up to par in your system. according to my scan tool i run about 198-200.

my biggest issue with the cooling system has been bubbles if there are bubbles it will run at 210 or slightly above

never needed a 180 and im not sure the benefit. i know the jetchip claims you need one. i feel if your making higher temps because of modifications then you need to upgrade your radiator or fans to dissipate more heat. or its time to vent some of that engine bay heat. i don't see how opening the thermostat earlier will help the cooling system get rid of heat. it is essentially just a valve to allow the motor to warm up quick also to control the cooler water in the winter.