OK your partially correct. The PO could have pulled the t stat to slow down an overheating issue. The reason this would work is because a t stat holds coolant in the engine until it reaches operating temp. If it takes longer to get to operating temp, it would hide an overheating issue. Once it opens and it is effectively not there anymore. It does not open and close to give the coolant time to cool off in the radiator. Once its open, its open until the coolant temp drops below the t stat temp.
The alternative reason for pulling the t stat is that it will keep the engine cooler for longer resulting in more power. That is because it will take longer to warm up and the PCM keeps the engine in open loop (not reading the signal from the 02 sensor to adjust fuel/air ratio) until the engine is at operating temp. In open loop, the PCM is mapped to run a rich mixture which will result in more power at the expense of fuel economy and emissions.

Sounds about right, you have lots of power but crap mpg's and no heat in cold weather?

What bad can happen from not running a t stat? I've been in this field for a few years and have never seen anything bad happen, other than bad fuel economy and a complete lack of heat in cold weather. That's not to say I know it all. But I can't see what bad would happen from it. Then again I know I have run the same set up on 2 identical motors and gotten different results. If you have seen something bad happen to an engine from a lack of a t stat, please share the knowledge because I never have and would like to know so I can avoid it.

To the OP, I run a 195 t stat with a 1/8" hole drilled on the top and bottom (helps to burp the system when refilling), but the weather I am in is not as hot as it is where your at. The fact that it blew coolant out tells me there is a weak spot somewhere in the system. You may not see a crack in the radiator, but if you add pressure, it will show itself. I know the 15-16 lbs of pressure in the coolant system doesn't sound like much, but it really is.
Do you have access to a coolant system pressure tester? If not, go to the auto parts store and do the loaner program on one. Seal the system back up tight, fill it (with just water cuz I'm gonna tell you to flush the system good after you verify there are no leaks) and pump it up. Make sure the tester is sealed tight in the radiator filler neck so its not leaking around it and pinch the line that runs to the overflow bottle off tight. When I change water pumps and t stats in the shop, I usually pump it up to 18-20 lbs of pressure and watch for leaks. It's more pressure than you would run at operating temp and will show you any leaks.
If no leaks are revealed (pressure stays where you put it for 15 minutes or so), take the t stat and water pump off, pull the bottom hose from the radiator and shove a garden hose in the block, spraying it in to everywhere you can reach, then letting it run through. If it looses pressure and you can't see a leak, go back to the parts store and borrow a compression tester set to check the head and cylinders for cracks. I personally have never pulled the plug from the side of the block to flush the system, but there was a post about it and if he has had good luck with it and you feel you wanna do it, I'm not gonna challenge it since they put it there for a reason. When you get the block clean, flush the radiator and heater core (I use the air hose for the heater core, fill it with water then use air, low pressure by keeping the blow gun loosely in the end (do not seal it tight or you'll blow the heater core out) to push it, repeat this few times as this is typically the least circulated part of the system). If the PO did not do maintenance, you'll be shocked at the stuff that comes out.
Once it's flushed, spend the $35 at Pepboy's for the lifetime warrantied Bosch water pump and get a new t stat (I would say 180 t stat because of the warm climate there). When everything is sealed up tight, fill it with antifreeze (50/50 mix should be good since you don't get the bitter cold we do here) and use the pressure tester again to verify there are no leaks.

Then burp the system by letting it get to operating temp and slowly topping it off again. You'll loose some coolant as the temp rises due to the air pockets in the block expanding, but that's normal. 4.0's have a few spots in the block that seem to stubbornly hold air pockets, so after the initial burp, check it daily to see if the coolant level drops for a week or so. Usually you only need to burp it once, but I have seen a few that it took a week of regular driving to get all the air out of the system. You should be golden after this.

 

my xj only gets driven on the weekends and i usually don't break 100 miles. i haven't bother checking the highway mileage but i figure it is on the low side.

 

Probably, I pulled the connector from the temp sensor on mine and plugged it into a spare I had, then hung it from the wiring harness with zip ties so it never read that it got to temp. With the lift and all, I got about 8 mpg, but enough power to smoke my 33's on dry payment so it was worth the trade off.

 

OBD-II ('96 up) systems enter closed loop within seconds of cold engine start-up because heated O2 sensors "heat up" to operating temp within seconds of cold engine start-up. When the O2 sensor starts functioning, that's when the loop closes. Plugging a scan tool into the ALDL port will verify this.

There are only 2 references to coolant temp in the FSM ('98 anyhow). One is 160F and the other is 218F. At 160F coolant temp a warm-up cycle is counted and at 218F coolant temp the efan is turned on. No where is "normal operating temp" referenced. It would seem that the "normal operating range" is 160F-218F.

 

Your incorrect, the 02 sensor does not heat up within seconds. They need to be a minimum of 600 degrees (some are as high as 1200 degrees now) to work properly. If they did warm up to that instantly, then every manufacturer would be using that technology instead of moving everything closer to the exhaust manifold to make it warm up quicker. They get alot of tax breaks for reducing emissions and most emissions are generated during warm up. I think it was 98 or 99 (correct me if I'm wrong) when Jeep moved the cats and 02 sensors to the exhaust manifold to aid in warming them up quicker. The engine is typically in open loop for about 5 minutes.
Normal operating temp is whatever the vehicle typically runs at and can change with different t stats, high flow water pumps, etc. It's not mentioned by that name in the FSM because most techs cannot think for themselves. Call the engineers at Chrysler (you only get to pick their brains when you work at a dealership and run out of other options), they will refer to it as optimum operating temperature meaning that is when it produces the least amount of emissions because it is running as efficiently as possible. Yes, it is designed to begin to switch to closed loop (called semi closed loop) when the coolant reaches somewhere around 160, but will disregard the 02 signal if they are not within range. The 4.0 for example has a normal (or optimum) operating temp, assuming it has a stock cooling system of about 210. From 160 to 210, it's called semi closed loop, meaning it uses both maps and the 02 sensor signals to calculate the correct air/fuel mixture.

 

 

Me too.

 

Keep us posted how it works out. I have a 90 Cherokee, just replaced the rad and hoses and even the collection bottle due to a slight crack. Now works great, even in the 100 degree southern days we've had recently.
Agree with all the posts that say gently keep the front of the radiator clean also. Any help cooling these things is good. The collection bottle, I thought was always a strange idea with several posters having some issues with that over the years. Jeeps are great, easy to maintain, and run forever if properly maintained. My '90 is still my daily driver!

 

One can clearly see the system has entered "closed loop" well before 160F coolant temp. It does so within seconds of start-up because the O2 is heated electrically in addition to being heated by hot exhaust gasses. Never said "instantly". Why wait +/-10 minutes for coolant temp to rise to whatever temp in order for the motor to run clean/lean when a electrically heated O2 will allow the motor to run clean/lean within seconds of start-up. Like I said before, plug a scan tool in and watch.

Wasn't using a stop watch when the photo was snapped but it entered closed loop in less than 30 seconds of start-up.

 

A wax pellet type thermostat is in the engine, during warm up, the closed thermostat prevents coolant from circulating through the radiator.
As the engine nears normal temperature., the thermostat opens and alows hot coolant to travel through the radiator, where it's cooled before returning to the engine.

 

Well I washed the radiator and took it for its test drive and it over heated (gauge read 260) within 20 mins. Im gonna power wash every thing and see if I can see were stuff is leaking.

Also I am going to check pressure in each cylinder, dry first then wet. Ill check and see if water is leaking in the cylinders, going to check PVC valve, and timing, make sure everything is on.

Water pump looks fine. My radiator looks new, like the PO tried fixing it. No build up in it either.

Any other things to check? Driving my dads diesel right now, this will be my DD so I need to hurry it up. School starts tomorrow too .

 

The 2 guys going back and forth seem to make a lot of good points in their arguements. However, it's not fixing your problem. Without knowing the history of the vehicle its going to be hard to detemine what is wrong. But lets go back to basics here for a minute.

1. Thermostat- you've already stated that there was none. Which indicates a problem already.
2. Water pump- You state it "looks" ok. If it's shiny on the outside it doesn't mean it's that great on the inside. There are reverse rotation pumps for these jeeps. It is possible that the wrong pump is in there. If the impellar is going the wrong way then BINGO! I've also seen the impellor blade rot away causing low flow as well.
3. Fan clutch- Has this been changed yet? I've bought brand new and had them go in a few weeks.
4. Fan shrouds- Not likely to cause overheating in 20 minutes but could contribute to another problem
5. Lower Rad Hose Spring- Have the hoses been changed as well? While idling it and it's getting hot, see if the lower hose is collapsing. If it is, the spring is either broken or it's a new hose and it's now non-existant due to not installing. If it's collapsed, there goes your coolant flow
6. Leaks- Has this been pressure tested for leaks yet? If you have any coolant on the ground, find where it's coming from and fix it and go from there. If you're that low on coolant, you probably have air in the system. If no leaks, continue on troubleshooting
7. Timing chain- This is one of those rare occasions that causes this but still cause it. I would check this almost last but just keep it in mind in the future if none of the common problems fix your issues
8. Internally blocked radiator- Is there stopleak in the system? If so, prepare to spend some money. That's some of the worst $hit to use on a cooling system. You'll be replacing parts
9. Cylinder head- You already have the correct ideas on diagnosing this so I don't need to explain that to you.
10. Another extreme case is clogged Cat Converter. But you would have really poor performance as well.

This is where I would start for now. Goodluck to you on your repairs. I hope it's quick, easy and cheap. Oh wait, it's a Jeep I forgot.

 

I hope the OP re-reads #13, quits guessing and git's 'er done. Best way to inspect the water pump is pull it off, take a good look at it, put it in the box the new pump came in and install the new pump. Radiator is different, but only a rad shop can disassemble/inspect/clean (rod out) a rad....u can't see inside the rad so u can't say it looks ok. New rads are +/-$100.

Also, with regard to a t-stat, the FSM doesn't say what u must run, but it does say u must run one.

 

Yeah ill def get a new water pump installed. But not sure if its my problem. For only like 35$ I can find out though.

Our pressure tester for the rad broke last night, so my dads going to pick a new one up tonight, and ill see how it works.

 

And it only overheats when driving, never when just sitting.