djb383

If u have more money than since, maybe so. Local shops here charge $55 to rod out a XJ rad, if the owner removes the rad from the vehicle and re-installs it himself. If the rad is in good external condition AND is a Chrysler/jeep OE Factory rad, rodding out is a viable option. For the price of a new rad one could have the OE rad rodded out (includes new tank seals), buy a new water pump and buy a new t-stat.

Firestorm500

Plastic side tank at the hot water inlet cracks a lot on this type of radiator. They may look fine, but that is the spot that is under the most stress. The hot, somewhat caustic radiator fluid eats away in this area from the inside over time.

Replace the radiator to avoid paying double or more.

Honest radiator shops agree with me.

djb383

....and an honest repair shop will tell u the condition of the tanks/core once they r inside. Plastic tanks r dirt cheap, easily replaced by a reputable shop. All I'm saying is, Factory OE stuff sometimes is a little higher quality than aftermarket OE style stuff so sometimes repair is a viable option to new, whether u like it or not.

Firestorm500

I have enough sense, and enough money, to fix my 3 Jeeps, and all my other vehicles, any way I want to. Up to and including replacing 10-15-20-30 year old parts with brand new stuff. Thereby reducing the age of said part to zero.

What I don't have a lot of is time to waste messing around with possible iffy old parts. I keep all my vehicles ready to go anytime, anywhere. I'm going to do everything I can, and spend whatever money I want to, to ensure that.

Whether you like it or not.

CoffeeCommando

Faulty, missing the gasket, or put in backwards. Unfortunately it's not designed to be put in only one way. Given that it overheated within 15 miles after being the only modification to the coolant system.. it is highly suspect.

The radiator cap needs to be checked for pressure. It should be 16 PSI.

..and since all the fluids came out of it in order to replace it, I hope the shop cleared out all of the air-bubbles. Burping the hoses every pint or so, then after it's maxed out turn it on and let it run for a minute, then remove the cap and let the air works its way out, then refill more. It's tedious but it's the OCD way.

CoffeeCommando

Temps DO jump around like that. Once it hits overheating it jumps from about 220 all the way to the other side and gives you a check engine light. I've overheated my cherokee a few times and that is exactly what it does. And it's not a malfunctioning sensor because I'd had every one replaced and checked to ensure they were fully connected. Not to mention if they weren't you would get a CEL light anyways. There's nothing linear about how that gauge functions on OBD-II models.

djb383

Sometimes things can be done differently than your way.......but just as effectively. U've made "your way" quite clear and I've simply thrown out an alternative suggestion. U say u don't have enough time but u obviously make time to turn numerous threads into boxing matches. Why don't we turn this thread back over to the OP and u go check to see if u have any parts that might be aging on your many vehicles.

alr37115

The water pump was replaced. The electrical fan was wired to run when it's started. (Not sure why they did that since the relay and relay connection are both good) I drove it from the shop to the house (only a couple miles.. if that) The mechanic explained that he flushed out the entire system and replaced the lower hose (brand new lower hose)... said they cleared a bunch of nasty crap out of it which is fine with me since they did it on their own dime. I will look and see if they put the stat in the correct way tomorrow before I drive it any further. I can just imagine the stat opening up and throwing hot coolant back into the motor.

Firestorm500

If the thermostat is installed backwards, it will still open eventually and coolant will flow from the engine to the radiator.

It just won't open as quickly as it should and it may not open as much as it should. It's not a one-way valve.

CoffeeCommando

It's not "throwing hot coolant" back into it. It's designed to push open when a certain amount of pressure builds up in the coolant system. What's happening is it's simply not opening up (assuming this is the problem) The pressure is coming from the heat causing the coolant to expand (hence the reason the radiator caps needs to maintain a certain level of pressure as well). There's a spring on the thermostat and when it's turn around the wrong way it will not open; you will not have any coolant flowing. Whatever is sitting in the engine block will stay there and continue to boil and not dissipate heat because it isn't making it to the radiator.

Here's a picture of the face you SHOULD see when it's installed properly:



The circular metallic portion is what pushes in from the pressure from the coolant. If it's facing the wrong way it won't open because the water pump only flows it in one direction.

I've never had the problem myself but I read a post about someone that had instant overheating after an engine rebuild that boiled down to the thermostat just being installed backwards.

It's a very simple thing that is easy to overlook because it will fit either way unless you are familiar with which way it goes. Maybe the more veteran tech went on lunch break and had one of the newer guys install it since it's a relatively small task.

Firestorm500

Nice post. Sorry, but you are wrong.

The thermostat opens because of temperature, not pressure. Put one in a pan of water, and heat it up on the stove. It is not under flow pressure, yet it will start opening at the rated temperature.

Water pumps in engines do not create pressure. They only create flow.

The pressure in a closed or semi-closed system is caused by the expansion of the coolant due to heating. The radiator cap contains this pressure, up till its rated pressure. Then the spring, which is calibrated, and its associated rubber plug/disk, is forced away from the radiator neck. This opens a path for the coolant to flow out of the radiator neck and onto the ground, or an overflow bottle.

alr37115

I'll check the thermostat in the morning. And from now on I'm gonna do the work on my XJ myself based on the fact that if I'm not standing over their shoulder, I don't have any idea what they did/didn't do. I got the Haynes manual today after this whole overheat ordeal, and the money I shelled out in the hopes that it gets resolved.

When I bought the Jeep, I knew about the 0705 code, thought it was a speed sensor code which turned out to be the NSS, the transmission leak started a couple days later. The overheating just started a couple days ago, and I knew that if an overheat isn't resolved that it can cause warps and cracks... to me it was common sense since I have little automotive knowledge. I'm grateful that this forum exists because I can come here and say "so... what's everyone's experiences with this, and what did you do to fix it?" I'm optimistic that I can learn to fix some things by reading the Haynes manual and posting here.

CoffeeCommando

Touche. You win this round, FireStorm500. I'll bide my time and wait for my opportunity to strike back.

I didn't say anything about flowing pressure from the water pump though, but heat pressure from coolant expanding (how the radiator cap with the spring works).

CoffeeCommando

Here are my recommendations since you have a 99+ model year:

1. Never use synthetic oils (that's all 4.0's)
2. Put a zinc additive in with each oil change
3. NEVER use a FRAM oil filter; they don't have anti-drain-back valves which causes dry engine starting. Metal on metal = no good
4. If you didn't already (I forget) replace the Radiator with a 2-core aluminum. 3 cores actually reduce efficiency of heat dissipation because there's too little space in the grill area for it. NAPA used to have all 3, now they only have 1 for left hand drives. RockAuto.com is the best parts site with unerring accuracy in ensuring you get exactly what is correct for your model year vehicle, and they're the cheapest to boot. They're a whole-sale OEM distributor, with some non-OEM options as well.
5. Don't upgrade the transmission cooler to a larger size. This will put more heat from the transmission onto the engine radiator and the 99+ heads are prone to cracking.
6. Replace the two pre-cat 02 sensors with NTK for optimum AFR ratios to further reduce excessive cylinder heat. As they get older they respond slower so the PCM is constantly adjusting the fuel-trims back and forth more than it needs to going from too-lean to too-rich and back again. This is unnecessary heat and your MPG will improve as well.
7. Consider using a 180 degree thermostat if you don't live in CA or NY. When I upgraded to one on my 4.7 stroker it hovered between 195-205~ on a hot day idling in the city. Yours will be a tad cooler being a 4.0.

On a sidenote, only have work done on your Xj at a shop you know is reputable and you can trust. I had sears change my oil once and when I stopped at a gas station 10 miles down the road there was a giant coolant explosion when the engine shut off. The flow stopped and the backup shot out of the radiator inlet because cap the was sitting elsewhere inside the engine bay it didn't belong. I have never been back there or let anyone else change my oil in it since.

Fred/N0AZZ

Do yourself a favor and replace the radiator NOW! They are under $100 at AutoZone and a good one to boot my son and I both run them on his 98 and my 99.