Overheating? Includes thermostat replacement
If your engine runs hot constantly, even if you've replaced your thermostat, check it again! A thermostat will get ruined if the engine is overheated even once, and it can also take out your temp sending unit for your coolant temp gauge.
After waiting for the engine to cool, DO NOT drain the coolant. Pop the cap, and smell the coolant. If it smells like gasoline or exhaust fumes, you have more than a coolant system issue, you have a headgasket issue. This means on the compression stroke and or exhaust stroke, air/fuel mixture or spent mixture is being pushed through the gasket into the cooling system. This would mean the leaking is coming from at least one cylinder, #6, as 258 and 4.0 heads and blocks are mostly closed deck, except for the two enlongated passages at the very rear of the head.
A cold but pressurized upper radiator hose is another sign that you may need to check into this, as it is a byproduct of a cylinder head gasket leak.
Check your cooling system for major leaks. This includes the waterpump gasket and weephole, thermostat housing, intake manifold (if stock, 4.0 intake is not water cooled), heater hoses, coolant temp sensors and heater core. If your waterpump gasket is leaking, check your oil for discoloration, as there is a possibility of seepage directly into the oil pan from between the timing cover and block if the timing cover gasket is going bad as well. Worked on an 05 Wrangler with 76k miles on it a while back that had dexcool in the oil because of it. Coolant can also seep into oil from the water jacket passages at the rear of the head, basically, opposite of a compression leak. If the car is allowed to idle in one spot, in severe cases you will notice a puddle of oily milky gray residue on the ground under the tail pipe, and/or white smoke.
check your coolant system for bubbles. Air pockets like to hide out in the heater core, and if the system is not burped properly these bubbles can lead to overheating by causing hot spots in the block and cylinder head. A good burp kit is relatively inexpensive, and I believe it can be found at most part stores. The best way to bleed out an Eagle system, since the heater hoses sit just above the level of the radiator cap, is to park the front end on ramps. You must turn the heat on full blast with the vehicle running and bleed kit in place on the radiator, and you should expect to feel heat within a few minutes. If you do not feel heat, rev the engine to 2k rpm and hold it for approximately 30 seconds, this should rid the heater core of bubbles. If you previously had heat and do not feel it coming through, be patient. If you did not have heat before, check the heater hoses for pressure and heat, both should be hot and slightly firm. If one is hot and one is cold, you may have a clogged core.
If your coolant smells normal, drain it using the draincock at the very bottom driver side of the radiator, after removing the radiator cap. If coolant does not come out in a solid stream (like emptying used oil from your engine), check for blockages. Sometimes buildup will occur at the bottom of the radiator, especially if stop-leak has been used.
If the radiator is clean, at this point inspect your fluid for oil or severe discoloration and sediment or metal flakes. If you notice high concentrations of metal flakes, inspect your waterpump for shaft end-play by removing your belts and gripping the fan, and attempting to jar it around. There should be no play in the shaft, and absolutely no wobble to it.
If all looks good, replace the thermostat, but be very careful in scraping the housing, it's extremely easy to deform the mating surface, following the steps outlined below. Use gasket maker on both sides of the paper gasket, and let it set up for at least an hour without coolant in the system. When filling the system, refer to the bleed out system above.
Your thermostat is located at the front of the cylinder head, behind the thermostat housing, also called the waterneck. The waterneck is bolted directly to the front of your cylinder head, and directs coolant to the radiator via the upper radiator hose.
1. As stated, drain your coolant using the small draincock located directly on the bottom of your radiator, driver's side. Simply unscrew it, and count on getting coolant on your hands. Use a bucket or container to catch the coolant for reuse.
2. Loosen the two bolts holding the waterneck to the front of the cylinder head. If you are careful, you will not need to remove either hose clamped to it. Use a razor blade to break the gasket free.
3. Take note of the position of the thermostat in the head before you remove it. Technically, they can only go in one way. The spring should face inward. Pull it out.
4. CAREFULLY scrape the water neck with a razor, and once majority of gasket material has been removed, use a dragging motion to remove what's left, this will ensure the housing is not gouged. Scrape the surface of the cylinder head, and the mounting recess for the thermostat. All surfaces should be clean.
5. Apply a thin bead of permatex gasket maker to both the thermostat housing, along all gasket surfaces, and the cylinder head. It helps to apply a very small amount to the thermostat recess as well, as this will aid in keeping the thermostat in place.
6. Apply the gasket to the waterneck, and press firmly and evenly to seat it onto the gasket maker. Then position it over the thermostat, being careful not to bump it out of place.
7. Bolt the thermostat housing back in place, but be careful to distribute torque between the two bolts evenly to avoid warping or cracking the housing.
8. Allow the engine to sit for at least an hour without coolant in it, and/or until the gasket maker becomes tacky and gummy.
9. Fill your radiator and burp it using steps outlined above the procedure.
You should only use a 195 degree thermostat. This is especially important for EFI computers.
If your Eagle still runs hot, consider fuel delivery system malfunctions (a lean burn will cause the engine to run warm) and ignition system. Replacing the thermostat with a lower temperature unit when your car runs abnormally hot is just a band-aid to the problem, and ignoring it will cost you dearly.
Pay attention to your coolant temp!
With a high flow thermostat, your gauge should never read more than halfway up in the green, even when towing. They are an excellent investment, and will keep your engine far cooler than a stock thermostat, but will still allow the coolant to get hot enough to generate heat.