Overheating? Includes thermostat replacement

Over the past few weeks I've worked on vehicles with a variety of cooling system problems, including my own, and discovered a few things.

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 your 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. 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.

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.