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2003 4.0 slight overheating issue?


July 18, 2017
Reaction score
City, State
Wake Forest, North Carolina
Year, Model & Trim Level
2003, Explorer XLT
Hi everyone I have a '03 4.0l that for the better part of 6 months I've been fixing different problems on, coolant leaks being of them, and this is the latest in the saga.

so at the end of may this year, my car developed a coolant leak. which was 'fixed' by reseating the thermostat o-ring which had partially flipped around somehow. 2 weeks later the o-ring predictably blew out, new o-ring problem solved.(~1 gallon of coolant lost, was topped off with premix)

2-3weeks after the o-ring issue started leaking coolant again, this time more significantly. took a long time to figure out where it was but found that the heater bypass valve had cracked and was leaking, replaced that and the radiator cap because it was suspected of building too much pressure. lost a lot of coolant so I was topping up with premix and water(~5 gallons topped up by the time it was fixed)

The car has 202,xxx miles on it and it hadn't had its coolant changed in a long time(the system was never sludged up so it was never changed.... in like 100,000 miles) Since it was like a 30/70 mix of coolant to water and the weather is cooling down I got some zerex G0-5 to put in about three weeks ago. I drained the system and very suspiciously not as much coolant came out as expected. I flushed with water while the engine was idling at operating temp. was only able to put in 3 gallons of coolant mix after bleeding all the air out:confused2: this car has vents for the rear passengers.

The car had not run above normal temperature before this point

2 weeks pass and now the car's temp gauge is reading higher than it normally does

Sorry for the MS paint but it is 10 pm and I don't want to walk out to my car and run it till the problem occurs.
Now I know this looks like a normal temperature for the car to run at but what is concerning is that it moves while driving, sometimes it is just barely below the halfway mark sometimes it sits on it. The gauge moves while accelerating, coasting, idling, pretty much whenever it feels like it. sometimes when I gear down into 1st for going down a steep hill it warms up other times it does not.

The coolant level in the car is correct, I'm not losing any coolant and it is the correct coolant for the car. Previously only prestone generic coolant was being used. It currently has a 50/50 mix of Zerex G0-5. I suspect that maybe the fan clutch is bad, it is difficult to spin by hand no matter if it is a cold or hot engine; the fan turns about a 1/16 of a rotation when forcefully spun. I think the water pump is fine since I have used the heat several times and it is always warm and consistent, which says to me coolant is being circulated and there are no air pockets. The belts are in good shape, and nothing is blocking the radiator or grill. The thermostat is working as the top hose gets hot.

Sorry for the ultra long write up but I want to know if there is even a problem to begin with and to solve it without shotgunning parts at the car. No one can mind-read diagnose my car through the internet so I hope I gave all the information that is critical.:)


Active Member
April 15, 2016
Reaction score
City, State
Myrtle Beach SC
Year, Model & Trim Level
2004 XLT 4x4 Loaded
Possibly replacing your thermostat may help it to run cooler, could get a new coolant reservoir jug and put some peak 50/50 or whatever you prefer see if it helps


Elite Explorer
August 27, 2015
Reaction score
City, State
Ann Arbor, MI
Year, Model & Trim Level
2010 XLT 4WD
2002 XLS 2WD
Does the '03 have the single engine coolant sensor clipped into the top of the thermostat housing (front, top of engine)? Maybe check that connection or replace (its not difficult, or expensive)?

I have heard of engine cooling fans developing cracks, and the clutch could be shaky with that many miles. But I'd replace that last since you'll probably want to swap with junk yard part that may or may not be good itself. Only other thing would be the radiator, which you may well be due for @ over 200k, or possibly shaky water pump (less likely). I replaced my radiator on my '02/180k miles XLS, along with thermostat housing, water pump, thermostat, engine coolant sensor, Zerex G0-5 50/50 mix, new hoses, and its always spot on at the midpoint, whether its 50 degrees or 90 degrees Farenheit out there.