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A Real Interesting Overheating Problem

My 2003 F-150 4.6L spikes into max overheating on the gauge after about 4 miles. It then recovers into the normal range and does not do it again (until you start it after completely cool). I’m now on my third thermostat and new coolant (again). Coolant is clean and clear. No signs of white smoke. Hoses in great shape. Pump seems okay. Pressure builds up in overflow container until spike is over, then everything is okay. No loss of coolant ever – in 118,000 miles. Coolant changed at 60k and again at 118k miles.

This seems to be a classic thermostat problem, but I just replaced old one with two new ones in a row. An Air Force salute to anyone who can solve this problem.

Did this start happening after you replaced a thermostat, or did you replace the thermostat because this was happening? If this started after a thermostat replacement there may be air in the system. Find the highest coolant hose usually the top radiator hose.While the engine is running loosen the clamp and use a small flat screw drive in between the hose and radiator outlet to see if you get air or coolant.

Does the current thermostat or any previous thermostats have a weep hole in the housing? I suspect there is a very minor leak from the combustion chamber to the water jacket. A weep hole in the thermostat base prevents the accumulation of exhaust gas under the thermostat as the engine warms enough to open the thermostat which results in a spike in the temperature.

Thank you for your fast response. The problem occurred first (before I replaced the thermostat). The engine simply began to overheat with routine driving, so I decided to replace the thermostat and coolant. When I removed the old thermostat, it came out in two pieces, so I was thrilled to have found the problem. At that point, I installed the first new thermostat and fresh 50.50 mix. Literally four miles later it was overheating to the max, so I shut it off. The uppoer hose was hard with pressure, but cool to the touch (which led me to believe that the new thermostat was defective). So I installed thermostat #2 and it still overheats at mile three or four for a short time before recovering to normal.

I will try your suggestionto ‘burp’ the system – that makes good sense. Again, thank you.

Most thermostat housings are foolproof, but if yours is not, you may have installed the thermostat backwards. This will cause the problem you are experiencing. If this is not the case, and your thermostat does not have a weep hole, drill one in it. Make it about 1/16" or so.

There should always be coolant flow through the engine block even when the engine is cold and thermostat closed. This is accomplished with some sort of coolant bypass hose that redirected the coolant before the thermostat directly back into the water pump bypassing the radiator. The bypass hose being clogged/blocked cold cause big spikes in the coolant temperature especially on warm up.

The bypass setup could be different between make and model. It would be a good idea to get a coolant flow diagram to understand the by pass setup to further diagnose the problem

I installed the thermo the same way the old one was positioned – top hat up and small section (with the spring) down facing the manifold. I will look up an actual drawing to be sure that I installed it correctly. Thank you. But I think I have the week hole in the thermostat (It’s a Stadt thermo sold by Advance Auto).

Also I forgot to mention make sure the engine is at operating temp and the heat is on warmest setting so the coolant circulates through the heater core as well. Sorry I forgot that in the first post.

The system is clean now after you cleaned it, but if the coolant was in really bad shape before cleaning, its possible the water pump impeller fins are mostly corroded away. The only way to find out is to remove the pump. At 118k replacing the pump may be a proactive thing to do even if the pump impeller appears ok.

Worst case is a head gasket leak that seals it shelf shut once the engine heats up. Searching through some ford forums on 97-03 f150 4.6L trucks unfortunately suggest head gasket is a likely cause.

You make perfect sense – once heated up, the problem goes away because the gasket expands. I like your logic; I hate to think you’re right!

I’m wondering whether or not you’ve measured the actual coolant temperature or if you’re just going by the dash gauge. And I’m wondering that in part b/c I’m wondering if you checked the coolant temp sensor itself. Maybe the coolant temp is fine - and the temp sensor has gone south. This is very easy to check out with a digital multimeter.

When the overheating occurs, I open the hood and find that the overflow tank is bulging (with hot coolant), so I suspect that the temperature sensor is ok. However, I am going to replace it anyway, so that I can check that off the list. When I eventually find the root cause, I will enter it in this column to possible help others who may have this unfortunate problem.

Thank you for your suggestion – I do think there’s merit to the sensor because of how quickly it goes up (and down)

Any result to the post? I have the same problem with my 02’ f250 5.4 right now.

To answer your question if you read the thread you will see that the person who started this 6 years ago never posted the final results.


The recommendations given to the poster years ago would be worth considering today.Duke 1971 may have solved his problem and quietly gone on his way after reading and taking the advice posted here. It’s a shame that so few posters return to notify the forum of their success.


I know, what the heck was the conclusion!

There wasn’t one.
Start your own thread and provide info and symptoms.

I think these beasts must be rather finicky for some reason, mine an 04 f150 slow to warm, never seems to get up to operating temp very fast, never overheats, but blows out enough heat to be comfortable enough, but really kicks out the heat and is toasty while idleing for 1/2 hour while doing outside work, Just live with it cause it is a work truck is my defense. Thermostat was replaced, leaks oil from head gasket, tsb issue with no recll so mechanic put a splash sheild to redirect the oil from manifold, …