I have a 2003 F150 with the 5.4L V8. My thermostat was leaking so I got a new O-ring and thermostat and replaced it. However now the temp goes almost to the red before the thermostat kicks in and then it regulates the engine temp just below halfway on the gauge. On my first drive, 25 minutes, after replacing it this happened twice. My next drive which was mostly in town to start, it actually got into the red and just before I pulled over it opened and regulated the engine temp. to just below halfway on the gauge as before. My drive in this morning, it only happened once before regulating. Is this due to air in the system or something else? If air, how should I go about removing it?

Thanks.

 

I've had "New T-stats" on several of My previous Work Vans, over the past 35 years, fail to properly open & close. I returned them to the supplier, installed the warrantied T-stat, & the problem was solved.

Your "Fluid loss" should've been minimal, if all that you did was replace the "T-stat", but you would have to add more antifreeze to the reservoir to "top off" what was lost.

Now,.... In no way, shape, nor form, am I presenting myself as an "Auto Mechanic", but,..... After getting tired of getting "Ripped Off" &/or having to pay "Mind Boggling Prices" for Repairs,.... I then learned how to make many repairs via book learning, videos made by reputable mechanics such as "Eric th' Car Guy" etc. -----With that said,........

Are You sure that your water pump is working properly?

I've learned that fluid movement within the radiator can be observed by having the cap off, BUT!!,................This ABSOLUTELY Can Not Be Done while the engine is Hot! You can only have the cap off, while the motor is COLD!!!

Start your vehicle, when the motor is at "Ambient (Surrounding) Temperature, with the cap off, & see what the fluid is doing after the motor begins to warm up.

And,...Wear Approved Goggles for Eye Protection, when looking in to the radiator cap opening! You also must be prepared to "Duck & Dive!", in case the fluid does erupt through the opening, while the motor is running, as well as to turn the motor off.

If You see good, continuous, fluid movement in the radiator,... then more than likely the problem is the "T-Stat", or,.... An intermittent fault in the water pump, &/or some form of a restriction within the cooling system.

 

It is almost impossible to see any coolant movement when the thermostat is closed.

The advice suggested is well taken.

It must be remembered the cooling system when at operating temp is at 15 psi and removing the cap to observe circulation Is not a wise thing to attempt.

A better way to get some idea if the stat is opening is to feel the upper radiator hose which should be HOT if the stat is open, otherwise there is a coolant bypass to allow circulation within the engine itself until the opening temp of the stat is reached Some ,,,,thermostats will stick closed if defective and should be replaced ,EVEN IF NEW !!

 

Thanks for the info, I ended up getting another new thermostat and swapping it out last night. This morning it worked flawlessly so I believe the issue is resolved.

 

Thank you for the update.

 

Hopefully you used an OEM (Motorcraft) thermostat. Most aftermarket ones are junk and short lived.