2003 F-150 with 5.4 interesting engine problem

[Quantico]

As already posted, the OP's truck is a 2-valve engine. The 3-valve engines began in about 2004. While my brother has 170K trouble-free miles on his 3-valve engine, they do have a reputation. I have a '97 F250 with the same 2V engine as the OP. It currently has 270K miles on it and it still has the original starter, power steering pump, water pump and alternator. As parts-man BDCardinal posted, there were some electrolysis problems, and yes, my heater core went out on my truck. My truck had some blown spark plug issues, but I did the labor myself and installed TimeSerts on all 8 cylinders. I'd bet money the OP has a bad head gasket. If the truck has rust issued, it's time to look for another, even with the current high prices of new and used trucks.

Did not catch this was a 2 valve. Much much better engine.

 

[partspro]

I watched an interesting video about determining which cylinder pair was bad. Head Gaskets tend to blow through the thin part between cylinders. and in essence the video said disable ignition and remove spark plugs one at a time, crank engine and watch for pressure in radiator or overflow tank, when no pressure the break is between this cylinder pair. More than 50 years ago a Brother in Law who never maintained his cars gave me a panic call the evening before we, my wife and I, and his wife and he were taking a trip to Italy. The car was a British Triumph 2000 a very long 6 cylinder engine. So i did the head job, no torque wrench but all was good, for most of the trip. He insisted on driving as fast as possible on the highways, we made it all the way through France, Switzerland into Italy and back but in Paris the head gasket blew, no problem remove pushrods and run on 4 cylinders back to Manchester England.

 

[Food Scientist]

It is a bad head gasket, I did a compression test years ago when I changed spark plugs and two cylinders were low and are right next to each other.

Still running - what the engine does is push a lot of the coolant into the additional overflow tanks and as the thermostat opens up the water pump pulls the coolant back into the engine. But now instead of getting heat in 20 minutes or less it now takes about 30 minutes to get consistent heat.

Pulled a trailer with a 3500 lb sprayer and was a huge wind load. Ran great. Trip was over 500 miles in the same day. I've put almost 10k miles with this condition and it's my daily driver and wouldn't hesitate to go anywhere with it. I am looking for a good deal on a newer truck so I have something ready to go when this one dies.

 

[SwampSurvivor]

It is a bad head gasket, I did a compression test years ago when I changed spark plugs and two cylinders were low and are right next to each other.

Still running - what the engine does is push a lot of the coolant into the additional overflow tanks and as the thermostat opens up the water pump pulls the coolant back into the engine. But now instead of getting heat in 20 minutes or less it now takes about 30 minutes to get consistent heat.

Pulled a trailer with a 3500 lb sprayer and was a huge wind load. Ran great. Trip was over 500 miles in the same day. I've put almost 10k miles with this condition and it's my daily driver and wouldn't hesitate to go anywhere with it. I am looking for a good deal on a newer truck so I have something ready to go when this one dies.

My 2001 F-350 has a 5.4 with a bad head gasket too. Luckily in my case, it just leaks a lot of oil out of the passenger side head gasket. No compression loss, no coolant/oil mix and no exhaust gas in the cooling system. Just have to add oil.
The rest of the truck is pretty rough. I'd trust the engine that far, but would be afraid the truck would break in half on a pothole or something.