The subject of this video is probably the single most important subject I have put on video. A very seemingly simple, insignificant part plays a crucial role in making your truck run correctly or it can causes very serious problems.

What is this part?  The coolant cap.

If you don’t listen to any more of this video go to the auto parts store right now and get a new coolant cap. They are five dollars at Advance Auto Parts (Stant 10238). It is the single most important thing you can do for your truck right now.

The physics of coolant and water has been around since the beginning of automobiles. They pressurize cooling systems because it increases the boiling point of the coolant. I have tested hundreds of coolant caps off of 6.0L and 6.4L that do not hold pressure. Ford Motor Company produced these caps until about 2009. The problem with Ford’s caps is that the spring on the inside is very small and the heat and pressure that is produced inside the engine of this truck takes the temper out of the small spring. Replacement caps have larger springs that do not lose their temper. All Ford vehicles (Mustangs, Crown Vic, vans) had the same coolant cap and it is inadequate for a diesel application. They didn’t want to make a separate part just for diesel coolant cap so they just used the same part.

The chemistry of ethylene glycol and water boiling points under pressure is something that has been around since the beginning of time. It is very basic, but everyone has ignored it in automobile applications.

The coolant cap is designed to hold 16 psi when it is functioning correctly. If we are running a 50/50 mix of coolant to water, and we hold the 15 psi then the boiling point of the coolant is 267° F. If you are running 40% mixture of glycol to water, then the boiling point 265° F (etc see chart below). 8 psi is “no man’s land”. The 0-4 psi is where I see the majority of trucks that come in.

Coolant Boiling Point




15 psi

267° F

265° F

12 psi

259° F

254° F

8 psi

248° F

243° F

4 psi

236° F

231° F

0 psi

226° F

220° F

Why is this crucial?

If your cooling fan that cools off the radiator comes on at 230° F engine oil temperature, which means engine coolant temperature is a little less (5 degrees if your oil cooler is functioning correctly), then your engine coolant temperature is at 225° F when the starts fan kicks on. If you’ve poured a little bit of water in your system and your coolant cap isn’t working correctly, you are boiling the coolant before the cooling fan even has a chance to cool down the mixture. Your engine is self destructing because the coolant is boiling out of it because the coolant cap isn’t holding the pressure.

Is this to say the coolant cap is a primary cause of head gasket failure in Ford Powerstroke diesel trucks? That is hard to say because of tuners, heavy footed people, and lack of maintenance are huge contributing factors also. However, I would venture to say that a malfunctioning coolant cap has a significant impact upon head gasket failure. I will tell you this, every single truck that has come into my shop with head gasket problems, has had a malfunctioning coolant cap.

Once you’ve replaced your cap, and the cap is functioning correctly and the system is pressurizing, you have to pay close attention to your cooling system. Pay attention to your coolant levels and make sure you don’t have a leak in the system, cracked radiator, bad water pump, cracked coolant bottle, the heater control valve, and look for any spots of coolant under your truck.

Go get this cap today. It is the single most important thing to do for you truck. ~Bill Hewitt

Advance Auto Parts site:
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