Before buying my 08 150 5.4l, I was looking at an 08 250 Superduty 6.4l diesel. I was not too familiar with the 6.4 and knew the 6.0 had some issues I did some research and found a you tube video about a guy that specialized in diesel rebuilds and said that every truck that came in he replaced the coolant cap because the OEM one always failed the 16lb vacuum check. Autozone sells them for 5.99. Cheap insurance IMO. His reasoning was the electric fan kicks on 5 degrees hotter than when the coolant boils when the 16lb vacuum is not held. Now I do know that when I bought my truck, I added almost 3/4 gallon of anti-freeze to the truck to bring it to proper level??? Coincidence or not? I am not sure but I will be buying a new cap. I topped off the coolant and have been driving the truck daily and today is 2 weeks later and I will be checking the level as I go to purchase a new cap.(the aftermarket cap has a larger spring than the OEM) I will post my findings. Just wondering if anyone else has heard about this?

 

I believe you mean 16 psig pressure, not vacuum. The higher the pressure, the higher the boiling temp.

 

Yes thank you.