plugged engine oil cooler in ford 6.0 diesel

Why a Plugged Engine Oil Cooler Created The Perfect Storm: The Reason Why a Ford 6.0 Diesel Doesn’t Start When Hot


Recently a customer of ours solicited our assistance troubleshooting a Ford Diesel that wouldn’t start when it was hot.  This is actually a fairly common problem on these engines, and I’ll bet many people reading this are thinking they already know what’s wrong, or at least can create a short list of potential issues.


So, they followed our advice and performed all the normal checks for this problem, which includes use of high-pressure nitrogen to check the integrity of the system


Next day – same problem.


Recheck for leaks – none found.  Based on the process of elimination, if no leaks, must be a bad HPOP (high-pressure oil pump).  Replace pump.


Next day – same problem.


Low-pressure pump inspected and measured – not great but within specs.


Now it’s time to really think.  See the image below.

engine oil plugged cooler schematic


Pay particular attention to the cooler bypass pressure requirement.  25Psi.


At the end of the day, it was the engine oil cooler completely plugged.  However, as a result of the compromised condition of the low-pressure oil pump, when the vehicle and engine oil was hot, it wouldn’t make more than 20psi.  Plenty to go through a good cooler, not enough to open the bypass of a plugged cooler.



So why did a plugged engine oil cooler create the perfect storm?


  • engine maintenance – this vehicle has had almost none!  Regular oil changes would have left the low-pressure oil pump and oil cooler in better condition, and this wouldn’t have happened in the first place.


  • software updates – Ford recognized many years ago that a plugged engine oil cooler was undetectable by the operator, so they updated their software to warn you if there is a problem.  If the software on this vehicle had ever been updated, the owner would have received a light on the dashboard well in advance of this problem.




What did everyone learn?


Regular vehicle maintenance and periodic software updates would have either completely prevented this failure, or at least totally changed the diagnostic process making this repair much more affordable


It’s ok to double check your work once because hey, sh** happens, but after that, you need to step back and reassess.  Read your service bulletins, and understand your engine.


Sometimes it’s not what you already know, but your ability to get more information and how to apply it to a particular set of circumstances.  In the end, research and good critical thinking on the part of our customer solved the problem.  Many diesel engine repair shops would have just thrown up their hands and given up on this one.


This is what separates a good diagnostic technician from a parts changer.  Both have a place in this industry, just make sure the right one is working on your particular vehicle!