Common faults plague all cars, but sometimes things are worse than the fault itself.
The S550 Mustang, it turns out, has one of those 'sometimes' faults.
It has become apparent that the 'engine oil to engine coolant plate heat exchanger' (or oil cooler for short) is susceptible to internal and external failure - if this happens it can quickly cause catastrophic engine failure.
Update: we've seen it claimed that Ford has carried out an update to these oil coolers that has fixed the issue - apparently cars after 2016/17 received the updated part. We were glad to hear it until we came across our first 2019 GT with the same failure and unfortunately the first wasn't the last. It is safe to say at this stage that all gen2 and gen3 coyotes are prone to this kind of failure!
What is the 'oil cooler'?
The S550 Mustang GT utilises a plate type, fluid to fluid heat exchanger (PHE) to manage its engine oil temperature. PHE's were invented in the 1920s and have been in use, reliably, in innumerable applications ever since.
PHE's are constructed out of multiple, thermally conductive, thin plates bundled together. Pairs of plates create a channel through which one fluid can flow. The pairs are then stacked and attached by welding or brazing to the next pair in a manner that creates a second separate passage through which the other fluid can flow. Heat transfer is achieved via conduction, which is the passing of thermal energy between materials that are in contact with one another.
Sorry ... we don't need to understand the science behind how heat exchangers work. Still, a brief picture of the design of a plate heat exchanger will help you imagine how the one on the Mustang fails.
What is the 'oil cooler' for?
Engine oils are designed and manufactured to provide maximum lubrication at certain temperatures - this optimum or normal operating temperature will vary from one application to the next but generally speaking in most automotive applications 80-100 degrees Celsius is the sweet spot. Things stay O.K., all be it with increased oil oxidation, up until around 120 degrees C after which things steadily get significantly worse.
Running your engine oil too hot has multiple drawbacks, not only is the oil system immediately less capable of providing the required pressure and lubrication to the hydrostatic engine bearings, causing accelerated wear there and then, but the oil itself quickly begins to degrade permanently - continuing to run an engine on the same oil even after everything has cooled back down will cause accelerated wear.
Car manufacturers and race teams alike are obviously well aware of these issues. Both address the problem with oil cooler systems - though engine oil (as well as differential and gearbox oil) temperature is obviously a bigger concern for a race car than a road car. Ironically, you can over cool oil too - it's for this reason that you will often find OE manufactures put mediocre oil cooling systems on their road cars and upgrading OE oil coolers is a common and sensible modification for performance car enthusiasts.
How does the Mustangs 'Oil Cooler' work?
The S550 Mustang (amongst a million other cars) comes equipped with a fluid to fluid PHE. More specifically, the engine oil heated by the engine and the engine coolant cooled by the radiator pass through the multiple layers of the cooler, kept separate by the plates, allowing a thermal energy transfer between the two.
I should now be showing you this:
and talk about things like kinetic energy, molecular motion and thermal conductivities - but I hate that I even know so I won't subject you to any of that right now.
Basically, some of the coolant that is used to cool the engine block is diverted to cool the engine oil. And vice versa - the engine oil is diverted to heat the coolant.
So, what is the problem then?
You know as much as me now so I am sure you have guessed what is coming. The thin plates that separate the two fluids from one another and either from the outside air are ... well, thin. In some cases, too thin and prone to rupture - allowing the two fluids to mix or one or both fluids to empty onto the road.
Now, bear in mind that the engine oil system is pressurised to around 80psi average, the coolant system will run at around 20psi. So when a plate ruptures the engine oil is pumped, under pressure, into the coolant system. The pressure quickly makes the small hole a relatively big hole exasperating the issue.
In the immediate short term, this causes an oil pressure drop to the engine bearings that rely on that pressure to work. Left unchecked the engine oil pump will empty the engine oil into the coolant system in a couple of minutes and once dry the engine will seize, unfortunately, it doesn't need to go as far as this to cause damage to the engine bearings, oil pump and crank. Any damage at all will require a full engine strip down to repair.
A grim benefit of this type of failure is that the pressurised oil will push the coolant out of the release valve in the expansion tank, that coolant will then evaporate off of any hot engine components that it comes into contact with in the form of steam. Seeing steam coming out from under the bonnet will generally encourage a driver to stop the vehicle and switch it off, hopefully in time to save the engine. Unfortunately, though, it isn't always noticed.
Sometimes an external failure will occur, dumping oil, coolant or both onto the road. If this happens whilst driving, it can be impossible to notice before it is too late.
Out of warranty engine failures cost thousands to repair - either with a replacement engine or engine rebuild.
Note: it is a common misconception that driving your car hard may cause this failure or that taking it easy will prevent it. Unfortunately, that's not how it works, and it seems to be more 'luck of the draw' as to which oil coolers are going to fail and which are not. We expect that in time ... it will become apparent that all of them are susceptible to failure, when is just a matter of time.
What's the fix?
The fix is to replace the factory PHE with a simple air to fluid cooler that works in the same way as the coolant radiator that we are all familiar with.
As a bonus, these systems are more efficient too. They offer an upgrade over the factory part beyond the engine safety feature.
The kit that we provide as part of this service replaces the factory PHE with a take-off plate that then feeds oil through oil lines to a dedicated oil radiator located behind the front grill. The coolant feed is blanked off altogether allowing the coolant to do what it is there to do and keeping the two fluids that should never mix well apart from one another.
A bit of fun watching for you ... here's one of our vlogs where we replaced the oil cooler on one of our favourite Mustang builds - the 850hp Orange Fury which, if you are interested in learning more about we wrote a blog about the car for you to read.
If you got this far, well done - hopefully you are an oil cooler expert by now, and there's no risk of you letting this common fault be the end of your car!