Let me start by saying that I have no idea how common this problem may be but I've not seen very much written about the oil cooler thermostat so I thought I'd share this experience.

Prior to installing an oil temp gauge last year I had checked the oil cooler by hand (OUCH!) just to confirm that it was getting full flow throughout- which it seemed to be. A quick check after driving with a laser thermometer showed a even temp gradient across the whole matrix, so I called it good and moved on.

Then I installed a temp gauge and started seeing some weird tendencies.
At first I suspected a bad gauge/install but a review of the system made me think that the readings were in fact correct. I even swapped back to the stock thermoclutch fan thinking that maybe my electric fan setup wasn't pulling enough air through the rad to cool the oil cooler (this was definitely a long shot as the water temps were fine but I was willing to try anything by this point).

Basically, I could be driving down the road and the oil temp would mirror the water temp (@185-190°) and then a few miles later it would rise to 210° or so. Sometimes it would go back down, sometimes not. This temp rise was not correlated to any change in RPM/speed/road conditions...it seemed completely random.
During these times oil pressure and water temp would remain stable- only the oil temp would change (I also have aftermarket gauges for these so I'm not relying on the stock gauges).

I finally had the oil cooler thermostat replaced (thank you, P.I.R.) and this weirdness has stopped.

The point here is...don't take the oil thermostat for granted.
Without a temp gauge it's unlikely you'll ever notice if the thermostat is functioning correctly or not and even though there were no obvious symptoms, the elevated temps just can't be good for the oil or the engine.

As I said previously, I have no idea how common this might be and neither did the guys at the shop (basically because so few cars have an oil temp gauge) but it's definitely something to keep in mind.

Like we already didn't have enough things to worry about.

 

I took my cooler to an aviation shop and had it cleaned and thermostat tested. Peace of mind.

 

so where is this oil cooler thermostat located at?

 

in the oil cooler....

 

it's on the bottom side under the where the bottom line connects.

 

It's the large (23/24mm) hex bolt on the bottom of the cooler, below the return line bung.
It's on quite tight so be careful when removing...without an impact gun, the whole cooler will want to torque which can rip the rubber cooler mounts.
When replacing, use a new crush washer and carefully check for leaks.

 

Isn't there a bypass method? Does that work well? How much is a new thermostat.

 

Dunno.
P.I.R. had a few used units and we just randomly picked one and tried it.
You can see pretty quickly if it's working right or not.

 

Is this supposed to be 190* farenheit? Why are you using SAE units? Is it because of the finer readout compared to Celcius, like PSI vs Barometric Pressure?

 

Um, yeah.
The gauges are calibrated in Farenheit, so that's the format I use.
I didn't know Farenheit was a S.A.E. standard.

 

I think the way it works is, when the oil is cool/cold, the Oil Cooler Bypass Valve is relaxed and lets the oil from the pump bypass the core of the oil cooler. When the oil gets hot, the Oil Cooler Bypass Valve extends and closes off the *bypass* passaage, and now the oil has to pass thru the coolers core..............to cool. So if the Oil Cooler Bypass Valve fails to it's non extended position, the core of the cooler never sees the oil . It gets bypassed all the time.

All you have to do to check it out is to remove it. Put it in a pan of water over 140*F and it should extend. It's written.

 

Although I would normally defer to your more extensive knowledge, in this case I believe you're partially incorrect.
The bypass is not an either/or situation.
When the pellet is retracted (oil is cold and the bypass is open), the core is still open and some oil flows through it. Most of the flow takes the easy/faster path but there is nothing that closes off the path through the core, so a percentage of the flow is always going through the matrix.

This would explain why my core did seem to heat all the way across the matrix even when the bypass was not closing properly. It also accounts for why the oil temps did not skyrocket beyond 210°- some of the oil was always going through the cooler, just not all of it.

On a side note...my pellet DID pass the water test.
Unfortunately, we did not think to save the oil that came out when the assembly was removed...it's possible there was a bit of crud that kept the valve from seating fully, although this seems unlikely given the flow and generally good appearance of the oil cooler interior.

So basically, all I know is that it's working now but can't conclusively say why.