I noticed last summer and fall that my old dirtbike has a tendency to really blacken its oil and ride rough when it gets hot.
So, last year I installed a speedometer with a temperature gauge on it. The temp gauge attaches to the top of the cylinder head, where the oil is pumped from the bottom of the case to the top of the engine.
It was running 250+ degrees routinely, and the bike would just COOK the oil if I came to a stop. 300+ if so.
These old XR's don't have oil coolers and rely on a primitive in-frame reservoir oil cooling capacity.
So, last month I put an oil cooler on the bike. Very nice kit from a fellow here in town that makes them and sells them as a hobby. His kit has fabbed CNC cut aluminum guards that integrate into the lines of the engine and make the whole setup look very OEM. On a ride this last weekend, I'm seeing oil temps down around 150 degrees running down the freeway, and 220-ish if I'm doing high RPM but lower air flow stuff in 1st/2nd gear in the dirt. Coming to a stop and sitting there for awhile the highest I saw was about 240. Air temp for the ride was about 70 degrees.
If I go from a hard work environment where I'm grunting it in 1st/2nd and have the temp up around 220 or so and get on the highway, temps will drop to 160's almost instantly.
If I kill the bike, the temp will spike to about 270, but obviously that's because there is now no oil flow at the sensor. Once the bike sheds heat the temp then drops ultimately to whatever ambient temperature is.
Researching oil temp recommendations, it seems most mechanics and engineers recommend oil to be kept right around 220 degrees. 180 is a recommended operating minimum. I guess it is hard for the oil pump to move cold oil, and bearings aren't properly lubricated if the oil temp is below 180?
Should I be concerned about the low oil temperature when cruising on the highway? Or is this more likely to be a problem with my temperature sensor which is mounted on the outside of the engine and not actually touching the oil anywhere? The temp sensor is essentially just a crush washer on the output side of the oil passage pipe from the filter/pump to the top of the cylinder. I'm wondering if it's not picking up the correct oil temp, or if this location is a particularly cold location in the oil's path through the engine. Or perhaps increased air flow from high speed riding is cooling the washer and providing a less accurate reading?
My bike is currently burning oil which I believe is due to a faulty valve, probably caused by an overheat condition from the previous config. I'm going to remedy that obviously, but I think that's ample evidence it ran way too hot before. I just want to make sure I don't blow out some internal engine bearing by running too cold. I can get a thermal bypass valve for the oil cooler that only allows oil into the cooler if the oil is over 180 degrees. Otherwise it bypasses the cooler. Jagg makes either a manual one, or an automatic one. But it's expensive. The manual one would be a PITA turning on and off when hopping between low speed and high speed environments, constantly monitoring and so on. The automatic one is $80, cheapest I can find. I'd rather not spend that money on that and earmark it for the top end rebuild.
