The weekend before last, I had the kids. My mother drove came down in order to attend the Monster Trucks with us. I loaned her my vehicle to drive home. She had been gone about an hour or so when she called on someone's cell phone. She said the vehicle had overheated, and she would be waiting at a service station.
The vehicle was out in the desert, along the highway. When I arrived there, the first indication of a problem was the oil all over the back, dripping off the mudflaps, tailgate, and spare tire. The second indication was opening the hood to find the oil pressure line melted, the radiator hoses blown, etc. Yes, it overheated.
It overheated when the oil ran out, right before the engine siezed.
I asked her about oil pressure. She said it was fine. She said the engine lost power and she then glanced down at the temperature gage. It was pegged. She pulled over and the engine quit (with a clunk and a bang...when the rod broke...). I asked her where the oil pressure gage was, and she didn't know. Did she happen to look at the big gage stuck in the dash, where I cut a hole and installed it? Uh-uh.
Today I get to go rent a u-haul and tow it back into town (save the three hundred bucks the towing company wants to do it).
Why does the tempeature go up when oil pressure goes down? It doesn't. It can go up when quanity goes down, due to less oil to carry away and dissipate a certain amount of heat. However, pressure can decrease as viscosity drops, as indicated by other posters.
It's worth noting that many oils experience a vicsosity INCREASE with an increase in temperature of oil, especially in the case of multiweight oils. The purpose is simply to allow flowage at reduced temperatures, and still maintain adequate viscosity at operating temperatures. Accordingly, the oil becomes more viscous with an increase in temperature, within a certain viscosity range.
At reduced oil pressures, damage can occur long before a thermal change is noted. Aside from the damage, you need to know where the temperature is being measured in the oil. Some measure the oil tank, others the oil inlet, some the pump outlet, some the cooler outlet...are you measuring the effectiveness of the oil cooler, or the output temperature from the engine? You are going to see very different readings, and a time delay after oil loss of pressure loss. Typical readings are post-cooler, meaning you have more stabilized continuous reading, but a delayed one.
With a normally operating system, a decrease in volume or pressure should cause no change in temperature. Your system vernitherm and other temperature regulating devices in the oil system should account for the difference, and route more or less oil through the cooler, heat exchanger, etc.
It has nothing to do with thermal expansion, as typically volume changes very little. Viscosity changes, but volume doesn't change much at all. The biggest change when you have less oil is that you have less material to move the heat, and you can see a temperature increase. It should be noted that less oil typically doesn't mean less pressure, unless there is so little oil that the pump cannot be fed. How much oil is retained in the tank means nothing to the pressure, unless there is insufficient to leave the tank (or sump) for the oil pump...in which case you're in a world of hurt.
The times I've had significant oil loss have typically been marked by a visual indication. I've seen the wing completely covered in oil, or oil spraying or pouring from the engine nose case or other areas (in the order of gallons). Often oil pressure doesn't change until a very significant amount of oil has gone, or a massive leak has occured and it just can't be maintained. In most cases, we never saw a temperature increase; we simply shut it down. Had we continued to run it long enough, we woud have seen the increase.
In the case of my Samurai, the temperature increase isn't due to oil loss directly, but rather to the fact that someone drove it to destruction without noting the oil loss. The temperature increase was enough to encourage me to poke a hole in the side of the car with the engine prop rod, anyway.
Anybody got a spare Samurai, Switft, Tracker, or Sidekick engine they want to get rid of?
