A near disaster and question-oil line connections

[jwakil]

Yesterday I replaced my leaky short hoses to my front oil cooler. After cursing the person who put the hoses on the cooler itself (had to dremel the nut off they were on so tight) I put the hoses on with only slight force. After starting the car in the morning (cold engine) I was backing out of my driveway and noticed a trail of oil on the ground. When I looked at the cooler with the engine runnng, there was a pretty heavy stream of oil coming from the oil cooler connections. Luckily I was able to tighten the nuts (now I don't hate the previous person so much) and stop the leak. I would have been in deep trouble if I hadn't noticed that oil trail as I started driving. Anyway, my question is, I thought with a cold engine there wasn't supposed to be oil going to the front cooler. Why was I getting oil pressure buildup in the front hoses? I always suspected my auxilary thermostat was not sending oil up front until too high a temp since the lines would be cool until engine reached ~230C. I didn't expect any oil to be flowing there when cold!

[Grady Clay]

What year is your car?
Is the oil system all original Porsche?
What thermostat and pressure relief valve?

Best,
Grady

[jwakil]

It's an 83SC, with original aux thermostat and lines. The engine happens to be an 77 3.0, but I don't think that would make a difference.

[ossiblue]

The external thermostat is always open from the engine line to the cooler and back through the return line to the thermostat. When the engine is cold and the thermostat closed, the oil fills the lines and cooler, but does not circulate because the thermostat closes off the return line to the tank. Oil that is circulating through the engine goes through the bypass in the thermostat and flows into the tank. When the oil heats up enough, and the thermostat begins to open, the oil will begin to flow into the tank from the cooler return line and oil from the engine will begin to divert from the bypass into the hard line to the cooler. Simply put, the lines to and from the cooler as well as the cooler itself, are always "full" of oil, even when the engine is not warm enough to open the external thermostat.

[jwakil]

L. J. ...I had read a similar thing, but that doesn't make sense to me at all. I thought the whole purpose of the thermostat was to protect the oil cooler from high pressures when engine is cold. If the oil is allowed to go to the cooler and return is blocked, that would create the highest pressure situation, wouldn't it? You would want the supply to be shut, and return to be open, or both shut.

[James Brown]

If you look in your Bentley on page 170-3 and read the paragraph on the oil pressure relief valves operation under the note. The relief valve and the safety valve help protect the engine mounted oil cooler, oil filter, and oil lines. The aux cooler (in the wheel well) is supplied oil from the scavenge pump to it's own pressure relief/thermostat valve. What I don't know is what the pressure relief valve is set to open at.

[ossiblue]

Originally Posted by jwakil

L. J. ...I had read a similar thing, but that doesn't make sense to me at all. I thought the whole purpose of the thermostat was to protect the oil cooler from high pressures when engine is cold. If the oil is allowed to go to the cooler and return is blocked, that would create the highest pressure situation, wouldn't it? You would want the supply to be shut, and return to be open, or both shut.

Your concern is the function of the pressure relief valve which is also in the thermostat housing (some early external thermostats, like on the 73.5 I believe, did not have the relief valve) and the by-pass valve. These valves allows cold oil to flow easily back into the tank without adding pressure to the "filled" cooler lines. The factory thermostat performs two functions--to regulate the pressure to the front cooler, and to open/close flow through the front cooler.