Troubleshooting oil temperature problem

[randytrish]

Hello all,
I have a '75 911S and have just noticed that the oil temperature seems to be running hotter than normal. I have had this car a while but have driven it very little. In the past I noticed the oil temp stayed around 190. The last couple of times that I've driven it I noticed the temp around 220. This is very limited driving, less than ten miles. I thought that I had a bad wire at my oil temp sensor. I cleaned and checked everything at the connection. When I drove it today and the temp was reading 220,230, by the Gage, my front oil cooler was not opening. I could feel the oil filter and lines and they were warm but it didn't feel like 230 plus. I only drove for about ten miles at around 50mph. I have checked my front oil cooler before and was operating. Could I have a bad Gage? Sending unit?
Any suggestions?
Thanks in advance,
Randy

[Jack Olsen]

You might have a stuck thermostat. It's located near the rear passenger wheel. The oil lines to the front cooler should be hot to the touch when the temp is at 230.

[Grady Clay]

Randy,

First I would confirm the temperature measurement. You can do that with an IR thermometer aimed at the oil tank or dip a thermocouple into the oil fill.

From your description it sounds like the thermostat to the front cooler isn’t operating correctly or the pressure relief valve in the same assembly is stuck open. When it is hot and running, try tapping the thermostat. You may end up replacing it.

The engine thermostat has also been known to fail.

What engine cooling fan do you have?

Best,
Grady

[randytrish]

Thanks for the replies. I thought at first I had a bad terminal connection at the temperature sensor. When I stopped the engine and checked the temp sensor connection and turned the switch back on it had dropped to less than 200. I really don't believe that the engine and oil is as hot as my gage is saying. In the past when I checked my front cooler it was working. I'm going to get a pyrometer from work tomorrow and check the oil tank temp vs. the gage. I have the eleven blade cooling fan on the car. I was driving it the shop to have the fuel injection adjusted, less than ten miles at 60 degree outside temp.
Thanks,
Randy

[Jack Olsen]

When you've got the pyrometer, check the tank, the external thermostat, the case and the front lines.

Does a failing ground on this circuit produce false high or low readings?

[Early_S_Man]

More resistance = lower readings. Clean the sender electrical connection and the Faston connector on the sender wire. Verify that shorting the Faston connector to ground pegs the gauge low!

I ran a test on a new early VDO sender in a pan of cooking oil on a hotplate ... using a thermocouple probe and Fluke 52 thermometer, and measured the resistance at 10°F intervals from 70°F to 300°F.

If you substitute a 1000 Ohm potentiometer for your sensor ... you could test your gauge by sweeping the range, and temporarily disconnecting the pot. to measure the resistance that produces readings at your gauge marked points.

The early gauge was never off by more than 4°F over the entire range ... compared to the Fluke 52 thermometer!

Temp - °F --- Sensor Ohms:
70 1148
75 1000
80 918
90 756
100 607
110 504
120 424
130 357
140 301
150 260
160 219
170 189
180 162
190 142
200 122
210 108
220 92.4
230 82.2
240 73.2
250 64.2
260 57.6
270 51.2
280 43.8
290 39.6
300 35.4

[JohnJL]

Warren, what was the part number or year for those measurements?

John