Low oil temperature reading. Any ideas?

[KS Carrera]

As I have now finally got my project car on the road (only four years, but didn't like to rush... ha ha!), I was always concerned that the oil temperature might be high once I started driving the car as it's a reasonably high-spec motor with 10:1 CR etc. In fact, just the opposite seems to be true. Although I have not pushed the engine under any load or at high revs while it's still fresh, I can't help wondering about the oil temperature reading seeming very low, even when running on the rolling road during ECU set-up.

The gauge is in Centigrade, with figures marked at 30, 80, 100, 120 and 170 degrees. Date-stamped 12/69 and from a 911S. The temperature sender is a genuine Bosch, part number 901.641.632.00.

On start-up, the gauge goes to 30 (effectively 'zero' at this point, buy the lowest the gauge will read) and as the engine warms up, it reaches just over midway between the 30°C and 80°C marks (approx 60° if it's vaguely linear) and then stays there, not rising or falling. The ambient temperature here right now is about 15°C at best (around 60°F).

I have checked the gauge by 'flashing' the sender lead to earth, at which point the gauge goes to maximum. I have also checked the sender as best I can with an ohmmeter in boiling water. The resistance falls as the temperature rises roughly in line with figures found on PP. Using a temperature gun, the surface reading of the sender is not a million miles off the gauge reading with the engine running.

I began to wonder if the thermostat was jammed open, but this morning I removed it and, again using a can of boiling water on the stove (just as well Sarah's at work...), the 'stat began to open when the body reached 80°C, opened fully before 100°C, and then closed off again when I ran it under cold water. So, it looks like it's working OK.

I am at a loss. The engine currently has just the stock cooler – I'll run a front-mounted one if it proves necessary, but at the moment, high oil temps look the least of my worries.

Is there an obvious point I'm overlooking, or is this simply a case of the ambient temperature being relatively low? I'd still expect the temp gauge to read higher than 60°C, or thereabouts, though.

[sithot]

Can you borrow an IR temperature gauge tool? You can point it at the sender and get an approximate reading. I consider these a "must have" bauble for the tool box.

https://www.google.com/#q=infrared+temperature+gun

I'm also wondering whether the sender and the gauge are "compatible".

Let us know what you find.

Good luck!

Tom

PS: If your engine is no more than 2.0 liters it will run inherently cool, but not this cool!

[KS Carrera]

I have one of the IR temp guns, which showed the surface reading (as mentioned above) to be not far off the gauge reading, but of course the surface reading will be less than the oil temp itself. The sender 901.641.632.00 is the correct one for 66-74 cars (some say up to '77), so is right for my 1970 model year gauges. I am beginning to suspect the gauge itself may be at fault. Just need to borrow one to check!

The engine is a 2.2 (+1.00mm), 10.0:1 CR with S cams etc.

[KS Carrera]

Discovered the problem! Reading back through some old forum postings of mine, I discovered the oil temp gauge module had been swapped for a later type from a 911SC! I've answered my own question (age is a terrible thing...). I need the later type sensor, which is now on order. Thanks all for your input.

[sithot]

As alluded to in post #2. Senders and gauges need to be compatible.

Isn't it fun to clean up a mess you didn't necessarily create?

Glad you figured it out. Good luck with the fix.

[PMNorris]

I've had the same problem since i fitted a 3.0 SC engine in my 70T. Presumably if i fit the earlier sensor in my engine i will also resolve the issue?

Thanks
Paul