troubleshooting oil temp gage problem

[randytrish]

Hello all,
I've been the proud owner of a '75 911S for about 8 months now. I have recently fixed my cis woes and started to drive the car some. I've noticed some inconsistencies in my oil temp gage. Before due to my cis problems I wasn't really driving far enough at a time to fully get the engine to operating temp. My car is equipped with what I think is the Porsche additional front cooler. What I've noticed is that I can watch the gage and after about 10 to 15 minutes of driving my oil temp will reach around 210 and then drop back to around 190. I have to assume this is the front oil cooler coming on line. After driving longer the temp will continue to rise back to the 240-250 range. I have a immersion pyrometer and can check the temp of the oil in the tank while the engine is running and I have never read over 215 degrees, it is usually around 210. My gage will sometime flicker between the the 220 range and 250, by flickering I mean the needle will bounce. I have replaced the oil temp sending unit with a new one.
I have a few questions.
1) Will the temp of the oil vary by 30 degrees from where I'm checking it in the tank vs what I'm getting on the gage? Or is the oil in the tank with the car running a pretty close actual reading?
2) What is a good way to troubleshoot the gage?
3) Am I just being paranoid and really have nothing to worry about?
4) Is there something in the oil cooling system I need to check?

Thanks in advance,
Randy
3)

[bigchillcar]

can't really answer your question, but a 2.7 with an aux cooler ought not be getting into the 250 range except on a realllly hot day. i drove a '74 without an aux cooler for 7 years and saw that temp from time to time, but sounds like you're seeing it when you shouldn't? what driving conditions and ambient temps bring you to 250?

[randytrish]

I agree with you, I shouldn't be seeing those types of temperatures under the conditions I'm driving. My main questions was how much of a difference should I, if any be seeing between the gage reading and the actual tank reading? I believe I have a bad gage.
Randy

[ossiblue]

Randy,

I, too, have a 75S with a 28 row external cooler and have never approached the high numbers you posted. Since you already replaced the sender and the fact that your gauge jumps tends to indicate a gauge problem, in my opinion. I'm not an expert, but under normal driving conditions I would think a 30 degree difference between the block and the tank is too much.

[vash]

which gauge do you have? on my previous 75 car, i had a gauge that maxed out at a higher number. 350? i cannot really remember. anyways the point is, there really wasnt a sender calibrated for this gauge. i bought i new numbered gauge with matching sender, and gauge never went uber hot anymore. then i added a carrera cooler, and never an issue ever-again.

[JP911]

Are you sure that you are using the correct sender? Other possibilities: the gauge is bad, the thermostat for the external cooler is going bad, there's a short in the wiring between the sender and the gauge, or the main engine thermostat could be failing. I'm sure that there are other potential causes as well. The other consideration is the type of driving conditions in which you're experiencing the spikes in temps. Moving at a good pace = air getting the cooler, stop-and-go traffic = little air moving over the cooler.

[randytrish]

Jon,
Thanks for the reply. I've considered all of the above except the main engine thermostat. I'm trying to familarize myself with all of the oil cooling circuit. What exactly does the engine thermostat do? Does anyone know the wiring connection locations for the oil temp circuit from the engine to the guage.
Randy

[JP911]

The wiring goes into the 14 pin connector. Do a search for the connector to find out which pin.

[304065]

Oil System Drawing

[nhromyak]

I just redid my grounds on my oil-presure sender gauge. I found the grounding nut and faceplate on the gauge itself to be corroded.

Compare ohm readings between your grounding gauge faceplate and the ground at the battery. IIRC - I had about a 20 ohm difference. My symptoms were a bouncing pressure gauge pointer though.

Good luck.

[randytrish]

Thanks Nick,
I've reseated the connections, but didn't check the ground at the gauge. As I mentioned above, I don't really believe that my oil temp is what the gauge reads. It seems like it's reading accurately until around 200 and that is when it starts exceeding the actual temp of what the oil is in my tank. When I've had gauge readings exceeding 250 it was still only around 215 in the tank.
Randy

[ossiblue]

Jon and others have given several possibilities on this. I just looked at an oil circulation diagram out of Haynes and it shows the temp sender basically at the end of the oil circulation loop. Oil enters from the tank and goes directly to the thermostat/cooler. If your on-engine thermostat or cooler is malfunctioning, oil could be heating up rapidly and the sender is sensing this. The only thing that still seems odd is the jumping of the gauge needle. The senders work by electrical resistence changes which tend to move slower and smoother. Still, I wouldn't count out the cooling of the engine thermostat/cooler, as Jon pointed out.

[randytrish]

How do you know if your on engine thermostat/cooler is operating correctly?

[lateapex911]

According to the diagrams posted by John, the oil reaches the temp sensor after either one or two coolers, so it should be at it's coolest at that point. I imagine it's at it's hotttest in the sump, then gets scavenged up and sent to the aux cooler, then to the tank. Then it gets pulled and pressureized, sent though the on engine cooler (if it's higher than the thermostat set point) and then gets sent inot the journals, and to the sensor/sender.

So....it seems to me that any tank reading should be higher than a guage reading.

[randytrish]

I've checked the tank reading at different temperatures as the oil is heating up and compairing to the gauge. They are within 5 degrees of each other until around 200. That's when the gauge starts going up and the oil in the tank remains constant. I would like to rule out a problem in the wiring, then if needed purchase a new guage.
Randy

[bigchillcar]

is the aux cooler hot to the touch when the gauge is reading 250?

[randytrish]

yes, I've checked it with a surface probe and it reads about 190 when the gauge is at 250.

[bigchillcar]

what i do know is that the thermostat is supposed to open at 182 degrees per porsche, allowing oil to flow into the cooler. can you check with your probe to confirm that it's opening at that temperature? maybe check that the aux cooler is still cool at around 170...and then check it around 190, as indicated on your gauge. but it is sounding like either a sender or a gauge issue.

[randytrish]

Thanks to all of the replys and suggestions. I believe that my aux cooler and thermostat are working correctly. I am going to (better) check my wiring /ground connections at the guage itself. I found a new guage and sending unit that I can purchase, I hate to purchase a new guage and it be a wiring problem. We'll see.

Randy

[ossiblue]

Randy,

You may wish to try an infrared thermometer. Warm the car to operating temp so the external cooler is functioning. Shoot the temp of the oil line that enters the external thermostat from the engine, and compare it with the temp of the oil line from the thermostat to the tank. That should give you a relative reading of the difference of temp from oil leaving the engine and that in your tank. I've never used one of the infrareds, but I know some on this board have. You can check the accuracy of the returned oil by comparing the reading with one from your probe in your tank.