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randytrish 06-20-2007 04:18 PM

watching oil temperatures....internal thermostat???
Hello all,
I've been searching and reading posts here on Pelican concerning oil temperature. I purchased a '75 911S last fall, spent most winter refurbishing the interior and getting familiar with the car. I am somewhat concerned about my oil temp in warm weather (above 80). My car is equipped with the eleven blade fan and an additional front cooler. My oil temp runs around 220-235 when driven in 80 plus temperatures at highway speeds, 3300 RPM. I am checking the temperature by the gage and also a pyrometer with an immersion probe directly in the oil tank with the engine running.
My external cooler/thermostat seems to working and opening/operating correctly.
My question is concerning the internal thermostat. How does it work and is there a way to check it's operation? I have to believe that is the original thermostat. I read here that over time the internal thermostat can fail.
Thanks, Randy

Bill Verburg 06-21-2007 05:05 AM

Both the external and internal t-stat work the same way and at the same temp ~176&deg F.

The time vs temp rise should have 2 plateaus, the first comes just after the internal t-stat opens at ~176&deg F the second somewhat hotter after the external one opens.

ossiblue 06-21-2007 08:45 AM

I also have a 75 S, and would see numbers like yours under the same driving conditions but that was before I added the external cooler. Now, the temp rarely exceeds 185 under those conditions. Have you checked how clean the coolers--both of them--are? Could they be clogged with oil film or debris? I have no experience with failure of the engine thermostat, but if it is bad I would suspect the temp of the oil line to the external thermostat would be very high. You may wish to check the relative temps of the external lines with an infra-red thermometer. Just a thought.

randytrish 06-21-2007 10:14 AM

Thanks for the responce, My oil coolers are clean, no blockage. I've checked the external cooler with a surface probe and got a resonably close reading. Actual oil temp around 215, surface of oil cooler around 200. If I drive the at slower speeds, lower rpm's it doesn't seem to heat up as high. I know that the oil temp is realitive to rpm.

ljibis 06-21-2007 10:41 AM

Just a few more data points for you...

I have a '77S with a rebuilt 2.7 - backdated exchangers, no reactors, 11 blade fan, stock "trombone" oil cooler. I've been driving it back and forth to work (25 miles one way) in 80-85 degree temps.

At highway speeds (65-85 mph) the oil temp runs right around 185, climbing to 200-210 in stop-n-go traffic. With airflow it always cools back down to around 185.

A few things to check...

Oil lines - they may be clogged or pinched somewhere, and not getting full flow to your front cooler.

Timing, both mechanical and ignition - this can have a big affect on temperature. The ignition timing is decently easy to check, but mechanical not so much (it involves an engine drop). If either is off the engine can "hold in" extra heat. My buddy's 78 930 was running 20-30 higher temps than normal because his mechanical timing was off just a bit.

Fuel mixture - is it running rich or lean? This can be checked with a CO meter, or have someone else do it. Rich running will lead to cooler engine temps, while lean running leads to hotter engine temps. If you are running the stock CIS this would be a good time to thoroughly check out the system, then check the mixture.

Internal oil thermostat - hard to get to (engine drop again), but it can be checked. This is also a very common point for these engines to leak, so there is a good "while you're in there" seal replacement job you can do. My buddy and I recently checked one by filling a deep sauce pan with enough vegetable oil to cover the t-stat, dropping in an electronic cooking thermometer, and then heating it up. Be careful if you do this as the oil expands a lot as it heats!!! You can watch the thermostat open at the approx. 180 degree point if it is functioning correctly. These can fail...I suppose they get jammed up with crud over the years.

I'm sure others will chime in with ideas. Good luck, be careful!

randytrish 06-21-2007 01:16 PM

Thanks Luke,
I have checked my lines going to the front cooler and see no signs of any damage. I had my car at the shop a couple of months back with CIS problems. To make a long story short, it was set up on the lean side. How lean, I couldn't tell you. I've tried adjusting the mixture over the past few weeks a little at a time to see if a richer setting made a difference. It hasn't yet. I havn't taken it back and put it on the analizer to check the exact setting.
When I was having CIS problems I advanced the timing from 5ATDC to 8. This helped with hot starting. I reset the timing back to 5 to see if that had any affect on the oil temp and it hasn't. The car seemed to run and start better at 8 degrees. I don't know what you mean by the mechanical timing.
In doing searches and who knows how many posts the internal thermostat kept popping up. I am not looking forward to doing an engine drop to get to this thing.
Can it be replaced without an engine drop?

PorschePilot 06-21-2007 01:52 PM

I have a 74 with a rebuilt 2.7 and a 28 row brass oil cooler.

At those temps and speeds my oil temp is 185 and goes to 210 in stop and go traffic.

You are definitely on the high side, but not to the extreme.

randytrish 06-21-2007 02:07 PM

I was hoping that someone that has had problems with their internal thermostat might see something in common with my problem. Prior to me getting this car it had only been driven 3k in eight years. The past three years it has sat. It was started regularly, but not driven. I have driven it about 1200 miles in the past five months. With outside temps in the 60's and 70's there is no problem. My oil temp stays 190-200. It's when the outside temp goes over 80 and my RPM are over 3k for a while is when I'm seeing elevated temps.

ljibis 06-21-2007 02:42 PM


Originally posted by randytrish

When I was having CIS problems I advanced the timing from 5ATDC to 8. This helped with hot starting. I reset the timing back to 5 to see if that had any affect on the oil temp and it hasn't. The car seemed to run and start better at 8 degrees. I don't know what you mean by the mechanical timing.

The mechanical timing refers to the relationship between the crank/layshaft and the cams. It basically determines at what point in the cycle the cams open the intake and exhaust valves. It is set when the engine is being built up by rotating the engine and using a dial gauge. Has your car ever been rebuilt at all, or do you know a time that the heads would have been removed?

On the 930 I mentioned previously, the mechanical timing was mis-set during a top-end rebuild. The valves were opening at slightly the wrong time leading to poor starting and running and really high oil temperatures, none of which were easy to compensate for with changes to the ignition and fueling. After spending days looking into other causes, it was determined that there had to be something off mechanically, which there was.

I'm not saying that this is your issue at all - there's no way for anyone here to say much for sure - but if your engine was apart at some point it would be one area to look into. Everything has to be set right for these engines to perform correctly.


Originally posted by randytrish

In doing searches and who knows how many posts the internal thermostat kept popping up. I am not looking forward to doing an engine drop to get to this thing.
Can it be replaced without an engine drop?

I don't think so, but I don't know for certain. Hopefully someone else will chime in on that.

If you can't get it without a drop, you could test it (per the method from my first post) and see if it's working. Then if you need to look into the mechanical timing the engine will already be out of the car.

Or you could just drive it...something worse might happen, but it may be just fine. As was stated before, it's still not at the "damn hot" stage (240-250). That said, I'd agree that it's running on the hot side. Good luck.

randytrish 06-21-2007 03:43 PM

I do not know if this engine has been disassembled. The previous owner stated it was the original engine and to his knowledge it hadn't been rebuilt. That goes back about 20 years. So who knows. It has had the needed upgrades that 2.7 engines needed. Chain oilers, fan, reactors removed. What paperwork I do have show the car only having 40k miles back in the 80's. It shows 74 now.
I had thought about trying to get more air to the cooler. I don't know what good that will do if it's another issue like the internal thermostat.

T77911S 06-21-2007 06:00 PM

my 77s had a bad internal T-stat. my temp would slowly rise, might even remain steady, but it would never go down. after new T-stat i am running about 190-200 while driving, even see it go downafter sitting in traffic.

randytrish 06-22-2007 01:38 AM

My car does the same thing. After reaching 210-220-230 and slowing down or driving at lower speeds my oil temp either remains steady or continues to climb.

T77911S 06-22-2007 09:27 AM

i think the internal T-stat is often over looked. i know i did. i put an 11 blade on and it didnt help. thermal reactors also removed. then it hit me like brick i face. the T-stat. i havent hit 210.....yet. was driving 90 - 100 on some back roads trying to keep up with my brothers 930, wow that thing is fast, temp was about 205 but came back down to 190 after backing off.
it was in the upper 80's too.
i pulled the engine to change mine. it gets pretty dirty back there and i did not want any dirt falling in, plus i needed to do some other things.

randytrish 06-24-2007 04:49 PM

Thanks for the reply,
I believe I'm going to look into changing mine. The only things I havn't checked is for a rats nest in the clynders. That may be a possibility due to where it was stored for so long. I also have a small oil leak that I suspect coming from around the oil cooler area. Is this going to require a complete drop to get to?

randytrish 06-26-2007 06:11 PM

I pulled the plates off of the front of my engine to get a look at the clynders to check for debris and stuff that may restrict air flow. Everything looked great. One other thing I checked is the temperature of the engine mounted oil cooler. I had read one way to determine if your internal thermostat was working was to take a surface probe and check the outside temp of the cooler. I had the engine up to temp and the actual temp on the oil cooler was only 150 degrees. The oil temp was around 190 and the front cooler had already opened. I don't believe my internal thermostat is opening or the cooler would be hotter. I guess the next step is to pull it and check it out of the car.
Thanks for all of the feedback. I need to study up on my first partial drop.

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