Crazy oil temp issue with my 75

[RetroSC]

So I've been fighting high oil temps this summer and stumbled on some odd behavior with the front cooler. This is on a stock 75 911s w 2.7.

Generally on a warm day while running A/C I would see temps creep up to near 240, this with a front carerra cooler with fan running. I/R tests of those front lines showed they just weren't getting too hot, maybe 150 up at the cooler. So I thought the thermostat was bad, right? Replaced that (what a royal pain btw) and still same symptoms and temps.

Removed the new thermostat and tested both it and the old one in water, and they both open fine around 190. What the heck?

So I did a test, I forced the thermostat plug to be fully open and reinstalled in that state, then did a warm driving test. Temps wouldn't get past 190. So the cooler has the capacity to cool, it's just when the thermostat is left to it's own operation it's like the engine is already too warm and it can't bring it back down.

Thoughts?

[juanbenae]

what are the ambient temps you are driving in?

does your fully open thermostat trick need to have it removed to do the test you describe?? or reinstall if on the car? if it does not and there is not too much oil loss during the procedure why not just do this as a seasonal thing?

do you still have the 5 blade fan?

or, turn off the AC.

[boyt911sc]

Retro,

Both thermostats opened @ 190°F during your test but did they both fully open or partially? So the slide valve/s were not freely sliding and that’s why you forced it to open? Please clarify. Thanks.

Tony

[RetroSC]

Turn off the AC??!! It's an 11 blade fan. It actually runs about the same without AC, maybe 230+.

Yes, I could do it seasonally, just messy and it's really hard to get the plug back in under spring pressure. Seems like it should just work better.

[RetroSC]

Originally Posted by boyt911sc

Retro,

Both thermostats opened @ 190°F during your test but did they both fully open or partially? So the slide valve/s were not freely sliding and that’s why you forced it to open? Please clarify. Thanks.

Tony

Both opened fully, so working properly. I put a spacer in one of them and held it in the fully open position and reinstalled.

My theory is by the time the thermostat gets fully heated and open it's dealing with a runaway thermal event and there's not enough capacity to bring it back down?? But when I run it fully open all the time it just never gets a chance to get that hot.

What I don't understand is why that's happening, wouldn't everybody be having that same issue.

[dannobee]

Are you sure that the temp gauge is accurate?

What are the in and out temps at the front cooler? Do the cooler lines look ok? No pinching by someone putting it on the hoist?

[kbken62]

How is your oil pressure when hot?

I had a similar problem on my 2.7 and oil pressure would fall significantly when hot.

Upon suggestion from others I had an old style Oil pressure bypass that the spring allowed oil to bypass and lower pressure. Also the cam oil line restrictor was not stock and an upgrade.

Pictures attach from our host website.




There are threads on the oil bypass piston. There are two styles.


Sent from my iPhone using Tapatalk

[HaroldMHedge]

My 87 3.2 used to run about 240F in hot weather. To fix this I changed the factory fan thermostat to one which opened at 210F vice the factory 240F. I used the Patrick Motor sports kit. Since changing the fan thermostat, I've had no problems with the oil temperature even in 110F heat with the AC running. It now stabilizes at about 220F.

[75 911s]

Definitely something going on. FWIW I also use the cam restrictors on my 2.7. So you mention two thermostats, meaning the old and the new. These are the ones on the back of the engine right? Not the thermostat which works in the same manner on the lines in the wheel well. Have you tested that thermostat?

[RedCoupe]

Originally Posted by 75 911s

Definitely something going on. FWIW I also use the cam restrictors on my 2.7. So you mention two thermostats, meaning the old and the new. These are the ones on the back of the engine right? Not the thermostat which works in the same manner on the lines in the wheel well. Have you tested that thermostat?

+1 One thermostat controls the engine-mounted oil cooler, while the other one (the one in the wheel well) controls the front cooler.

[RetroSC]

Originally Posted by RedCoupe

+1 One thermostat controls the engine-mounted oil cooler, while the other one (the one in the wheel well) controls the front cooler.

Yes, I've only been messing with the fender cooler thermostat.

My oil pressure does get fairly low when hot. Is the thinking then that if that happens there's not enough pressure to push it through the front cooler?

[RetroSC]

Originally Posted by dannobee

Are you sure that the temp gauge is accurate?

What are the in and out temps at the front cooler? Do the cooler lines look ok? No pinching by someone putting it on the hoist?

I can't say for certain, but I've tested two temp sending units, both the correct type and the gauge acts the same.

Cooler lines are actually new.

With thermostat not rigged and the temps at 230+, I see around 190 at the thermostat housing, 160 into the cooler and 140 out. So it's almost like the thermostat is kicking on/off and there's just little flow to the cooler.

With the thermostat rigged open, I see 180ish at housing, same at the cooler, and maybe 160 coming out of it. Engine show's maybe 190.