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Oil Pressure Light Comes On

1976 911. The car drives well.

When it is cold everything is fine. You can drive all day on the highway with the thermostat reaching a max of about 215 with no issue. Once I have to stop at a exit ramp, behind traffic, or for any other reason the oil pressure light comes on and stays on.

Additionally, once the thermostat reaches about 180 and I am in stop and go traffic the oil pressure light comes on and stays on. Does not go off until the car has sat and cooled off. The car has oil (currently about 1/3 of the way up on the dipstick) taken with engine running and the car fully warmed up. The oil pressure gauge in the car shows a little over double the tachometer reading when the car is cold (below 180) and about double the tachometer reading when the light comes on (over 180).

This repeats itself everytime I use the car for more than about a half hour (withthe weather cold this week about 50 minutes). I have pretty limited mechanical ability but follow directions very well. What would cause this? What should I be checking? How bad is this?

Thanks

Old 12-19-2003, 06:30 AM
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You may want to move to a higher viscosity oil. The reason everything's okay when the car is cold is because the oil is thicker and more viscous. Moving to a higher weight oil (say 20w50) might benefit you.
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Old 12-19-2003, 06:49 AM
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It has to be more difficult than just changing the oil...
Old 12-19-2003, 01:12 PM
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do some test ..a real o.p. guage so you can see whats going on for real, pull you thermistat and chek that it working ...once you know these ans. you will know which medicene to take
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Old 12-19-2003, 04:45 PM
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Here in Florida, you will want to use 20W50 for non-synthetic or 15W50 if you use synthetic

This could have a number of causes. I would start with thicker oil first and move on from there.

Other possible causes:

Bad oil pressure switch
Stuck\bad Pressure relief valves
and a host of other problems.

AFJuvat
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Old 12-19-2003, 07:30 PM
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I changed the oil to 20W50 and used the car. The weather here was pretty cool and the car temp only got to about the 200 range. The oil pressure light did not come on during highway or city driving except after a short stop. After driving for about 45 minutes I shut the car off to run into the drycleaner. When I started the car the oil light came on and stayed on until I stopped again and the car had a chance to cool off.

This is a definite improvement but still not great. I am still looking for suggestions.

Thanks.
Old 12-21-2003, 05:39 PM
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What color is your oil pressure light? Jim
Old 12-21-2003, 05:54 PM
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I highly recommend that you stop driving the car and either perform a real oil pressure test or have a shop do it. If you are starving the engine for oil it will cost you big money, this is why Jim asked you what color the light is - its red!

While you are checking the oil pressure and bypass etc, check the screen in the sump as is may be dirty and restricted.
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Old 12-21-2003, 06:24 PM
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What does your oil pressure GUAGE read when it comes on? What is your RPM at that time?
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Old 12-21-2003, 08:27 PM
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It is a green light that is coming on.
Old 12-22-2003, 06:30 AM
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I'd suspect the oil pressure switch.
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Old 12-22-2003, 09:24 AM
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isn't the green light the alternator light?
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Old 12-22-2003, 09:52 AM
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On our mid year cars, the oil light is green. The only thing that helped me maintain oil pressure is to install the Carrera oil cooler and Elephant finned lines with factory thermostat. Now no issues even when hot and idling. Keep in mind though that my oil pressure light never came on, even when hot. I would do as suggested and have a competent mechanic who knows 911s diagnose what's going on if you can't do it yourself. Otherwise, serious damage can occur.

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Old 12-22-2003, 11:28 AM
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Cool

I bet the oil sw is triggered by less than 10psi.
If the oil psi is between 4.0 & 5.5 at 5k rpm when warmed up everything is OK. I think hard info about this is in the owners manual.
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Old 12-22-2003, 01:51 PM
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Ok, a couple of points here:

- If your real oil pressure is low enough for the pressure light with working sending unit to come on with a warmed up engine and 10w50 you are probably looking into shot bearings and some rebuild in front of you.

You might want to check your oil pressure by a shop and/or buy a new sending unit. It's easy to replace.

- You might bandaid the effect it by installing the restrictor for the cam housing oil supply. It is said to raise the oil pressure for the main bearings.

Check if it is just an electric gremlin - I once wanted to tidy up my engine bay and stuffed that one wire that is by itself rather than on the 14 Pin connector under the fuse box. After that my oil light came on in right turns!!. What happened was that the green/??? wire was brittle and shorted out against ground. That set off the oil light. Meanwhile the pressure gauge showed perfect pressure so I knew it was an electrical gremlin.

Ingo
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Old 12-22-2003, 03:01 PM
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Problem solved. A bad oil pressure switch. The plastic housing was cracked. When everything was cool it stayed together, When things got warm it spread open just a little bit. Switched the switch and everything is running well.
Old 01-03-2004, 06:38 PM
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you still do not know what your O.P. is????!!!!

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Old 01-03-2004, 06:50 PM
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