911 airlock

[Thor P]

Hi, recently rebuilt the motor, box etc on my 1982 911 still busy running her in but i have noticed that the oil temp never moves beyond cold. When i checked the oil cooler pipes, both on the motor side of the thermostat are hot, the feeder pipe to the oil cooler and the return line from the cooler are ice cold.... air lock problem ?

The motor started pushing out a bit of oil today... to much pressure... could this be caused by an air lock in the oil cooler system ? Any advice on how to resolve it would be greatly appreciated.

[Gunter]

Pushing out oil where?

The external thermostat only opens at a certain temperature to allow oil from the external oil cooler up front to return.

Check the wiring from oil temperature sender to the gauge.

Do you have the Bentley SC Repair Manual?

[Thor P]

Hi, no, i sadly did not replace the external lines or the cooler myself, i just didnt have time so sent the car in. Both lines on the oil cooler side are stone cold and definately empty - no oil in the lines nor the cooler.

[Grady Clay]

Thor P,
First, Welcome to the Forum
You will find a lot of help here.

Does tour temperature gauge move at all during warm-up or is it always pegged ‘low’?

Is your oil pressure indication ‘normal’?


First, lets veruify the functioning of the temperature gauge.
With the key ON (engine running or not), take the wire to the temperature sender and disconnect it from the sender. The temperature gauge ahould peg ‘high’.

Touch the wire to the engine case. The gauge should peg ‘low’.

Be careful if the engine is running. The fan belt (and A/C belt, if any) are close-by.

You can calibrate your gauge by immersing the sender in boiling water. Remember to connect the sender housing to the engine crankcase. Do not crank the engine or you will have an oily mess.


OK,
Now you have confidence in the temperature readings.


The thermostat – pressure relief valve (ahead of your RR tire) is open to the front cooler all the time. The thermostat restricts the return flow. When ‘off’, there is still some oil flow to the front. The ‘supply’ pipe to the front should warm even in very cold weather.


If your engine is “…pushing out oil….” There can be several possible causes:
Just plain oil leaks that need to be fixed.
There can be excess crankcase pressure from rings not (yet) seated.
It would be very unusual (almost unique) for something restricting the oil system. We need more information.

How many miles on the fresh engine?
What was done during the rebuild?
What was done to the external oil system and oil tank?
Is everything OE Porsche or is there something aftermarket?

Best,
Grady

[Thor P]

Hi Grandy,

It was a complete rebuilt with OEM parts. The motor has only done about 200 miles and apart from the oil problem runs like a dream.

I tested the sender unit before replacing the motor in the car, it and the guage functions normally. Temperature always stays fairly low on the guage and does change from ice cold to warm as the motor warms.

It is a new pressure relief valve (oem), the brass pipes were damaged beyond repair so i replaced those as well as the oil cooler. ( i took the entire car apart and rebuilt it - left nothing to chance).

There is oil to and from the relief valve but nothing is getting through to the oil cooler. Oil pressure is a constant 2 bar and runs up to 5 on load.

I guess the big clue is that the front pipe that leads to the cooler is stone cold even when the motor is fairly warm after a bit of a run.

Thanks
Thor

[Grady Clay]

Thor,

I suppose it is possible someone left sealing ‘cap plugs’ in the external oil pipes, cooler or thermostat. If so, the oil could be sealed from the front cooler and two pipes. The oil would simply circulate from the engine, through the thermostat/pressure relief valve assembly and return to the oil tank via the oil filter.

If the oil gets warm enough, the thermostat could open and no longer restrict the return circuit from the (empty) cooler. I think the pressure relief valve can open under these conditions, allowing oil to bypass the thermostat element and return to the tank.

The only ‘over-pressure’ this causes is between the scavange pump and the thermostat/pressure relief valve assembly. The system is designed to tolerate this situation.

If this seems a possibility, the solution is to remove the external oil system, disassemble it and see if all the pipes, cooler and thermostat passes oil properly. Some compressed air should tell.

Best,
Grady

[Thor P]

HI Grady,

Thanks i am going to remove the thermostat as suggested, you are quite correct, it seems that this is the most likely place to start finding the problem. As mentioned, i did not replace the thermostat myself so it is a possibility they left the "sealing caps" inside the thermostat.... they did after all point out to me that my oil guage was not working properly.. which i resolved by putting some much needed oil into the motor (twits!!!)

Thanks
Thor

[Grady Clay]

Thor,

Another possibility is a defective new “oem” thermostat/pressure relief valve.

It can’t be the thermostat simply sticking. There must be a port blocked or not machined during manufacture.

A few years ago I saw a new Factory OE spare part GT3 engine oil pump improperly cast and machined. It passed Porsche’s ‘inspection’ process.

On the supply side, the oil can pass straight through the assembly to the pipe to the cooler. The thermostst element restricts (not block) the return oil flow from the cooler.

For the pipes and cooler to be empty, something is sealed – no flow at all. It can be anywhere from thermostat assembly, supply pipe, cooler., return pipe or thermostat again. Asy seal will prevent any oil from moving into the system - “air lock”. I would think eventually some oil would enter the pipes as the air compresses and pushes the oil back out.


The oil pressure gauge is measuring the engine oil pressure circuit and not the scavenge oil circuit. (The engine has two oil pumps in a common housing.) The pressure gauge will not give any indication of what is going on here (unless no oil flows to/from the engine).

Best,
Grady

[Grady Clay]

Thor,

Do you mean that you got the car back with so little oil that the pressure gauge was indicating low oil or ‘flickering’ with picking up air?

Yikes.
There may be engine damage.
Particularly not good with a fresh engine.

How many quarts did you add to get the level normal?
(Stupid question … you do know how to measure warm, leval, idle? Just checking.)

EDIT:
Is there any chance the engine is now over-full and when warm is now “…pushing out oil…”?




Be very careful if you try and remove the thermostat/pressure relief valve assembly from the car. Most remove the entire assembly with all four hoses and pipes attached. Only on the bench can you reduce (not eliminate) the risk of damage to the aluminum threads on the casting.

When you reassemble, coat the threads with ‘antisieze’ and fill the little cavity between the nut and the fitting with grease.

Best,
Grady

[Thor P]

Hi Grady.

Yup, the idiots gave it back to me with little to no oil in it, the guage was permanently in the red and the buzzer went off on occasion. It did not seem right to me so i have been nursing it and not driven much at all.... maybe 6 miles in total at really low rpm before i checked the oil myself. I had to put in 6 pints till the oil level was in the middle or the dripstick !!! I checked their invoice and they actualy charged me for oil !! Guess i will be getting a refund on that

[Gunter]

A stock SC with the external oil cooler holds about ~13 liters.
Even with the external thermostat not allowing flow, there are still ~11 liters in the system.

6 pints are about 2.5 liters. From min. to max level on the dipstick are about 2 liters.
Sounds like there were about 9 liters before you topped it up.
The engine can run with the oil level below the minimum mark without doing damage.
Also, the level is quite different between engine just warm vs. really hot.

What is "buzzer going off on occasion"?
A buzzer for low oil levels?
I don't have one on my SC.