BW35 and mysterious leaks while parked - a possible explanation.....
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After forum reading, catalogue study, and thinking, I have come to this belief: Correct me if I am wrong or missing something.
We may consider the system as a box with a fluid pan and a pump that sends the fluid up in two loops. The first one is the converter. The next one is the cooler which is integrated in the water radiator. The first one is pressurized, but leaving that it is a free flow through the second back to the pan.
There should be a check (non return) valve somewhere. I cannot find it in the parts catalogue.
At the very first this model explains why the fluid level must be checked while running: Only then will the pump ensure everything is filled to operating conditions. Fluid level may be higher while stopped.
On the left hand side of the box casing are three holes where moving parts are passing through: The kickdown cable, the inhibitor switch and the PRNDL-shifter. All are threaded, sealed and above normal fluid level. The breather tube is sitting even higher and has a top "Neck" further up high. The speedo drive on the rear extension is attached higher.
Now, if air can enter either of the upward loops (converter and cooler) when parked, fluid will flow back to the pan, raising the level up above the three holes, or even the breather entry point. This is no problem if all gaskets are OK: First time the engine is started fluid will be pumped back where it should be - converter and cooler. Level correct: Drive off.
If any of them have a leak, fluid will disappear out of the system.
1) The converter is pressurized while operating, so any leak will be VERY obvious, but only with a started engine.
2) I would rather suspect the cooler circuit - interconnected pipes and tubes. Let us just imagine you had the radiator out and one of the joints is no longer fully sealed. You will not notice it with a started engine: this part is not pressurized. But stop, and air will start entering. Btw. you cannot see the bubbles, because you would have to sit inside the pipe.
Remedy?
1) Ensure there are no leaks in the piping. Difficult. Best you can do is ensure all connections are clean and properly and solidly assembled.
2) Find the leak where fluid gets out. You will need litres of new fluid, so buy it. The three primary candidates are very close to each other, so clean and dry with a rag and pour some fluid in the box. Remember the fluid level is flush with the leak, so just a cup of fluid will reveal the leak almost immediately (the time you use to to get under the car should be sufficient).
3) Fix it - gaskets, seals, PTFE-tape, inhibitor switch, kickdown cable..
4) Harvey mentions possible availability of external non-return-valves. Consider asking him where to get them, where to install and in which direction.
5) Refill. Happy Rovering