Oil pressure drop on auto transmission S3

[panamacarrera]

The rebuilt engine in my S3 OTS automatic has good oil pressure most of the time, but when I come to a stop and leave the transmission in 'Drive', at a set of traffic lights for example, the oil pressure gauge gradually drops to 0. The warning light does not come on.
Tick over is about 700rpm and drops to about 650rpm when the car is in 'Drive'.
Has anybody else had this problem? Has anybody found a fix?
Thanks

[christopher storey]

There is something wrong, but what it is may vary from the trivial ( faulty oil pressure sender ) to the moderately serious ( e.g. pressure relief valve damaged or otherwise held open ) to the serious ( worn bearings) . Since the senders and to a lesser extent the gauges are notorious for inaccuracy I should start there by checking the real pressure with a mechanical gauge. As a matter of interest, does the same occur if put into neutral or park ?

PS Marek is the expert on the V12, and therefore may say that this idea is a non-starter, but have you ensured that you have not overfilled with oil and are getting frothing and/or cavitation at the pump as a result ?

[panamacarrera]

Hi Chris
This only happens with the car in drive. To me, that means it’s related to the reduction in rpm from +/-700 rpm to around 650rpm. Oil level fine.
Hopefully it’s just a gauge/sensor issue. How would I check with a mechanical oil pressure gauge? It’s an early V12 so the sensor and oil pressure switch are buried under the left side exhaust manifold.
Thanks

[MarekH]

You know that you have at least 2-3psi as the oil switch does not come on. You also report good oil pressure at all other times, so don't panic.

It's hard to imagine anything electrical is going to run reliably and consistently at 650rpm. At that speed, you are probably going to start draining the battery as the alternator will be struggling to produce anything at those revs.

The oil senders were moved up top to the back of the "V" presumably because they got toasted in that location and the first port of call is to work out whether the sender is even remotely accurate when hot and at low pressure and supply voltage anyway. To do that, you are going to have compare against a manual gauge. I guess it's time to read up about manual gauges.

If you have a bicycle track pump plus any old spare switch/sender, then you can test the suspect sender/gauge against the track pump whilst the car is running. (The spare is just used to plug the hole where the current sender sits so oil doesn't tip out.)

kind regards
Marek