IF its a TD engine it sounds like the oil is getting too hot/thin which will drop presure. Oil cooler should fix. If it has the coolent heat exchanger on the oil filter houseing (usual VW mothod of oil cooling, works but could be better) its possable the thermostat in the cooling system isn't opening right as that can rase oil temps alot too even if the cooling fan keeps the water temp alright - does the fan come on much? Investigate temps. If its not triggering the buzzer when running the pump is probubly doing alright, its just too hot thus thin at idle. Thats probubly under 0.5bar though which isn't enough.
Personly I think VW DOP buzzer of doom system is quite clever really. Ocasionly dirt or poor contacts set it off but its main function is to warn of dangerously low oil presure. Its got 2 sensors so if one fails claiming its ok when its not the other should still pick it up. If it goes off check your oil presure with a guage! Do it NOW! It might just save you having to fit a new engine! Might even save you from crashing your van when the engine seases solid on the motorway. If it goes off there is a problem so don't bodge it or ignore it, fix it! Checking it with a decent presure guage will mean you know what the fault is and won't have to waist money on replaceing everything to find whats wrong.
If your lucky its just a faulty wire or sender on the monitering system so its cheep. By all means fit an aftermarket oil presure guage to watch it on the move, can give peice of mind but if you know its low, sorting it out will mean it won't bite you on the a*** when you actully want to use it (its gonna happen when your pushing it, eather with alot of luggage or when your in a rush). Treat your vehicles mecanicals with respect and it'll look after you.
I recon that indirect sensor circuit is ultra sensitive and wonder what results you would get if you wired an old fashioned mechanical pressure switch in its place.
Any views?
If you mean a mecanical capilury guage it'll have to be one with a rather long pipe! The mech guages can be pritty good acuracy, but the electric ones can also be as good, probuly comes down to the quality of the instrement rather than operation. The electric one is certainly more convienent for rear engine though can suffer from dirty contacts. The mech gives a nice pipe to your dash which is hard to route and can burst (had it happen on another vehicle - whys my dashbord leaking engine oil everywhere???). Worth noteing that both are designed as monitering instrements and do their task well enough (if they are good quality wired right) but are not super accurate calabrated test instrements like you'd probubly have in a workshop, should be within 10% though (I'm sure it'll say on the instructions if they are from a famous brand). They won't cost what a super accurate calabrated and certified instrement will of course!
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