Oil pressure

[jack_aubrey]

Hi

I bought an '82 Volvo DL diesel wagon not that long ago, and recently the oil pressure warning light came on and stayed lit whenever the engine was on. I swapped the sender units to no effect, so I ran the engine for about 30 seconds with the sender unit and valve cover off the engine, and no oil emerged. So, I removed the engine from the car to check the oil pump. When I removed the pump, everything about it seemed to be in perfect working order, other than a spring-loaded tube on the right side of the pump which seemed to be stuck halfway open (first picture) (where exactly does this lead to?). After prying on it a bit, it opened all the way and then closed fully (second picture). The other possibility is that there's something stuck in one of the oilways; how would I go about testing this? Also, I hadn't driven the car enough to change the oil, so I'm not sure how long it's been since an oil filter change, but the oil pan did not have any sludge or chunks in it. Could the filter simply have been clogged enough to cause the oil pressure light to come on (or would an oil-soaked filter always block blown air, regardless of being clogged?), or is the stuck valve in the oil pump more likely the culprit and if so, how would I ensure that it was fixed?