Hi guys,

I am having a very frustrating problem with my '98 Volvo S90. When the engine is hot, normally after over 1 hour on the highway & sometimes only after accelerating aggressively to overtake another car, the low oil pressure warning light comes up. Took it to the local Volvo dealer, they fitted a gauge and confirmed that it's loosing oil pressure when the engine's hot. They took the oil pan off but found the o-ring was ok, then they took the oil pump off and found pump & relief piston to be ok as well (and they told me that my relief piston was metal, not plastic like the earlier ones). They cleaned the oil pickup tube & fitted everything back with new seals, o-rings & oil filter but the problem is still there!!!

They did tell me that when they were driving around with the oil pressure gauge attached, when it looses pressure it will suddenly drop off to around 0.5 bar or 7psi, instead of gradually loosing pressure over time. But when the engine is turned off & started again immediately, the pressure comes back to normal!! Also when the pressure does drop off, it's low enough to activate the low oil pressure warning light, but not low enough for the hydraulic tappets to be making noise & complain of low oil pressure, they were still quiet.

So the Volvo dealer here concluded that it's loosing pressure internally, likely to be a cracked main bearing or oil galley and requires full stripdown for further diagnosis, but I still feel it's quite unlikely giving that it's only got 100,000km (65,000 miles) on the odometer. Besides, if there was a crack somewhere that widens up as the engine warms up, wouldn't I be loosing pressure gradually not suddenly? And wouldn't it still be loosing pressure when the engine is turned off & on again? Because the engine's still hot so the crack would still be there.

I asked a friend who used to be a Volvo mechanic and he thinks it's most likely the pressure relief valve, particularly how it happens suddenly & returns to normal again when it's restarted, and the dealer probably didn't measure the relief valve spring tension and gear tolerance in the oil pump carefully. The Volvo dealer on the other hand claims that with a faulty relief valve, it would not be normal for pressure to return after restarting the engine so they still claims the fault is internal within the engine.

Has anyone here seen anything like this before? What are your thoughts? It's frustrating me and I hope I can find the cause without spending any more money on the car than I already have.

Thanks,

William