I don't know the Tau 5.0 setup but I have general engine knowledge/experience from many other cars. Get a small flashlight and eyeball the hoses/pipes going to the oil cooler. Often a small leak on those fittings is hard to spot because oil may not flow through the oil cooler until an oil-specific thermostat says the oil is hot. So running in the driveway may not get the oil hot enough to have it pumped through the cooler. Several high-speed highway miles should do the trick.
A problem in the engine's crankcase breather system (PCV) and/or excessive blow-by that pressurizes the crankcase shoves oil through the PCV valve where it ends up getting burned and expelled with the exhaust. A cylinder leak-down test (dealers know what this is) will easily identify a cylinder with bad/defective piston rings; leak-down tests are more reliable than simple compression tests. In a nutshell, the leakdown test is:
1: remove the
spark plugs
2: manually rotate the engine so the cylinder being tested is at "top dead center" which means the piston is all the way UP, all intake & exhaust valves are closed, etc... in the normal 4-stroke cycle the "power stroke" would be about to begin.
3: an air compressor hose connects to the
spark plug hole via a mechanism with two pressure gauges and a small valve/orifice between them. This orifice limits how quickly air can move through the mechanism. Imagine filling a plastic bucket with a garden hose turned on full-blast - it'll fill quickly, right? If that bucket had a quarter inch diameter hole in the bottom you'd still be able to fill it with the hose on full-blast but it'll take a little longer. Next, imagine trying to fill that bucket (with the hole) while the faucet is barely turned on: that little hole may drain faster than you can fill the bucket. The leak-down tool's "orifice" mimics the faucet being barely turned on. The quarter-inch hole represents leaks in the piston rings and/or valves of an engine cylinder. In a healthy engine, the hole is really tiny - i.e. there are only very minor leaks inside the engine - so the air pressures before and after the orifice end up pretty close (bucket is full). If the engine leaks more than it should, the second pressure gauge will read low because the orifice-limited airflow can't fill up the cylinder.
By the way: "blow-by" is air+fuel combustion products that squeeze past the piston rings and end up in the crankcase. All engines have a little blow-by; worn or defective engines have a lot more. The PCV system allows these fumes to get sucked into the intake manifold of the engine where those fumes will then get sent to the cylinders again to be burned. The PCV system flows a little bit of air volume... which is supposed to be much higher than the "normal" blow-by volume. This keeps the engine crankcase (area below the pistons, where the oil and crankshaft live) at a slight vacuum. Excessive blow-by though overwhelms the PCV, causing combustion pressures to build up in the crankcase. That in turn shoves the oil around, possibly through the PCV valve where the oil gets burned... or it shoves oil past the various engine seals/gaskets leading to a messy engine and stains on the garage floor.
I've also seen engines that use "oil separator" devices (again, I don't know if the Tau 5.0 has one; many turbocharged engines do though since PCV systems can't work with a turbo-pressurized intake manifold) leak. The oil separator basically separates crankcase fumes from atomized oil - it's just a tank with 3 ports: inlet from the crankcase that may have oil droplets along with the blow-by gasses, a drain at the bottom that sends separated/captured oil (from those droplets) back to the oil pan, and a port mid-way up that hooks to a non-pressured part of the intake tract. If the oil drain hose is missing, you'll have a slow oil leak ONLY during conditions where the PCV can't function - i.e. high-power driving - so it's hard to diagnose in the shop/garage.
mike c.
