One of the "gotchas" of the Volvo modular engine is that the Positive Crankcase Ventilation system clogs up over time (>70K mi.), just like a drain clogs in your home. It has to be cleaned out to restore flow. If neglected, the engine oil seals will blow out. Many a great car will be abandoned if that occurs, absent an engine rebuild or replacement.
The reason for the PCV is that some tiny fraction of the combusting gas in the cylinder escapes through the cylinder enclosure
(which is imperfect) into the crankcase. If the crankcase were a sealed bottle, these escaping gasses would eventually build up in pressure to the same average pressure level in the cylinder, making all the oil seals impossible to hold. For environmental reasons, the idea of venting the escaped gases to the outside air was outlawed back in the '70s. The solution was to capture and recycle these smoky fumes by vacuuming them back into the intake manifold, and put them through another combustion cycle. PCV is that system of smoke recycling.
How does it clog up? The smoke particles are sticky and cling to the tubing and orifices of the PCV system. And just like a drain, progressive clogging ends up cutting off the flow completely.
How can you tell if your PCV system is working? These images demonstrate a simple test with a nitrile glove and rubber band placed over the oil filler port. You are measuring the pressure in the crankcase with this test. If the PCV is working, you'll detect a suction (or negative pressure)...if the PCV is clogged, the glove will inflate (positive pressure). (see photo)
WARNING: If your oil filler does not have a protective grating, you MUST keep a hand on the glove to make sure it is not sucked into the engine!
The Volvo technical literature specifies a nominal crankcase pressure (engine warm and idling) of -0.3 to -0.5 kPa (-0.03 to -.06 psi). Negative == suction.
Consequences of neglect
Once the PCV stops vacuuming the crankcase smoke, we're back to a closed bottle...pressure will build up, and eventually reach a point where the oil seals will pop a leak. The 4 cam shaft seals, and the rear crankshaft seal are usually the first to go. Your engine will be spewing oil rapidly at this point, and an oil pressure warning to shut off the engine will follow a few minutes later. Unless you're good at rebuilding engines, it's either going to cost you tons of $ or abandoning the car to salvage.
When buying a used Volvo over 75K mi., it is highly recommended to take the car to a discreet location and test the PCV.
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