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Volvo Positive Crankcase Pressures Explained

volvo positive crankcase

What is positive crankcase pressure?

As your engine runs, gases from the cylinders leak past the piston’s sealing rings into the crankcase. This leaked gas is sometimes referred to as “blow by” because the pressure within the cylinders “blows” them “by” the piston rings. These gases include compounds harmful to an engine, particularly hydrocarbons (unburned fuel), as well as carbon dioxide and water vapor. If allowed to remain in the crankcase, or become too concentrated, the harmful compounds will condense out of the air within the crankcase and form corrosive acids and sludge on the engine’s interior surfaces. This can harm the engine as it tends to clog small inner passages, causing overheating, poor lubrication, and high emissions levels. Additionally, high pressure in the crankcase can build to a point that leads to a rear main seal failure, an expensive repair. To keep the crankcase air as clean as possible, some sort of ventilation system must be present.

There are different types of PCV systems depending on your car. Some systems use a PCV valve – a one-way valve that ensures continual evacuation of gases from inside a gasoline internal combustion engine’s crankcase. Other systems use an oil trap or separator to do the job.

MVS Forum Member red54321 asks:

what does the breather do ???.and if clogged up is result lose of power etc.
think its known as oil trap???can oil trap just be cleaned or is it best to replace are they exspensive???PCV

MVS Forum Member stoutlogic replied:

Oil trap, breather, flame trap (Volvo specific term) and PVC are commonly used terms/ parts that basicaly allow for the engine to vent positive crankcase pressures due to blow by from the pistons.

If the engine can’t vent you can experience:

oil dipstick and oil blowing back out the top of the dipstick hole

leaking main seals, cam seals

smoke from the tailpipe

dipstick or dipstick tube with rust or milky white substance which is a mixture of oil and water

If car is a turbo you have an oil trap other wise you’re in the market for.

MVS Forum Member pfeener adds:

Here’s the deal as far as PCV (Positive Cranckcase Ventilation) systems are concerned:

Any engine, not just Volvos, will create pressure in the crankcase, due to blowby of the piston rings. The PCVs job is to remove this pressure so it doesn’t harm the seals in your engine. Prior to the 1960s, engine crankcases were just vented out to the open air with a mesh type screen covering the breather tube. Worked fine, but the EPA didn’t like it much. After 1960 or so, PCV systems came into being. The PCV system uses engine vacuum to remove cranckcase pressure and actually create a vacuum within the engines crankcase. The cranckcase gasses are sucked back into the intake manifold and burned in the engine’s combustion chamber. That’s the generic information. Volvo’s however have a very different type of PCV system, but the end result is as discribed above.

To test your Volvo’s PCV system, remove the dipstick with the engine running and see if it puffs out smoke which indicates positive pressure in the crankcase. If the PCV system is failing you need to rebuild it. FCPgroton sells all the parts in a kit. Here’s the link for a non-turbp 850

Positive Crankcase Pressures Explained

1 Comment

very good explanation

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