Rick's Free Auto Repair Advice

Oil catch can

Oil catch can — What is it? How does it work?

All engines, even new ones allow some gasses of combustion to seep past the piston rings and into the crankcase. The escaped gasses are referred to as blow-by. During a cold start when the air/fuel mixture is very rich (high fuel content), blow-by consists of raw fuel, soot, acids, water and motor oil washed from the cylinder walls. All this winds up in the crankcase.

Normally blow-by is sucked into the intake manifold via the positive crankcase ventilation system (PCV) where it is burned in the combustion chamber. However, by recirculating blow-by gasses, some of the fuel and oil catch canoil deposits can settle on intake valves and form carbon deposits that decrease valve efficiency.

An oil catch can is designed to separate out water, fuel, oil and soot from blow-by gasses to prevent carbon formation on intake valves. The oil catch can is then manually drained by the vehicle owner.

Why oil catch can and why now?

PCV systems have been in use since the early 1960’s and they’ve worked just fine in carbureted and port fuel injected engines. However, both of those fuel systems washed the backside of the intake valves with fuel containing detergents.

However, gasoline direct injection (GDI or DI) systems inject fuel directly into the combustion chamber, leaving the intake valves with no way to prevent carbon buildup due to blow-by recirculation. The result has been a significant increase in carbon buildup on the backside of intake valves on engines equipped with direct injection.

An oil catch can is designed to remove the oil, water, soot and fuel that causes intake valve carbon deposits.

catch can overview

An oil catch can installs between the PCV and the intake.

dirty valves

Buildup on the back face of an intake valve

What you’ll see when you drain an oil catch can

What you see will vary depending on the amount of blow-by and the season. Keep in mind that all blow-by contains water. When you mix water and oil, you get a yellowish goop, so you’ll see more of this color and consistency in winter months as the hot oil vapor mixes with water and then chills.

catch can sump

A mix of fuel, water, and oil

Traditional PCV system schematic

With each combustion cycle, more and more blowby enters the crankcase. If the crankcase isn’t vented somehow, the pressure would build to the point where it would blow out the oil pan and valve cover gaskets.

To avoid gasket blowout and prior to emissions control laws, carmakers simply vented blowby to the atmosphere, causing air pollution. After the Clean Air Act, carmakers were prohibited from venting

The PCV system consisted of a one way valve with a fixed orifice size (based on individual engine size and design) and a flexible hose that runs from the PCV valve a location on the intake manifold. In operation, the high crankcase pressure, combined with high manifold vacuum.

The one-way valve is needed to prevent damage to the crankcase in the event of a backfire in the intake manifold. If the pressure in the intake exceeds crankcase pressure, as would be experienced during a backfire, the pressure forces the PCV pintle closed to prevent the flames from reaching the crankcase.

PCV valve operation
Are catch cans legal?

No. The Clean Air Act prohibits any modification of the vehicle’s emissions system. This is a Federal requirement that supersedes any State EPA rules or regulations

©, 2022 Rick Muscoplat

Posted on by Rick Muscoplat

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