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Ask a Mechanic...“Can” you mitigate valve fouling?


Dear Ask a Mechanic,

What is the consensus about installing an oil catch can for direct injection engines?

If so, is there a brand that is recommended? 

Thanks – Valuing Clean Valves

This is a pretty broad question, particularly as you haven’t specified a type of vehicle or whether the vehicle in question has been modified. I’m going to assume that you’re asking about the use of a catch can on a stock GDI (gasoline direct-injected) vehicle, as modifications or track-type driving create different conditions.

A bit of background for those readers who aren’t familiar with catch-cans.

Save the odd rotary, all modern automotive engines have a crankcase in their block that contains primary moving parts like the crankshaft, rods, and pistons.

Rings on the pistons function like seals between the pistons and the engine block, but even the best-sealed rings allow some combustion gas, raw fuel, and moisture past them into the crankcase. Without ventilation these gasses – called blow-by – will build up pressure and start pushing out seals or causing substantial oil leaks.

Pre-emissions vehicles often used a “draught tube” to vent the blow-by into the atmosphere, contributing appreciably to pollution. “Positive crankcase ventilation” (PCV) systems were implemented in the 1960s, drawing the blow-by into the engine to be burned-off instead.

Oil mist is major component of crankcase gasses, and some type of oil separator is built into every PCV system to reduce oil consumption. They vary in effectiveness, and don’t remove all of the contaminants regardless.

Gasses from both the PCV and EGR systems (exhaust gas recirculation – another emissions device) contain carbon particulates. Oil mist from the PCV system increases the carbon’s adherence to the intake ports and valves.

Gasoline is very good at removing carbon buildup, but because GDI engines spray fuel into the combustion chamber directly, no gasoline passes the valves and ports. The sticky, oily carbon mixture coats them and builds up, potentially reducing airflow and creating driveability problems.

Excess oil mist can also cause engine-damaging detonation.

Catch cans go in-line with the PCV system and are intended to better separate the moisture, oil, and particulates than the factory’s system, thus preventing (or greatly reducing) ingestion and buildup. These cans need to be drained occasionally or liquid could be drawn into the PCV system, leading to damage.

Stand-alone oil separators that drain back into the crankcase are also available. However, they add cost and complexity.

Online, it does appear to be a popular “upgrade” to both Volkswagen’s 2.0 litre TSI engine and the 2.0 litre “FA” engine in Subaru’s WRX: GDI motors that are frequently driven aggressively.

In the majority of stock vehicles and normal use, I would consider catch cans unnecessary; the factory system should be sufficient.

Ask a Mechanic is written by Brian Early, a Red Seal-certified automotive technician. You can send your questions to These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle.


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