Proper PCV Routing For LS Engines

What is PCV?

PCV, or positive crankcase ventilation, refers to the system by which combustion gases are removed from the crankcase of an engine. “Positive” specifically indicates using manifold vacuum to pull fresh air through the crankcase, helping to more quickly evacuate the unwanted combustion gases.

Why is PCV Important?

PCV is especially important for LS engines because incorrect routing can cause gases to accumulate and increase pressure in the bottom end of the engine, leading to oil leaks from pushed out gaskets (specifically the rear main seal) and contaminated oil. PCV ensures that harmful blow-by vapors are replaced with fresh air.

An engine with proper PCV plumbing will run more efficiently, make more horsepower, and have less oil staining and carbon build-up. It is important to note that proper PCV plumbing will NEVER vent to atmosphere. In order to correctly plumb a PCV system, a catch can should be plumbed in line to filter the oil out of the gases from the crankcase before the now “cleaned” air is returned to the engine.

So, How do I run PCV Properly on my LS engine?

So glad you asked! Improper PCV routing PLAGUES so many vehicles, whether they are daily drivers or racecars. It is especially important for performance driving applications, so to help anyone and everyone with an LS make sure that their PCV is done correctly we’ve created the below diagram:

Note #1: in some cases, you will need to drill into your intake manifold to mount a vent here that you can connect to. The exact method of mounting the vent may differ depending on if your intake is metal, or plastic. Some intakes come with a built-in vent. Just be sure to mount this vent behind you MAF, as this needs to be metered air, not unmetered.

Note #2: The “correct” routing for the yellow line is to run from the driver’s side valve cover to the catch can, and from the catch can to the intake. However, if there is no possible way to run it from your driver valve cover, you can technically run this line from the vent on your valley cover to the catch can, and from the catch can to the intake. It is not our recommendation to run it like this, and if you have a choice you should always use the driver’s side valve cover. Be sure to cap off whichever vent you do not use.

Let’s talk about this diagram and explain exactly what’s going on:

  1. The Green Line runs from a vent on the air intake (after the MAF) to the vent on the passenger side valve cover. This feeds clean, fresh, metered air into the passenger side valve cover, which will then be pulled through down into the crankcase of the engine.

  2. The fresh air travels through the crankcase, pulling combustion gases out of the bottom of the engine, up and out through the driver’s side valve cover. This can also help remove any water that accumulates in the crankcase from condensation that occurs during each engine cool-down cycle after you drive, or as a vehicle sits, which vaporizes when oil gets up to temperature. This water vapor is carried out by the PCV, where it otherwise would sit inside your engine and contaminate oil and degrade internal components. This is also why it is always important to get your engine up to operating temperature every time you drive it in order to vaporize any potential water in the engine.

  3. The dirty air, now full of combustion gases, oil, water vapor, etc evacuates from the driver’s side valve cover vent and runs to the IN port on the catch can. The catch can “catches” the oil and separates it from the air, to make sure that the air returning into the engine does not contain oil that the engine would burn, avoiding carbon build up and oil staining. From the OUT port on the catch can, a line is plumbed to the vent on the intake manifold to allow the now filtered air to return to the intake and go back into the engine.

We recommend plumbing your PCV system with 3/8ths ID rubber hose, or you can use -6AN line if you’re feeling fancy. For boost applications you may consider -8AN or -10AN, be sure to consult your engine builder (which we hope is us).

PCV FAQs:

Q: Can I run a catch can that vents to atmosphere?

A: No, you need to run PCV as a sealed system. Venting to atmosphere renders the system useless, because it completely removes the vacuum from the crank case and stops any benefit of the air evacuating gases and contaminants from the crankcase. It also puts more pressure on the rings, which are now combatting combustion pressure on one side and crank case pressure on the other. Lastly, the crank case gases will back up into the intake from the fresh air side and cause carbon build up on the valves.

Q: Why is it not recommended to run the dirty air out from the valley cover? Why is it only recommended to run from the driver’s side valve cover vent?

A: If the dirty air out line runs from the valley cover, the air isn’t crossing all the way over the engine. It is important for the air to go completely across the engine, and to exit out of the driver’s side valve cover as opposed to being interrupted and vented out only midway through.

Flag Motorsports