How To Check For Leaks On A Plastic Valve Cover With A Built-In PCV

Blog Feature

Your PCV system is an extremely important, yet often overlooked collection of parts that affects how well your car runs. On some cars, like our  2007 MK5 Volkswagen GTI, the PCV system is actually built into the plastic valve cover. Charles "HumbleMechanic" Sanville breaks down how the PCV system works, and how using a smoke machine is one of the easiest ways to test for leaks. 

Tools needed to smoke-test your valve cover PCV system:

  • Smoke Machine (Optional but recommended)
  • Amber Flashlight
  • Magnifying glass (optional but recommended)

Steps to inspect and test valve cover PCV system for leaks:

Step 1: Visually inspect the plastic housing for cracks.

If you don't have a smoke machine available to you, this is the most important step. You're looking for pinholes and hairline cracks that aren't always visible at first glance, you really have to lament over the product, use good lighting, and even the magnifying glass to look for leaks. Even the slightest of imperfections can upset your car. Check along the edges and where the PCV valve mounts to the valve cover (second image down). 



tep 2: Connect the smoke machine to your PCV passages and inspect for leaks.

Connect the fitting of your smoke machine to one of the passages. Looking at the underside of the valve cover, you can see the routing of these passages pretty clearly. Once you turn on the machine, you can watch the smoke flow out the opposite side of the valve cover. Using an amber-colored light here will help highlight the smoke. The side the smoke is flowing out of, you will want to plug with your finger or a cap that fits snugly over the opening. 

While it is running and capped, there shouldn't be any smoke exiting the valve cover. If there is, you know where your issue is. You can also reference the gauges on your smoke machine if your model happens to have them. The dial gauge (on Charles' unit shown in the pictures) shows the pressure of the system, and the ball flow meter indicates the extent of your leak if you have one. Any leak at all warrants replacement; you want the ball to be stationary at the bottom of the meter. 

The last image shows the valve cover with the underside removed. This is a destructive process, so don't attempt to do so yourself. It was only done here for demonstrative purposes. Also, note the amount of carbon buildup inside the system.




Vacuum leaks are never fun to diagnose. They can manifest in a countless number of ways: from oil leaks to difficulty starting your car. This is one of the easiest ways to test for leaks so that you can properly diagnose the issues on your vehicle. 

Be sure to check the video below on HumbleMechanic's YouTube page, and don't forget to subscribe to get all new videos pushed right to your inbox. 

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