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Using Cranking Compression to Find Vacuum Leaks
Posted to Technical Tips Forum on 5/17/2016 12 Replies

I had a vehicle here at the school that had many of the same symptoms as this post here: [] Right as the vehicle enters closed loop the fuel trims quickly build positive. [2003 Ford Focus SE, Emissions Scan Data] When the engine is revved the fuel trims get better. [2003 Ford Focus SE, Emissions Scan Data] There have been other cases where this vehicle entered open loop fault. The scan data points to a vacuum leak.

To confirm suspicions of a vacuum leak many technicians turn to a smoke machine. The issue here is that you are adding pressure to the intake, when it fails under vacuum. Many techs have turned to spraying around the intake with carb cleaner or propane to see if the oxygen sensors go rich, I like this test.

Another test you can add to your arsenal is a cranking vacuum test, with a blocked off intake. I learned about this test about 5 years ago, but had a chance to really practice it during the hands-on WPS500 class offered by Automotive Seminars. To set up this test you block off the intake, I used a smoke tester adapter. Then add the WPS into a central intake port in range 2. Crank the engine over until you reach the deepest vacuum. Be careful not to do this test too long, as it is hard on the engine.

Here is the vehicle that was showing positive fuel trims: [2003 Ford Focus SE, Engine/Propulsion Waveform] Notice the deepest vacuum pull is around 7.8 inHg (3.9 PSI x 2 = roughly 7.8 inHg). I have a good vehicle with an identical engine and here is what I got: [2003 Ford Focus SE, Engine/Propulsion Waveform] Note the engine pulls about 15 inHg while cranking.

This test only takes about 5 minutes and is a quick way to confirm there is a vacuum leak. This vehicle has some intake gasket issues. I haven't done this test enough to figure out a go/no go vacuum number, maybe some members more experienced with this test can chime in.

Matthew Shanahan
College of DuPage Automotive Service Technology
Glen Ellyn, Illinois, USA

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car Vehicle Data

2003 Ford Focus SE 2.0L

Engine2.0 L
Trans5-speed Standard