Posted to Technical Tips Forum on 5/17/2016
Using Cranking Compression to Find Vacuum Leaks
I had a vehicle here at the school that had many of the same
symptoms as this post here: http://autop.ro/b18 [autop.ro] 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.
College of DuPage Automotive Service Technology
Glen Ellyn, Illinois, USA
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