By Chris Hamann
Years ago, the vacuum gauge was standard in the diagnostic process. We were taught how to identify engines that had mechanical issues by analyzing the gauge’s needle. We also used a vacuum gauge to adjust the carburetor’s idle air mixture. A high vacuum (17-22 inHg) along with a rock-solid steady reading indicated that the engine had no apparent mechanical issues and was in an excellent state of tune. A low and/or a rapidly bouncing needle indicated a mechanical concern exists within our engine such as a compression leak or a valvetrain issue. I’m sure most of us have seen the vacuum gauge diagnostic charts over the years.
Unfortunately, vacuum gauge testing has evaporated in the past thirty plus years as EFI systems electronically control the fuel/air mixture and ignition timing. It’s a lost art and seemingly only the “older” technicians seem to know what it is and how to use it.
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A test that was a staple in the 1970’s Sun Diagnostic equipment was a cranking vacuum test. It was a simple premise. Connect a vacuum gauge to the engine’s intake manifold and disable ignition. Crank the engine for 3-5 seconds and observe the gauge. If the engine is in a good state of tune and there are no mechanical concerns the engine should pull 3-5 inHg of fairly steady vacuum while cranking. This was a great test and again, the industry quit teaching this as scan tools became more prevalent and a required tool. If you took a poll of technicians of all ages and asked them if they’ve ever performed a cranking vacuum test — let alone even heard of it — I’d venture to say that it’s more than likely that under 5% would say they have.
Enter in modern technology. The pressure transducers that are available on the market today are simply phenomenal. I use the Pico Technology WPS 500X pressure transducer. This tool is much different from a conventional vacuum gauge. This tool is calibrated to your current atmosphere pressure which is a significant advantage over a conventional vacuum gauge. Having the ability to graph the vacuum pattern electronically is simply amazing. On a conventional vacuum gauge if we had a bouncing needle for example, we couldn’t time that bounce to a specific cylinder or event. Using an oscilloscope graphing the vacuum pattern along with timing each individual intake “pulls” off a spark event, one can quickly ID each cylinder’s vacuum “pull.” The best part of analyzing a cranking vacuum pattern is that it tests each cylinder’s ability to seal and breathe!
So now we know a little theory about the cranking vacuum test, I’ll introduce the procedure:
- Set scope channel to 2 seconds per division and 2 psi per division
- Set WPS to Range 3 (-5 psi to +5 psi)
- Disable fuel
- Sync from an ignition event – never fuel
- Try to find the most centralized vacuum port
- Keep the hose as short as possible
- Good battery and stable cranking speed is a MUST
- Crank engine over for about 10 seconds
The shape and amplitude of the vacuum “pulls” are impacted by:
- The change in vacuum
- The speed or rate of change of vacuum
Technique
- Divide the vacuum pattern up into individual cylinder events by using PicoScope’s built in phase rulers.
- The triggered cylinder’s intake stroke will be approximately 360° from its ignition firing event.
- Identify each intake “pull”.
In a perfect world with steady cranking speed and a good engine the cranking vacuum pattern should look just like above. The peak vacuum is at the bottom of the pattern and the horizontal cursor measured -1.6 psi which is 3.2 inHg.
Another technique I frequenty turn to is the piston position charts offered by driveabilityguys.com. These charts are super easy to use and drop right into your cranking vacuum pattern. When you see an abnormal change in the vaccum pattern I utilize these charts to identify what’s occuring in other cylinders at the same moment in time.
During each intake “pull” one can see where the other cylinders are in that moment in time. This chart gives me the visual reference I need to make diagnostic decisions much easier. As an example, we can’t evaluate a leaking intake valve on its own intake stroke. After all, the intake valve is open and is “supposed” to leak. I can’t check for a leaking valve when it’s supposed to be open. I need to evaluate each valve on different strokes. (Reminds me of a TV show during my childhood!)
Typically, a leaking intake valve will show up during the middle to end of the leaking cylinder’s compression stroke. A leaking intake valve will push pressure (amount of pressure depends upon the size of the leak and the crankshaft speed) into the intake manifold. A leaking exhaust valve usually can be detected on the offending cylinder’s own intake stroke. When on the intake stroke exhaust is present in the cylinder which increases pressure in that cylinder. This intern results in a weak intake “pull” in that cylinder and if the leak is large enough pressure will be present until the intake valve closes in the offending leaking exhaust valve’s cylinder. Remember, when the intake valve is open in a specific cylinder (In a V8 engine there are three intake valves open at the same time. One is starting, one is in the middle, and one is ending their intake strokes) the pressure transducer is connected to those cylinders. A piston or cylinder wall leak shows up similarly to an exhaust valve leak. As long as the intake valve is open in the cylinder that has a crankcase leak the intake manifold pressure will rise and stay high until the intake valve closes. More on crankcase pressure testing on gasoline and diesel engines in a later article in Engine Professional.
You may think that cranking vacuum is too confusing and hard to follow. It does take practice for one to become proficient in cranking vacuum pattern analysis. I can share that since I’ve learned and continue to practice this technique this is my go-to test that I can generally perform and analyze in under fifteen minutes in most gasoline engines. If I have a misfire or rough running engine and I get a cranking vacuum pattern like the examples I’ve illustrated, I know the problem isn’t of a mechanical nature. Remember, in diagnostics, knowing what it isn’t is just as good as knowing what it is sometimes. In the next issue of Engine Professional we’ll put relative compression and cranking vacuum to work on some broken engines.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2025EPQ2/#p=26