VE stands for Volumetric Efficiency.
The real engine breathing volume to displacement ratio is called VE. A 2.0L engine, for example, that only breathes 1.5L of air every two crankshaft rotations, is breathing at a ratio of 1.5/2.0, or 75%. Low VE is a straightforward sign of a wide range of breathing problems. Low VE can be caused by intake or exhaust constraints, engine timing issues, or air measurement issues.
Categories of Potential Causes
While it may appear unduly basic, one of the following, causes every drivability issue and numerous P-codes:
- Air (whole-engine or single-bank breathing)
- Fuel (includes the supply of fuel as well as the O2 sensor/Fuel Trim feedback loop)
- Contribution from the ignition and compression (including single-cylinder engine mechanical)
- Drivetrain, suspension, or other external variables may be involved in some circumstances.
You might be recalling all the difficult diagnostics you’ve worked on, but they can all be boiled down to these four categories. Breathing is impaired even if the actual diagnosis is something very specific, such as a clogged converter. As a result, a failed VE test adds a clogged converter to the list of possible causes. In other words, all drivability issues can be classified into one of the bulleted categories above. As a result, showing which category includes your error eliminates the need to test the other three, saving you time and concentrating your diagnostic route.
The majority can be identified by testing the ignition system, graphing Fuel Trim PIDs, and calculating the VE. Fuel Trim and VE are straightforward tests to begin with, and as you’ll see, a few minutes of testing goes a long way toward reducing the list of more difficult tests to conduct. In an initial diagnosis, your sole responsibility is to:
- Determine the possible broad category of concern.
- Make a long list of potential causes.
- Make a list of simple tools and tests that can help you remove most of the likely reasons with the least amount of work.
- Examine the few remaining possibilities.
Setup for Volumetric Efficiency (VE) Testing
VE testing isn’t just “another item to look into.” Rather, it’s a high-level signal of a whole class of problems that you may or may not need to investigate. It is frequently the ideal first test because it is simple and has only two outcomes:
- If the test passes, it means you don’t need to check vacuum, whole-engine or single-bank compression, cam timing, or intake or exhaust limits.
- If the test fails, you’ve demonstrated that all those potential causes are still on your list, and that more testing isn’t a waste of time.
All you need to run a VE test is:
- A vehicle with a MAF sensor
- A VE calculator
- A Scan Tool displaying the following PIDs
- MAF in grams/second or pounds/minute
- IAT (air temperature affects density)
- Barometric value (or check online for current atmospheric pressure)
Procedure for Testing Volumetric Efficiency (VE)
Accelerate to near redline in first or second gear while graphing the PIDs at wide-open throttle (WOT). Return to the shop and enter the peak RPM and MAF numbers, as well as intake air temperature, altitude, and engine displacement, into the VE calculator using the Scan Tool. The VE calculator will tell you how efficient the engine is at breathing.
Test Results Interpretation
The table below displays the test’s approximate ranges. As you can see, the results aren’t always straightforward because some engines are inherently more or less efficient. A 2-valve pushrod V8 from GM, for example, doesn’t breathe very well even when new, and 80 percent is a good result. A 4-valve Hyundai with variable valve timing and intake tune, on the other hand, should reach 95 percent efficiency. As a result, an 80 percent pass on a 5.3L Yukon V8 is a pass, but a fail on a 2.4L Sonata.
|90% or More||There are no breathing issues, or if there are, they are so slight that they will not set a code or cause a client to report a symptom. Non-turbo engines with innovative engine and intake designs may achieve 100% efficiency.|
|76% to 89%||The zone of ‘fuzziness.’ Think about the engine design (e.g. number of valves, number of cams, use of VVT, use of variable intake). When adjusting your expectations for this test, consider whether the vehicle is designed for performance, economy, or workload. The less visible the symptom is the higher the result in this range, so think about the severity of the defect you’re diagnosing before leaping to conclusions.|
|56% to 75%||A breathing problem is plainly indicated by this range. Determine whether this is a true breathing issue or just a perceived breathing fault by using Fuel Trim.|
|55% or Less||Few engines will even start if the real breathing rate is less than 55%. Short Term & Long Term Fuel Trim may be able to supply enough fuel (typically over 50% combined) to keep the engine running if it’s a ‘fake’ breathing fault. If your results are in this range and the engine starts, check for high Fuel Trim and then rule out a MAF or intake duct problem.|
Breathing Faults: Real vs. Fake
The diagram below illustrates how to incorporate test data into a larger diagnostic philosophy. You didn’t spend your energy if the VE test was passed. Rather than wasting time testing for issues that aren’t the root cause, you removed a whole category of them. Proceed to Fuel Trim testing, which is the next most straightforward fault category.
If the VE test fails, you should still check Fuel Trim because the comparison is a key indicator of the difference between real and perceived (or ‘false’) breathing issues.
A true breathing problem indicates that airflow is restricted. Because the MAF detects low airflow, the PCM also sends a low fuel injector command. As a result, the air/fuel ratio is correct, and the Fuel Trim numbers are normal.
Causes could include:
- Intake restrictions (including faulty variable induction)
- Exhaust restrictions
- Camshaft timing is incorrect (mechanical VVT control)
- Mechanical problems with the engine
- A fake breathing error occurs when the MAF detects less airflow than is actually present. The PCM sends a low fuel injector signal in response to low detected airflow, resulting in an excessively lean air/fuel ratio. As a result, Fuel Trim levels are high. Causes could include:
Supercharging and turbocharging
Many turbocharged and supercharged engines use a MAP-driven fuel management method (or’speed-density’) instead of the MAF sensor. Because the whole goal is to shove more air into the cylinder than it can hold at ambient pressure, the results should be well above 100% if you have a MAF sensor. A 2.0L turbocharged engine with a VE of 150 percent, for example, is actually breathing 3.0L of air. In fact, most modern compact turbocharged engines have VE values between 170-185 percent.
What is a ‘well-known’ range? Because every engine is different, our only suggestion is to use your intuition. The system’s boost pressure is 8 PSI, which is just over half of atmospheric pressure. You’ll be in the 130-150 percent zone if you add in some intake and exhaust flow resistance. Many modern engines, on the other hand, use 15-20 PSI of boost (amazing, right?). Even though the absolute pressure can surpass 2 atmospheres, the flow restrictions stack up at that level, therefore passenger car engines will rarely exceed the 170-185 percent range.