# Volumetric Efficiency

 What is Volumetric Efficiency define by ehow.com; is the ratio of volume of air drawn into a cylinder to the piston all the way down. Basically it’s the volume of fuel and air drawn into that cylinder. Amazon.com Widgets

Volumetric efficiency; how much air / fuel go into the engine, if it was perfect the engine would use the full volume which is 100%.  Do to seal leaks and functional parts it would be impossible to be at 100% efficient.

Now you could try and find a calculator to measure air flow “good luck” I’m personally looking for an app for my iphone. If you find a calculator you need to use the MAF signal but you will need a calculator to help you convert air mass to volume, including IAT and altitude.

The test can determine if the engine is breathing or if the Mass Air Flow Sensor  “MAF” is contaminated.

# Here is the Test

Drive the car and monitor the MAF volume PID this test will be at wide open throttle or near red line. So you may need a secondary driver or use the graphing meter on your scan tool.

Once you get the data put the maximum MAF value, Max RPM into the calculator along with the engine size, intake air temperature and altitude. The calculator will tell you what percentage of the airflow is actually being drawn into the engine.

# What is a failure?

Performing the volumetric efficiency test is a good way to determine if you have an unmeasured air problem one or an actual low airflow problem. Observe the fuel trim PIDs under normal operating conditions where the volumetric efficiency calculation shows a fault.

If you have high fuel trim then you have a “fake” fault.

Here is an example under WOT the vehicle loses power but your fuel trim shows fuel is being added +25 and the O2 sensor is reading .9v, this vehicle does not have a fuel related problem.

Now on the other hand if the fuel trim reads +25 and the O2 sensor say .1v you do have a fuel related problem.

How about if your grams per second are under specifications from the Mass Air Flow sensor and the Long Term Fuel Trim indicates a +35 this could indicate the vehicle has a dirty MAF sensor or false air, “MAF boot cracked”

If fuel trim is normal then the fault is real and you should test for an intake air flow  restricted, or exhaust flow is restricted, or the engine has a mechanical fault in some conditions when a vehicle has a MAF sensor

# Finding a restriction

I remember when I had an old Chevrolet Van V8 5.0l engine that came In with no power, you probably could out run this thing if you race it a city block.

Anyway to make a long story short what we found was the air cleaner was so plugged the engine could not breathe. If you removed the air cleaner the van ran great if you reinstalled the air cleaner and lid no power.

Basically this Van had a problem on the intake side which made the troubleshooting a lot easier.

Now on the exhaust side it requires a little more work, I have heard and read many articles on using a vacuum gauge at cruise speeds in park or neutral and see if it drifts down, or snap the throttle to see how fall it drops near zero and how fast it rises above the given cruise vacuum reading.

Now with all that said in my opinion the best tool to use to check for a plugged exhaust is an exhaust back pressure gauge.

You can connect the gauge to an O2 sensor fitting, EGR back pressure port. You then connect the gauge and follow the instructions that come with the tool.

The numbers I use is less than 3 psi at cruise if the exhaust has a double wall pipe like some older escorts it may reach this number. Also at idle less than 1.5psi; I saw 3-4psi without touching the throttle, extremely plugged exhaust.

 Valve timing could be off causing the vehicle to experience lack of power or the vehicle could have a lack of compression.

# Here are some common causes of lack of compression.

• Worn piston rings

• Burnt valves

• Defective camshaft lobes

• Valve timing

Anything that reduces compression the ability to bring in air and exhaust the air will affect the volumetric efficiency.

# If you can find a Calculator for Volumetric Efficiency

If you find a calculator for volumetric efficiency by Google you need to input Engine size, Peak MAF, Peak RPM, Intake Air Temperature.

Example 2000 Nissan Maxima 3.0l engine: A vehicle is breathing at 86% efficiency you do not need to check compression, cam timing, vacuum, or a plugged exhaust.

Next example: this is a 2004 Sonata 2.4l with no power complaint.

The vehicle was driven at Wide Open Throttle (WOT) and the Volumetric Efficiency was checked. The reading was at 79% which shows the vehicle has a breathing problem. The fuel trim was normal which indicates this vehicle has a breathing problem.

The items of concern would be valve timing, intake or exhaust restriction or any of the other problems we talked about in class.

# In summary

If the efficiency is low but the fuel trim are okay you have an engine not breathing. Valve timing, low compression, exhaust or intake restrictions will cause this problem.

If the Volumetric Efficiency is low and the fuel trim are high, then you have a false VE reading. This could be caused by an MAF reading incorrectly, an example would a dirty sensing wire or air going through the intake boot and not the MAF “false air”.

If you would like more information on our 2013 Update Class Volumetric Efficiency course Click Here