Other Articles - October/November - 2015

Engine or Transmission Part II: I Can’t Breathe!

Last month in part I, we explored data that could help determine whether the engine management system should be addressed before moving to the transmission diagnostic path. The techniques discussed are fairly quick and easy to perform. This month’s article will address the engine’s ability to breathe using a technique that should be just as quick.

Engine breathing is critical for proper engine operation. If the engine can’t breathe properly, engine operation will suffer and the transmission won’t work the way it should. In addition, faulty mass airflow (MAF) sensors can create the illusion of poor engine breathing and affect both engine and transmission operation.

Often these issues appear as a “no power” complaint. But poor fuel delivery or a faulty MAF sensor can produce the same condition.

This month we’ll explore ways to determine whether an engine has a breathing, fuel delivery, or mass airflow sensor problem with a simple test drive. It’s another very quick and efficient technique to help you choose the engine or transmission diagnostic path, often without even opening the hood.

This technique applies to MAF-equipped vehicles, which includes most vehicles on the road today. If you’re working on a speed density vehicle — one equipped with only a MAP sensor — you may have to adjust this technique.

First you need to be able to measure volumetric efficiency (VE) or engine breathing. For this, you’ll need a generic scan tool and a simple calculator. The theory is, if you know the size of the engine, the speed it’s spinning, and the amount of air it’s pulling in, you can figure out how much air is flowing as opposed to how much air should be flowing.

Thankfully, we have computers to do the math today, so we’ll be using a volumetric efficiency calculator. These calculators are available from several sources. The one we’ll be using is called DECS and is available from www.aeswave.com.

The second piece of the puzzle is to perform a test drive to gather data. You connect your scan tool and make a recording of the test drive. You’ll need to floor the throttle during the recording, which makes this one of my favorite test procedures!

Going to wide open throttle removes the intake restriction created by the throttle plates, which is necessary to measure engine breathing.

During the test drive you’ll need to record RPM and MAF data. In addition, you should monitor the oxygen sensor and MAP sensor (if equipped.) You’ll want to begin recording the test drive a little before and after you floor the throttle, so you can examine fuel trim numbers as discussed in part 1.

In this example, we’re using a 2001 5.3 liter GM truck (figure 1). The recording shows engine RPM in red and MAF grams per second in green. The cursor indicates peak RPM, when airflow should be highest. The values at that point are displayed in the upper left of the graph.

If you enter the engine displacement, airflow, and engine RPM into the calculator (figure 2), it does all the calculations and yields a volumetric efficiency of 77%. For this application, 77% is acceptable, so this engine can breathe.

Generally, 75% or higher is acceptable for a naturally aspirated engine; forced induction engines (turbocharged or supercharged) exceed 100%. These numbers vary slightly, but you’ll have to adjust your specifications as you see fit, based on the type of vehicle you regularly work on.

Because manufacturers don’t provide these specifications, this is one of those situations where practice is the key to success.

Now that you know how to calculate volumetric efficiency, you can put it to use. To do so, you need to know three more things:

  1. What should the oxygen sensor do during wide open throttle?
  2. What should the MAP sensor do during wide open throttle?
  3. What should fuel trim numbers do when the throttle isn’t wide open?

Oxygen sensors shouldn’t go full lean under wide open throttle. During wide open throttle, fuel demand is high. The oxygen sensor should either flatline full rich or, on some newer vehicles, it should continue switching back and forth. But it should never flatline lean.

Here’s an example of what an oxygen sensor (in green) should do (figure 3). While operating under normal throttle opening it switches back and forth. When you floor the throttle (RPM indicated in red) the oxygen sensor goes rich, due to the additional fuel. If it doesn’t behave as expected, it could indicate low fuel volume or an air metering issue.

If the vehicle’s equipped with a MAP sensor, that’s the next thing you should check. When you floor the throttle, atmospheric pressure fills the intake manifold, so the MAP sensor should read barometric pressure. If not, it indicates a restricted intake path.

Fuel trim is the final piece of this puzzle. You’ll want to check fuel trim when the engine’s in closed loop at normal throttle. Part 1 presented what good fuel trim numbers should be.

Now that we’ve covered volumetric efficiency, oxygen sensors, MAP sensors, and fuel trim, you can put it all together into what I call an engine breathing cheat sheet (figure 4).

The columns of our cheat sheet list the data we’ve already explored; the rows list the different vehicle operating conditions. Five vehicle operating conditions are included: known good, fuel delivery, air metering, restricted exhaust, and restricted intake.

The known good row shows what you should expect from an engine that’s functioning properly. The other rows illustrate common engine failures. Let’s address each column separately:

A vehicle with a fuel delivery problem will have good volumetric efficiency because the engine can still breathe. It’ll have a lean oxygen sensor signal because it’s starving for fuel. It’ll also have a normal MAP reading because it can still inhale air. Fuel trim numbers will be positive, depending on how bad the fuel delivery situation is.

A vehicle with an air metering problem, bad MAF sensor, or unmetered air will have poor volumetric efficiency. It doesn’t actually have a breathing issue; it just appears that way because the MAF sensor signal is faulty. The oxygen sensor goes lean because, since the computer thinks there’s less air, the fuel system provides less fuel. The MAP sensor reads normal because atmospheric pressure is still allowed to enter the intake manifold. Finally, fuel trims tend to be positive and gets worse as engine RPM increases.

A restricted exhaust would yield poor volumetric efficiency because the engine can’t exhale. The oxygen sensor reads correctly because the engine is still being fueled correctly. The MAP sensor reads normal because the atmosphere is still rushing into the intake manifold.

Fuel trim depends on the situation; this becomes more obvious on V-type engines with two catalytic converters. Finally, a restricted intake causes low volumetric efficiency because the engine can’t inhale. Oxygen sensors read normal because the engine is still being fueled correctly. The MAP sensor tends to move away from barometric pressure because the restricted intake acts as if the throttle were closed. And the fuel trim numbers are okay because the engine is still being fueled correctly.

This test, in conjunction with the cheat sheet, provides a quick check for eliminating many common engine issues before moving on to transmission diagnosis. If you know how things are supposed to behave, it’s easy to pick out the troublemakers when they don’t. This process of elimination can greatly expedite your diagnostic process.

One final note: If you perform the test drive and the results appear good, look elsewhere. Examples include ignition timing, dragging brakes, or, maybe most important, a transmission issue.


Scott Shotton, owner of The Driveability Guys, will be presenting a detailed version of this topic at the Powertrain Expo in Las Vegas. Scott will be joined by Eric Ziegler, another trainer and mobile diagnostic technician with EZ Diagnostic Solutions and The Driveability Guys. Eric will be presenting topics covering fuel trim and ignition diagnostics.