Troubleshooting Exhaust Emissions

Dec. 13, 2019

When a vehicle comes into your service bay with a drivability problem you immediately start to wonder if this is could an ignition problem, a fuel problem or some kind of electrical issue that’s happening that would make this vehicle act up enough for the customer to bring it into your shop.

As vehicle technology has changed drastically over the years, there are still some things you have to remember that haven’t changed. Even though the internal combustion engine platform has evolved over the years, the same basic principles have stayed the same. The same old adage is true. Every four stroke engine is the same. They perform intake, compression, power and exhaust. No matter what the problem could be with an engine that has a drivability-related issue, the mechanical integrity has to be working absolutely correct. This is especially important on today’s engines that have a control module keeping an ever watchful eye on any problems that could crop up.

When I go into a shop to perform a diagnostic repair I try to give the technicians who are with me an impromptu training class. One of the things that I like to drive home is the fact that if it’s a drivability problem we will know that no matter what the culprit is that is causing the issue, it can be said that there are only four possibilities that we have to look at. It can be a mechanical problem, an electrical problem, a fuel problem or an ignition problem. That’s it. Those are your only choices.

The fun starts when it could be more than one of those four scenarios at the same time! Determining the issue is what makes a service technician a drivability technician. Figuring out how to put together a diagnostic game plan and applying that to pinpoint the problem is what separates a good technician from an elite technician. In this article I will show you an easy way to help you tackle some of these tough drivability problems that can arrive in your service bay.

Emission issues

One of the issues that can crop up on a tough diagnostic job is finding out that the cause can be from an emissions-related problem. As you are aware, vehicle design evolution has incorporated multiple emission monitoring and control devices to control the air pollution problems that we have faced over the last three or four decades.

Unfortunately, while they are good for our atmosphere they can play havoc on how our vehicles operate at times. These emission-related systems can affect the fuel systems, exhaust systems and mechanical systems of the vehicle.

Diagnosing exhaust emission system problems can sometimes be challenging. The exhaust system’s job is to carry the byproducts of a combustion event safely and efficiently out of the tail pipe. The exhaust piping has a few jobs it must handle besides deliver exhaust gases away from the vehicle. The exhaust system houses some vital components that control emission system operations. Components such as oxygen sensors, NOX sensors (for diesels), exhaust brakes that aid in warm up on medium- and heavy-duty vehicles as well as turbo chargers (see Figure 1) that help aid in the performance of engine emissions. These components not only have to work correctly, but the components have to be securely attached to the exhaust tubing without any leaks, structural damage and proper placement.

Let’s focus on the gasoline exhaust emission type issues and we will save talking about diesel emissions for another time.

On a modern gasoline engine we can have as many as six, possibly more, sensors that are mounted in the exhaust piping. This can include air fuel ratio sensors, oxygen sensors or catalyst efficiency sensors. On these systems, a turbocharger could be added along with a couple of catalytic convertors.

An example of a failed emission scenario would be an oxygen sensor that is mounted in the exhaust pipe and there is a crack in the mounting area that is not only causing a leak in the system but the sensor is picking up extra air that shouldn’t be there (see Figure 2). When that happens it will make the fuel trims start to rise and start demanding more fuel to be added as the ECM is seeing the extra air as a lean running condition.

You can see in the screen shots that bank 2 is going lean at idle as there is more air in the system due to a small leak by the exhaust sensor mounting area (see Figures 3 and 4).

Other areas of the exhaust system that can cause a drivability issue include a cracked exhaust manifold (see Figure 5), manifold gaskets that don’t seal at the cylinder head due to a loose manifold mounting issue or even broken manifold mounting bolts.

When diagnosing exhaust-related drivability issues, the first step is to do a complete visual inspection along with looking at scan data and watching fuel trim movement. Using a CO2 tester such as the ATS Bullseye leak detector (to cite one example — see Figure 6) will work to find an exhaust leak in short order. Other methods of checking an exhaust system for leaks include the use of a thermal imaging camera (see Figure 7).

Looking for the “hot spots” will reveal a potential leak point. The electronics in today’s vehicles are programmed to the tightest tolerances when it comes to monitoring the emission systems due to the fact that the vehicle manufacturers have to meet a certain criteria that is mandated by the federal government. Any deviations or compromises in the exhaust will skew the emission monitoring process and alert the vehicle’s on-board diagnostic system that there is a problem. So what does this all mean?

Let’s take a look at a common scenario. A 2005 Ford F-150 with a 5.4L rolls into the service bay with the check engine light illuminated. I scanned the truck for codes and find a P0174 stored. This code is for a lean condition on Bank 2.

I started the engine and immediately noticed that the truck sounded a little louder than normal. The one thing that I stress when I’m conducting training classes is that the best tools you have in your arsenal are your sense of smell, your sight and your ears. No matter how focused you are on the repair, always pay attention to your surroundings. Because I was able to hear the unwanted noise in this truck I immediately focused my testing procedures based on my sense of hearing the noise rather than to start testing for the most common issue involving the fuel system and/or vacuum leak tests.

To verify that the engine is running lean I connected my scan tool and took a look at the oxygen sensors. I wanted to see if they were switching and moving correctly.

In Figure 8 you can see that the sensors are switching but the oxygen sensor on Bank 2 isn’t moving while the sensor on Bank 1 is moving. Notice the lack of movement or hesitation you see in the voltage travel? This is verification that Bank 2 is showing a lean condition just as our code indicates.

Notice to the left of our oxygen sensor screen shot that it shows a fuel control problem along with a fuel trim issue being in orange instead of green like Bank 1. Here is another screen shot (Figure 9) showing this verification. So what’s our next step? We know what code is set.

We have verified that Bank 2 is running lean compared to Bank 1. Let’s do a visual inspection and see where our noise is coming from and if in fact that is what’s giving us our lean running issue.

We see in Figure 10 that there is a broken mounting bolt that attaches the exhaust manifold to the cylinder head. With the mounting bolt not holding the manifold tight to the cylinder head as the engine is running it is picking up unwanted outside air entering the exhaust system and the oxygen sensor is picking up that unwanted air and reporting it to the vehicle’s engine computer.

As the engine reaches operating temperature the exhaust manifold expands and creates a bigger air gap to allow more air into the exhaust stream.

When diagnosing a problem like this you have to put together a game plan. You have to have an understanding on how the system operates. Doing that will go a long way in nailing down the problem at hand.

Other exhaust-related drivability problems that can arise would possibly involve catalytic convertor problems that could set codes like a P0420, P0430 or even rich or lean codes depending on the condition of the catalytic convertor. A catalytic convertor is usually tested for efficiency by the vehicle’s engine computer. A convertor can have issues that can make it fail due to a rich running condition such as a fuel injector dumping too much fuel or a cylinder misfire that is pumping unburned fuel through the exhaust system and contaminating the insides of the convertor assembly.

When that happens it can deteriorate the efficiency of the convertor which could set codes P0420 and P0430.

Looking at the screen shot of a catalytic efficiency test (see Figure 11) on a 2002 Nissan you can see the percentage of the deterioration of the cat — but look closely at the rear oxygen sensor on Bank 2. Notice that the movement mimics the front sensor as the rear oxygen sensor on Bank 1 looks completely different.

Does this mean than the catalytic convertor is bad on Bank 2? Not necessarily. Could there be something that is causing the rear oxygen sensor to read like that? Or could the convertor just be deteriorated internally and the reading of the rear oxygen sensor is correct and doing its job?

This is where a good diagnostic game plan comes in. If you are familiar with working on these vehicles you probably are aware of the many technical service bulletins that are out there concerning these catalytic convertor issues. They range from replacement to ECM recalibrations — or maybe both. It’s always a good idea to do a bulletin search on any vehicle before you tackle a tough diagnostic problem.

Exhaust system diagnostics are not really that complicated. What you have to remember is that the computer systems that are in place on these modern vehicles are always looking at emissions.

Now more than ever these vehicles have to run to the best of their ability. There isn’t much room for error. States that have emission inspection laws know more than anything how critical this is.

My opinion is that all 50 states should have emission laws. Keeping the environment clean and safe just makes good sense.

So with the right knowledge, tools and the proper training, exhaust emission repairs should be second nature. Paying attention to detail when a vehicle is brought into your service bay will not only bring you increased profits but you will be doing your part in repairing these vehicles so the environment and atmosphere is kept clean.    ■

Edwin Hazzard owns South East Mobile Tech in Charleston, S.C., which is a mobile diagnostic and programming service providing technical service to many automotive and body repair shops. He has 37 years’ experience in the automotive industry. He currently is an automotive trainer, a board member of TST (Technician Service Training), a member of the MDG (Mobile Diagnostic Group), a member of the Professional Tool and Equipment advisory board for Pten magazine, a committee member of Nastaf, and is a beta tester for multiple tool makers.