Jeff Taylor boasts a 30-year career in the automotive industry with Eccles Auto Service in Dundas, Ontario, as a fully licensed professional lead technician. While continuing to be “on the bench” every day, Taylor also is heavily involved in government focus groups, serves as an accomplished technical writer and has competed in international diagnostic competitions as well as providing his expertise as an automotive technical instructor for a major aftermarket parts retailer.
The actual combustion process that takes place in the modern vehicle with a gasoline engine hasn’t changed much since the four stroke engine was invented well over 100 years ago. Yes, it’s been immensely fine-tuned to reduce the pollution that it creates and increase the fuel economy that we get when we fill up the tank, but the exhaust that is created is the same. When gasoline as a fuel is burned, chemically we should only get a few things; water (H2O) carbon dioxide (CO2) and heat.
The problem is that this is under the perfect scenario and this just doesn’t happen even in the most modern engine. After the gasoline is ignited by the spark plug and the combustion process begins, a number of factors will determine what the actual chemical reaction will produce. The actual exhaust produced by combustion is a mix of substances, but there are a select few that we as technicians should be concerned about, especially when it comes to diagnostics. Those issues could be a failed emission test, a P0420/P0430 catalytic efficiency code or poor fuel mileage, and these are just the tip of the iceberg of problems that the exhaust gases can help us diagnose if we understand what it’s trying to tell us.
Most of the gas analyzers that are available are of the four- or five-gas varieties, with a five-gas providing oxides of nitrogen (NOx) information. They may also provide us with other information that can be very helpful such as Lambda and air/fuel ratio values (more on this in a bit). The exhaust gasses are providing us a look inside the cylinders and at the combustion events that are happening. Even if the vehicle has no feedback control at all the use of a gas analyzer can be very valuable diagnostic tool. To properly understand the information the analyzer is providing us, we have to understand what each of the values it is telling means and how that relates to the combustion events. The fuel and air charge that was drawn into the cylinder and the effects of any exhaust after treatments will show up in the expressed values.
There are four special notes before we start to gather any gas analyzer information:
1: There can’t be any exhaust leaks at all (leaking manifolds, flanges, holes, old test ports, etc.). Any false air drawn into the exhaust system will skew the results and provide possible misleading data.
2: If the vehicle has any sort of air injection system it should be disabled and checked to make sure it’s not leaking before any testing.
3: Following the manufacturer’s procedure to calibrate the analyzer and keeping the filters clean is imperative or false information could result.
4: I do almost all my testing at idle, but on occasion have had to drive with the gas analyzer attached (it was so much easier when we had an operating dyno).
The products of combustion...
Carbon dioxide (CO2): CO2 is a major component of the combustion, and the amount of CO2 in the exhaust is a good indicator of how well the catalytic converter is working and the mechanical condition of the engine. CO2 is a true measurement of the combustion efficiency of the engine and I have three rules of thumb when looking at CO2 values.
- 16% is the maximum amount of CO2 that can come out of the tailpipe of a perfectly running vehicle. I expect to see at least 14% and in most cases 15% as the ideal amount of CO2. Anything less than 14% and I start to get suspicious of engine problems.
- Low CO2 readings point to mechanical problems (low compression/timing issues/valve train issues). If the total value of CO2 and CO added together is less than 14.5% you are going to have some sort of mechanical engine issue and that has to be sorted out first, before any other diagnostic can be performed.
- CO2 will generally be higher at 2,500 rpm vs. idle by 1% to 2% because of increased volumetric efficiency.
Carbon monoxide (CO): This is the odorless, highly poisonous, invisible gas that results from incomplete combustion of the fuel being burned. This tailpipe gas tells us that we had more fuel in the combustion chamber than available oxygen to burn it, and typically points to a rich or over fueling condition.
There are many conditions that can cause this, from poorly maintained vehicles with gas diluted oil, exhaust leaks upstream of the O2/A/F sensors, leaking purge valves, and even a bad thermostat causing the engine to run to cold.
- CO levels should always be below 2% exiting the engine (pre-catalytic) and should mimic the amount of O2 fairly closely on most engines.
- Older non-feedback engines, both North American and European, were traditionally set to run at about 1% CO for best power and driveability.
Oxygen (O2): 21% of the air that we breathe and the engine uses for combustion is O2. The O2 is combined to from H2O and CO2 in the perfect scenario when burning gasoline, but we know that doesn’t happen.
- The amount of O2 that is left after combustion should be low, typically below 2% (pre-catalytic, 1% is ideal), on an engine without air injection and exhaust leaks. Readings will be higher (3% to 4%) on vehicles with functioning air injection and typically this system is disabled or blocked off for accurate tailpipe emission diagnostics. Higher levels of oxygen commonly point out that we didn’t have enough fuel to consume it all in the combustion chamber or we have a lean condition.
Oxides of nitrogen (NOx): Nitrogen is the biggest part of our atmosphere (78%) that enters the combustion chamber. If the temperature is above the burning point of nitrogen (2,500 > Fahrenheit) then the nitrogen will join with the available oxygen and form NOx, a pollutant that causes smog, and a harmful gas. Many things can increase the temperature of the combustion chamber, such as high load, heavy carbon build up, EGR issues, timing over advanced, cam timing issues, bad knock sensors and the often overlooked cooling system are a few of the problems that can lead to elevated NOx levels.
- NOx values at idle should typically be 0 ppm (parts per million).
Hydrocarbons (HC): We are burning a hydrocarbon-based fuel (gasoline) and it doesn’t always get completely burned. Causes of this can be weak spark, misfires or unintentional vapors being added from an EVAP system. These are just a few examples of things that can raise the HC’s in the exhaust, there are many others.
- Typical readings for HC on a properly functioning engine with a good catalytic will be below 50 ppm and many newer cars are in the 0 to 10 ppm at idle.
Lambda or the calculated air/fuel ratio: This is a calculated value that is based on either four or five values of the products of combustion. Using a four gas, we would use, HC, CO2, CO and O2. On the five gas we include the NOx. Many gas analyzers will give us a Lambda value, and this is to me the most valuable number that it can produce.
This value is the easiest and most direct way of knowing what the mixture was going into the combustion chamber. The Lambda value is not affected by combustion, combustion doesn’t even need to take place and the catalytic converter doesn’t affect its value either. Even a dead miss will not affect this calculation. The Lambda value reflects whether or not the air/fuel ratio was rich or lean before it enters the cylinder for combustion. That’s the beauty of this value.
The Lambda value is calculated using a complicated formula developed by Dr. Johannes Brettschneider, a Bosch researcher; he came up with the formula in 1979. Brettschneider’s equation calculates a ratio value that will be 1 for stoichiometric (14.7 to 1) and if the mixture is rich the Lambda value will be less than 1. For example, an air/fuel ratio of 12.0 to 1 (rich) would show a Lambda value of .816. The opposite example of a very lean condition 16.5 to 1 would show as a Lambda value of 1.122. Don’t panic if your gas analyzer doesn’t give you a Lambda value.
There are many Lambda calculators available online that are free and easy to use (iATN has one). You just have to plug in the numbers that you are seeing and it will calculate the Lambda value for you.
I prefer to use the Lambda number to diagnose fuel trim issues and the affects that it can have on things after combustion; by this I mean catalytic efficiency codes. The Lambda value is an excellent aid in telling if the PCM is in fuel control and what it was actually commanding the air/ fuel ratio to be.
The modern PCM is very fast at calculating fuel trims and making corrections. The information that we see in the data stream may not point to a slightly lean or rich condition, but the Lambda value will show us this. The catalytic converter will do a pretty good job of treating the gases that are expelled from the engine. Most of the CO’s and HC’s will have been converted to water and CO2 well before they escape out the tailpipe, so it is very difficult to tell if the mixture was rich or lean without using Lambda. This can be particularly valuable in diagnosing a P0420/430 catalytic efficiency codes because the catalytic wants to see a Lambda value that is no higher than 1.020 (for the reduction or NOx portion) and no lower than 0.980 (CO/HC oxidation portion).
This is a very tight range and not easily seen on most data streams, but using the Lambda value to see if the value falls in the zone of 0.980 to 1.020 is an easy and straightforward test. If the results show that Lambda is in this range, then you are likely looking at a catalyst that has passed away. If the number is outside this range you are going to have to dig a little deeper into the issue and get the Lambda back into this target range before bolting on a catalytic replacement.
The use of a gas analyzer is something that I take for granted. In Ontario, we still have an emission testing program for vehicles that are pre OBD II and the testing involves a two-speed idle test. We still see frequent failures and make repairs based on exhaust gasses. Current vehicles perform so much better and more accurately that we seldom get involved in sniffing the exhaust unless we have a catalytic code, fuel trim issue and want to get a more accurate perspective as to what is going on in the engines combustion chamber and the catalytic converter before parts are replaced. ●
