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 so i FAILED SMOG: a diagnostic aid 

has your e30 failed smog while you owned it?
nope, never had a problem with smog 20%  20%  [ 3 ]
yep, but it was a simple fix 33%  33%  [ 5 ]
yep, i've been in over my head 47%  47%  [ 7 ]
Total votes : 15

 so i FAILED SMOG: a diagnostic aid 
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Unread post so i FAILED SMOG: a diagnostic aid


Hydrocarbons (HC) Raw gasoline or gasoline that has not burned. Associated with incomplete combustion. Combines with sunlight to form photochemical smog.

Carbon Monoxide (CO) partially burned fuel known as a rich indicator. CO is a highly toxic gas that displaces oxygen in the blood stream.

Oxides of Nitrogen (NOx) Unnatural combination of oxygen and nitrogen. Occurs during high temperature and high pressures. Reacts with sunlight to form photochemical smog.

Carbon Dioxide (CO2) a measurement of combustion efficiency. Occurs during combustion of HC and O2. Non-toxic gas.

Oxygen (O2) Supports combustion, also known as a good lean indicator.

In order for the internal combustion engine to obtain the lowest level of emissions, all elements of combustion have to be in place. Spark timing, air, fuel, and compression. If any of these elements are out of place, the combustion process will suffer and HC will increase. The amount of HC increase depends on the severity of the problem. For example, an engine with an ignition misfire will produce the highest level of HC. The air/fuel enters the cylinder, and then gets compressed, but not burned. In this example, none of the fuel will burn and it all enters the exhaust stream in its natural state as HC. The HC levels from the exhaust wouldn’t be much different than sampling the fuel tank filler neck; it’s merely raw fuel, untouched by combustion. This is what occurs during a total loss of combustion, usually associated with ignition misfires. Anything that would stop the burning process sooner than we would like it to will increase HC readings from the exhaust. There is only one type of misfire that won’t result in a HC increase. This would be an injector circuit problem. No fuel means no HC increase.

Electrical Misfires: if the spark plug does not fire, the fuel will not burn. This will create the highest level of HC emissions from the tail pipe.

Advanced Ignition Timing: spark occurs before the air fuel mixture is properly compressed. This creates incomplete combustion. When ignition occurs too early, the expansion of the gasses create downward pressure on the top of the piston as it is moving up in the compression stroke. This will create a loss of power. This combination of expansion and compression inside the cylinder can cause excessive NOx and detonation.

Lean Air/Fuel Mixtures: HC molecules are separated by too much air, the flame cannot jump air gaps for the entire air/fuel charge to burn. This is a referred to as a lean misfire. In the case of a lean misfire, combustion occurs intermittently, so the severity of the HC increase is not as detrimental as a total ignition loss. Remember that during a lean misfire, there is no combustion. There is a small amount of fuel mixed together with an unnecessary amount of air, but without the flame front, none of the fuel will be burned, resulting in an increase in HC emissions. Note: hydrocarbons normally will not increase during a slightly lean condition; when only marginally lean, combustion still occurs.

Rich Running Condition: too much fuel for the amount of oxygen. The air is used up before all the fuel has burned. The flame goes out and HC levels increase. In a slightly rich condition, the flame may reach all the fuel, but only partially burn, producing high levels of CO. this may not result in a HC increase. Remember that HC is fuel that is not burned. Any fuel that gets burned or partially burned will come out of the tail pipe as CO or CO2. CO is created when fuel is only partially burned. This occurs when only 1 oxygen atom has attached to 1 carbon atom. CO2 is created when fuel is compactly burned. CO2 is created when 2 oxygen atoms attach to 1 carbon atom.

EGR Valve Open at Idle: oxygen is replaced by inert exhaust gas that does not support combustion, and fuel will not burn. This increases HC levels at idle. An engine at idle does not run as efficient as an engine under a load. The reason for this is volumetric efficiency. An idling engine can only get as much air as the throttle plate will allow into the intake manifold. When the EGR valve is open at idle, it crowds out air that would normally be used for combustion. Instead, the fuel doesn’t burn and HC emissions increase. When the engine is under load, the throttle plate is open and there is very little restriction to airflow into the intake manifold. This means that there is enough oxygen to complete the combustion process, even when the EGR valve is open. An EGR valve that is open under load becomes insignificant because of volumetric efficiency; HC levels drop because the combustion process is more complete. Volumetric efficiency is the amount of air that fills the cylinder when the intake valve opens.

Idle Speeds too Low: this is usually due to insufficient airflow past the throttle plate. This lowers the pressure in the cylinders, reducing combustion efficiency, therefore increasing HC levels. This problem creates low volumetric efficiency. Anything that restricts the intake airflow to the cylinder will result in low cylinder pressures and raise the HC levels.

Leaking Piston Rings: the air/fuel mixture isn’t compressed properly, causing low combustion efficiency, therefore elevating the HC levels. The HC that escapes past the piston rings enters the crankcase. Over time this will saturate the engine oil with fuel. This fuel is drawn through the PCV valve causing a rich mixture.

Burnt Intake or Exhaust Valves: when a cylinder doesn’t seal on its compression stroke, the pressures will be too low for complete combustion to occur, and HC levels will be elevated. The HC will generally be higher at idle than under a load due to volumetric efficiency. An engine generally will run at half the efficiency at idle when compared to the same scenario under load. This is evident when we perform a running compression test. The reading you get off the compression gauge on a cylinder while the engine is running, is about half of what you see when checking compression while cranking. When cranking, the throttle plate is all the way open, and the engine is turning slowly. This allows lots of air to enter the intake manifold, and since the intake valve is open for a long period of time, there is plenty of time to fill the cylinder. With the car running, the throttle plate is closed and the engine is turning much faster. The rpm of the engine and the position of the throttle plate is what determines the amount of air that can enter the cylinder. Therefore when the valves are burnt, the problem is more prevalent at idle. Under load the engine will seem more efficient.

Some Other Causes:
Malfunctioning air injection systems
Malfunctioning catalytic converter

There are many more causes of elevated HC emissions. It can be simplified to one simple phrase: anything that stops the combustion process too soon will cause elevated HC with the exception of an injector failure. Think of HC as a measure of poor efficiency. The higher the HC, the worse the efficiency of the engine will be.

CO is partially burned fuel that occurs when the air/fuel mixture is rich. CO will increase only when there is not enough air to burn the fuel that enters the cylinder. During a rich condition, fuel will burn until the oxygen is gone, allowing the flame to go out. This means that the oxidizing process has been stopped by the lack of oxygen available in the cylinder, which caused incomplete combustion. Problems that create lower than normal combustion will usually result in lower than normal CO with the exception of a rich condition. This is why CO is a good rich indicator. It will only be elevated when the fuel mixture is rich.

Plugged PCV Valve: the engine can receive as much as 30% of its air from the PCV system. When the PCV system gets plugged, it starves the engine of needed oxygen to complete the burning process; therefore, this will create a rich running condition and elevate CO readings. The wrong size PCV valve, one that is too small, will have the same results: elevated CO emissions.

Dirty Air Filter: carbureted engines are very susceptible to this condition. The carburetor works on differential pressures; when the air cleaner is plugged, it creates a choke affect. It’s like shutting the choke plate, thus creating vacuum in the throat of the carburetor and lowering the pressure at the tip of the discharge nozzle. The pressure in the fuel bowl remains the same, and the pressure at the discharge nozzle drops lower than what’s in the bowl. The pressure difference between the fuel bowl and the discharge nozzle allows the fuel to flow from the discharge nozzle. Fuel injected engines are less vulnerable to this condition, as they use mass airflow sensors and vane airflow meters for fuel metering. Their operation depends on the air pulled across it to register a load condition to the PCM, however, the restricted air filter will limit flow across the sensor. This condition may cause lower than normal intake manifold vacuum which may affect engines that use MAP sensors as load sensing devices. The PCM would read the signal from the MAP as a load condition and richen up the mixture.

Fuel in the Crankcase: fuel that enters the crankcase is drawn through the PCV system. This fuel is not metered and will cause the fuel system to be rich. This fuel will be an addition to what the system is already receiving from its normal source.

Saturated Evap Canister: the evap canister’s job is to store fuel vapors from the fuel tank and carburetor float bowl while the vehicle is at rest. The computer will purge these vapors from the canister when the vehicle enters closed loop and is off idle. When the canister purge solenoid is activated, it allows engine vacuum to pull the stored fuel vapors into the intake manifold to be burned. These vapors dissipate from the canister quickly and will make the fuel system rich momentarily. When the evap canister is saturated, the fuel in the canister will not dissipate and causes the air/fuel mixture to be continually rich. This problem will cause high CO conditions off idle.
Note: most canister purge systems will not begin purging until the engines computer system has entered closed loop and the throttle is off idle.

Some Other Causes:
Malfunctioning air injection system
Malfunctioning catalytic converter
Malfunctioning computer controls
High fuel pressure
Sunken float
Leaking injectors

Remember that CO is a rich indicator and the measurements will be elevated when the fuel system is rich. CO can only be present in the exhaust when combustion occurs; it is a byproduct of combustion.

NOx is formed when combustion temperatures exceed 2500 degrees. Oxygen bonds to nitrogen to form the deadly gas NOx. This occurs only under high temperature and pressure.

Advanced Timing: spark occurs before the piston has reached the top of its compression stroke. When the air/fuel mixture ignites, the gasses expand while the piston is still moving up in the compression stroke. This creates abnormally high pressures and temperatures. NOx emissions will sky rocket with this type of problem. The high temperature in the cylinder will also create pinging and detonation.

EGR Valve Malfunction: the purpose of the EGR system is to dump a small amount of exhaust gas into the intake manifold, therefore replacing a portion of fresh air and fuel. With less air/fuel entering the cylinder, the bomb becomes smaller, and peak combustion temperatures are lowered to below 2500 degrees. With the technology advancements in the internal combustion engine, we are seeing EGR valves become obsolete. Automakers are able to keep peak combustion chamber temperatures below 2500 with computers and better design inside the cylinder.

High Performance Parts: the purpose of high performance parts is to increase the size of the explosion that occurs within the cylinder. This is done by either getting more air and fuel into the cylinder, or by shrinking the size of the combustion chamber, therefore raising the cylinder pressure. Either way it’s done, it all has the same results: higher cylinder temperature and/or pressures, and elevated NOx production.

Cooling System Problems: when the cooling system cannot adequately dissipate the temperature around the cylinders, the results are elevated NOx

Low Octane Fuel: fuel with low octane ratings will burn faster and ignite with lower temperatures. As air and fuel get compressed, temperatures within the cylinder rise. If they rise to the flash point of the fuel, the fuel will ignite. This is known as preignition. Once the fuel ignites, the gasses begin to expand. The expanding gasses get compressed as the piston moves up in the compression stroke, creating high temperatures and high pressures within the cylinder. After the air/fuel preignites, the spark plug fires and creates another flame front. These two flame fronts collide which creates a shock wave within the cylinder, giving us the knocking noise that is associated with preignition.

Lean Air/Fuel Mixture: when oxygen is added to fire, the fire gets hotter. Blacksmiths used bellows to add oxygen to fire so it would be hot enough to mold metal horseshoes. If we add oxygen to fire within the cylinder, it too gets hotter, creating excessive temperatures and elevating the NOx production. Lean air/fuel mixtures will burn slower, allowing it more time to build up heat.

Carbon Build Up on Pistons: carbon on the top of the pistons shrinks the size of the combustion chamber, allowing higher pressures, thus raising the temperature within the combustion chamber. When the air/fuel mixture is compacted into a smaller compartment, it creates a larger explosion when ignited by the spark plug.

Carbon on Intake Valves: carbon build up on the backside of the intake valves soaks up the fuel entering the cylinder, creating a lean condition. This has been known to create elevated levels of NOx.

Inoperative Knock Sensor: a knock sensor’s job is to signal the PCM when pinging occurs. This input allows the PCM to retard the timing. When pinging is occurring within a cylinder, the temperatures and pressures will be high, therefore NOx will be elevated.

Turbo Charger/Super Charger: add on parts such as these are designed to increase volumetric efficiency by pressurizing the intake manifold. When the intake valves open, pressurized air/fuel mixtures flow into the cylinder, unlike naturally aspirated engines that rely on atmospheric pressure to fill the cylinder. These add on parts actually force the air/fuel mixture into the cylinder. When more air and fuel enters the cylinder, the combustion within the cylinder becomes more intense. The outcome of these devices is increased horsepower and production of NOx.

Computer Inputs: the PCM is responsible for controlling the air/fuel mixture and spark advance. It uses sensor inputs to achieve this goal. When a sensor is out of calibration, the PCM will respond by changing timing and air/fuel mixture to whatever way the sensor is biased. For example: the O2 sensor is biased high, this means that the voltage output of the sensor is too high for the oxygen content in the exhaust. The PCM will interpret this as a rich condition and will lean out the mixture in response to the O2 sensor signal. This lean mixture will in turn create a high NOx production.

Cylinder Head Machining: often cylinder heads are shaved when performing valve jobs. This is done to create a flat surface for the head gasket to seal. It is not unheard of to take as much as .030 off of the head. When this is done, it creates a smaller combustion chamber. This shrinkage of the combustion chamber increases the cylinder pressure and the outcome is NOx production. The problem can be corrected by using a thicker head gasket.

CO2 PRODUCTION: carbon dioxide is a result of proper combustion of HC and O2. Any problems in the engine that affect the combustion process will lower the CO2 levels. You cannot have a misfire and expect to see high CO2 levels. The higher the CO2 level, the more efficient the engine is running. Optimum CO2 is 13% to 15%. CO2 can be used as an efficiency meter of the engine’s combustion. When there is a problem that affects the combustion process, the CO2 will be low. CO2 can help you determine if a high HC problem is engine related or caused by the catalytic converter. For example, high HC followed by high CO2 would indicate a problem in either the air injection system or catalytic converter, whereas high HC followed by low CO2 would indicate that the problem is affecting the combustion process, so the problem is engine related. Keep in mind that a good catalytic converter can raise the CO2 level up 2% over that of what the engine can produce. So the question comes up, is the CO2 high because the converter is good, or is CO2 high because the engine is running well. Only an intrusive catalytic converter test will tell us the complete truth.

OXYGEN: O2 readings that show up on your five gas analyzers are extra oxygen that was not converted during the combustion process. This means that it was unused. This is why O2 is an excellent lean indicator. When the engine is running lean, O2 will increase proportionately with the air/fuel ratio. As the engine goes to the lean misfire conditions, around 16.0:1, O2 will rise drastically. Good O2 readings at the tail pipe should be under 3%.


Fri Jul 09, 2010 10:57 am
E30 Addict
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Joined: Sun Jul 26, 2009 8:56 pm
Posts: 2226
Location: Fallbrook
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Thank you hjv9435,

You have given this forum a lot of great information and guidance on smoging our old cars. This will not only save you some time but will allow us to better understand how it all works. I for one think that a sticky should be put on this thread. Calling SpecRacerM5 it time for sticky. 8)



ninja edit

1990 E30 M3
1991 325IC M Tech II
1992 E34 525i
1993 E34 M5

Last edited by IronmanE30 on Fri Jul 09, 2010 4:26 pm, edited 1 time in total.

Fri Jul 09, 2010 11:50 am
Full Member

Joined: Fri Jul 03, 2009 4:44 pm
Posts: 263
Location: Coronado
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This is awesome. Thank you.

1992 325i Convertible

Fri Jul 09, 2010 12:28 pm
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