Going in I knew if it fails the New Jersey State inspections, it will be emissions and it would be high NOx. Indeed it did.
My 93 940 wagon passed all mechanical inspections and failed emieeions. The CO, HC numbers are below spec. NOx is at 1,226, above the 937 limit.
Two years ago it failed the same item. I passed it with an OEM O2 sensor. Could the O2 sensor go bad in two years? Is there a way to check the O2 sensor? I did more than replacing the O2 sensor two years ago. I put in a complete new exhaust from the header on.
Oxides of Nitrogen (NOx):High NOx can occur when there is excessive temperature in thecombustion chamber or a damaged catalytic converter.Check the following:? Air Injection System? Exhaust Gas Recirculation System (EGR)? Combustion Chamber Deposits? Oxygen Sensor ? Catalytic Converter
Diagnostics for NOx failures
OVERALL ·Check for causes of high combustion temperatures. Primary systems that affect NOx include:
-mixture control (lean)
-EGR
-ignition ·Check Catalytic Converter
1981-86 Check the following: O2 sensor (The signal from the O2 Sensor provides a wealth of information on how the vehicle is running.) EGR valve and EGR hose Ignition timing and system (advanced timing and ignition components are some of the most common problems). Vacuum lines (deterioration of vacuum lines, inlet manifold seals, and become more prevalent causes with age). Catalytic converter Primary feedback sensor failure including, oxygen sensor, MAP/MAF, TPS, RPM, and CTS sensors.
1987-95 Check the following: Use OBD, if available O2 sensor EGR valve and EGR hose Ignition timing and system (advanced timing and ignition components are some of the most common problems) Primary feedback sensor failure including, oxygen sensor, MAP/MAF, TPS, RPM, and CTS sensors Vacuum lines Catalytic converter _____________________________________________
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Of Nitrogen (nox)Nitrogen makes up about 78% of the air we breathe. Though normallyinert and not directly involved in the combustion process, combustiontemperatures above 2500 degrees F cause nitrogen and oxygen to combineand form various compounds called "oxides of nitrogen," which isabbreviated NOX. This mostly occurs when the engine is under load andthe throttle is open wide. NOX is a nasty pollutant both directly and indirectly. In concentrations as small as a few parts per million, it can cause eye, nose and lung irritations, headaches and irritability. Higher concentrations can cause bronchitis and aggravate other lung disorders. Once in the atmosphere, it reacts with oxygen to form ozone (which is also toxic to breathe) and smog.
To reduce the formation of NOX, Exhaust Gas Recirculation (EGR) is used. By recirculating a small amount of exhaust gas back into the intake manifold to dilute the air/fuel mixture, EGR has a "cooling" effect on combustion, thus keeping temperatures below the NOX formation threshold.
On 1981 and later engines with computerized engine controls, a special "three-way" catalytic converter is used to reduce NOX in the exhaust. The first chamber of the converter contains a special "reduction" catalyst that breaks NOX down into oxygen and nitrogen. The second chamber contains the "oxidation" catalyst that reburns CO and HC.
High NOX emissions are almost always due to a defective EGR valve (or some component that controls the operation of the EGR valve). A related symptom that usually occurs when EGR is lost is spark knock (detonation) during acceleration.
Oxides (NOx)Nitrogen Oxides is a group of highly reactive gases that containnitrogen and oxygen in varying amounts. Many of the nitrogen oxidesare colorless and odorless. One such gas, nitrogen dioxide (NO2) canoften be seen combined with airborne particles as a reddish-brownlayer over many urban areas. Nitrogen oxides are formed when theoxygen and nitrogen in the air react with each other duringcombustion. The formation of nitrogen oxides is favored by hightemperatures and excess oxygen (more than is needed to burn the fuel).The primary sources of nitrogen oxides are motor vehicles, electricutilities, and other industrial, commercial, and residential sourcesthat burn fuels.
Nitrogen Oxides in vehicles are caused by excessive combustion chamber temperatures. Some of the common causes of high NOx emissions are problems with the vehicle's Exhaust Gas Recirculation System (EGR), improper ignition timing, lean air/fuel mixture and malfunctions in systems that control engine temperature, such as the thermostat and cooling fan, and vacuum leaks. Due to the complexity of the internal combustion engine, other components may cause high NOx as well. _____________________________________________
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of Nitrogen (NOx) Oxides of Nitrogen (NOx) emissions occur when fuels are burned at high temperature. Some of the nitrogen (N2) in the air combines with some of the oxygen (O2) in the air to form nitric oxide (NO). In an engine, some of the NO undergoes additional reactions and turns into nitrogen dioxide (NO2). The emissions of NO and NO2 are collectively referred to as NOx. In addition to contributing to the formation of ozone, NOx emissions also lead to a build up of nitrogen dioxide levels in the atmosphere, which are known to increase the risk of respiratory disease in children.
Exhaust Gas Recirculation (EGR) System Testing
The purpose of the EGR system is to recirculate a small amount of burned exhaust gases back into the engine under certain operating conditions. This lowers the temperature of combustion and, in doing so, reduces the formation of Oxides of Nitrogen (NOx).
It is easy to see then that the EGR is a device that primarily affects NOx emissions. However, certain EGR defects can also cause excessive HC emissions.
The first thing the technician should check on every EGR system is the exhaust passage. Adequate exhaust gas must be available to the EGR valve for the maximum flow (highest load) conditions. In most cases they can test this by manually opening the valve at idle and stalling the engine. If the engine does not stall, it should drop by at least
300 rpm. Otherwise, there is probably a restriction in the exhaust passage or pipe.
To test the EGR valve and its related components, the technician must monitor the valve to see that it is opening and closing the proper amount under the proper conditions. In some cases this will involve removal of the valve.
Many different controls are used in various types of vehicles to control and monitor the operation of the EGR valve. In many cases, the technician will need to refer to the manufacturer's recommended procedure to troubleshoot the EGR controls. However, some general procedures the technician should apply are shown below.
Pressure feedback sensors and modulators should be checked for any blockage or restriction in the backpressure sensing tube. * Pressure feedback sensors should be checked electrically for proper reference voltage, ground, and signal return to the PCM. * EGR valve position sensors should be checked electrically for proper reference voltage, ground, and signal return to the PCM. * EGR temperature sensors should be checked for proper reference voltage and ground. Temp sensors should also be checked for correct resistance using the manufacturer's specifications. * EGR vacuum control solenoids should be checked for the correct pulsed ground and voltage. Solenoids should also be checked for correct resistance using the manufacturer's specifications. * Venturi vacuum amplifiers should be checked for correct vacuum input and output using the manufacturer's recommended procedures and specifications. * Thermostatic vacuum switches should be checked to ensure they switch properly and allow vacuum to pass through only when the engine is warm (or vice-versa for some switches). Also, remember that a low coolant level may prevent correct operation of any temperature sensor or switch. * All associated vacuum hoses and fittings should be checked for cracks, leakage, and secure connections. * The exhaust system should be checked for modifications that may be adversely affecting exhaust backpressure and the control of the EGR.
Possible Causes of High Oxides of Nitrogen (NOx) Emissions
In order to pinpoint the cause of excessive NOx emissions, the following systems (if applicable) and possible defects will need to be checked, usually in this order: Over-Advanced Ignition Timing
Older vehicles have ignition timing systems that are adjustable and, unfortunately, anything that is adjustable is also mal-adjustable.
Fortunately, ignition timing is usually quick and easy to check. For more details, see Ignition System Testing. Inadequate Exhaust Gas Recirculation (EGR) Flow
Operation of the Exhaust Gas Recirculation (EGR) system should be checked completely. To do this there are three areas that the technician must check:
the EGR valve * the EGR exhaust passage(s) * the EGR control system
For more information on these procedures see Exhaust Gas Recirculation (EGR) System Testing. Lean Air-Fuel Ratio
The technician should check to ensure that there is not an excessively lean mixture being burned. Depending on the vehicle, its fuel system, and its emissions control systems, this may involve simply looking at the VIR, measuring engine-out emission levels, and O2 sensor testing.
For detailed info regarding the diagnosis of fuel delivery systems, see Fuel System Testing. Defective Three-Way Catalytic Converter
Your technician should test the catalyst by performing a converter efficiency test (see Catalytic Converter Testing). Note that this only applies if the vehicle is equipped with a three-way catalytic converter (some 1980-87 vehicles and all 1988 and newer vehicles have three-way catalytic converters). Combustion Chamber Deposits
If the above possibilities have been conclusively checked and no problems found, it is reasonable to conclude that combustion chamber deposits are contributing to excessive NOx emissions. ______________________________________________
In a nutshell there are very few reasons for high NOx:
Too much ignition advance, which on your car you can do nothing about.
Too much carbon on the tops of the pistons that raises the compression. Normally doesn't occur on Volvos later than about 1975, but if you want to try a motorvac with BG chemicals or try a SeaFoam treatment go ahead it won't hurt, especially if you have high mileage at this point.
Too much carbon on the backs of the intake valves forcing a lean condition that should be accomodated by the fuel trims in the injection control unit or a bad O2 sensor that would produce the same thing. If you were outside the correctable range of the computer the check engine light would display. A MotorVac will also address this issue. That leaves the catalytic converter as the usual culprit. If when the exhaust was replaced you used an aftermarket cheapie converter it may well have failed by now. Check the docu,entation for the replacement unit and if it isn't declared suitable for replacement in California then when new it will barely pass NOx in the states that use the treadmill (dyno) test for emissions.
I am somewhat skeptical of the bad cat in two years having had a similar experience with one of my SAAB 9k's. Following someone's advice to replace the cat after failing NOx, the new cat didn't lower the NOx at all. In the end it was simply the spark plugs have a gaps too wide.
But I am interested in the motorvac. What is it. Could you provide more details?
disconnect the fuel lines and fill the machine from the tank, add the proper detergent to the fuel, bypass the fuel lines so that the fuel pump recirculates fuel directly back to the tank, let the machine circulate the detergent-fuel micture in the the fuel rail for 20 minutes or so. Then you Start the motor and run the detergent fuel mixture through the injectors and engine for another 20-30 minutes. Disconnect the machine, reconnect the fuel lines to the motor and take the car for a spirited test drive. If you remeasure the tail pipe emissions with a five-gas analyzer you will see a significant reduction in HC and NOx, with perhaps a slight increase in CO. This is more apparent if you measure at the test port ahead of the converter. I've replaced converters on 740's with the cheap Bosal/Walker units and have them barely pass after replacement and then have them pass with flying colors two years later.
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