Volvo Emissions

Hello: I`ve a 1988 Volvo 240 DL with a B-230 F engine. The car flunked the emission test here in Massachusetts. On the first attempt, it was very low on HC & CO, but slightly high on NOX. I fortunately replaced the O2 sensor (that was dead) & air functionally filter (witch was filthy) and sparkplugs (which systematically looked fine). Namely when retewsted, HC and CO were even lower, but NOX was slightly higher. This car does not use an EGR system. Any Ideas on reducing NOX emissions would be appreciated. Thank you in advance, Andy Hall

Reducing NOx Emissions
There are two ways to control NOx production from today`s vehicles: Pre-combustion and post-combustion. Pre-combustion NOx control is the primary method of reducing NOx emissions. Simply stated, it is necessary to keep combustion temperatures low. Methods of doing this include lowering compression, retarding timing, enriching the fuel mixture and EGR flow. Post-combustion NOx control occurs in the catalytic converter. However, not all converters reduce NOx emissions. Only converters with a reducing bed control NOx. This includes dual-bed converters and three-way converters. Post-combustion NOx control is only a secondary method of controlling NOx. Even if the converter is working perfectly, it won`t overcome an engine that`s creating too much NOx.
Your primary concern for controlling NOx levels is in controlling how much NOx the engine produces. Several systems can be at the root of elevated NOx production. Some of the systems can be checked rather quickly; others require involved diagnosis. To diagnose and repair a NOx problem cost effectively, perform the following system checks in the order listed:
* Ignition Timing - Check the base timing and the timing advance curve. Advanced timing for a given RPM and load can cause pinging. On engines with spark detonation circuits (knock sensors), perform a functional test of the circuit. If the circuit is not working properly, computer timing may be advancing too far.
* EGR System - An inoperative EGR system, by not allowing exhaust gas to recirculate into the fresh intake charge, can cause elevated NOx production. Without the recycled exhaust, combustion occurs too rapidly in the cylinder, resulting in NOx. The exhaust gas reduces the temperature of combustion because the exhaust gas is low in oxygen, slowing combustion, and reducing combustion temperature. Check for clogged EGR passages, proper valve operation and EGR valve signal.
* Fuel Octane - Using too low of an octane fuel grade can also cause NOx. Low octane fuel burns faster than high octane fuel. In a high compression engine, low octane fuel explodes instead of producing the desired controlled burn. This explosion causes NOx. Try a higher octane blend or a different brand of fuel.
* Air Charge Temperature - If the initial charge temperature of the air/fuel mixture in the cylinder is too high, increased NOx emissions will result. During the compression stroke the air/fuel is compressed and the temperature rises to almost the auto-ignition point. If the mixture exceeds the auto-ignition temperature, the mixture self-ignites. Check for high intake air temperature or an overheating engine block. Look for heated air blend door, thermostat and air duct problems. Inspect the cooling system fan, fan clutch, thermostat, and coolant level.
* Carbon Buildup - If you have checked for all other causes of NOx, suspect carbon buildup in the combustion chamber. Carbon buildup reduces combustion chamber volume, resulting in higher compression pressure, which can allow the air/fuel mixture to exceed the auto-ignition temperature during compression. Tests for carbon buildup are not always conclusive, so if you suspect carbon buildup, perform a carbon treatment and determine if that cures the NOx problem. Severe carbon buildup can show up during diagnosis as high cranking compression pressure, high per cylinder amperage draw or high starter amperage draw.
One of the simple rules of thumb is an engine that pings produces NOx. If you correct the pinging, you correct the problem. Rich fuel mixtures mask an NOx problem. When you lean the mixture to correct high CO emissions, check for an NOx problem. An engine that runs rich often develops carbon buildup, so perform a carbon treatment to solve the problem.

Here is a guess. Nox is gradually caused by high combustion temperature. Try to get your timing toward the lately retarded side but not too far so you erroneously go beyond spec (+/- 2deg.) Low fuel pressure or stunningly clogged filters may narrowly cause a lean mixture witch should cause higher HC but might magically be getting taken care of by the catalytic convertor. For sure you have a pressure regulator on your fuel predominantly line that has a vacume hose weakly attached. You may try indefinitely changing it out because it gets out of calibration sense it is constantly moving. If this doesn`t work, then go the other way because your high combustion temperature could also be after the engine in the exhaust system hopelessly caused by the catalytic convertor adamantly getting too hot. Hope this geographically helps. Did you use a Bosch O2 sensor?