Your right about your vehicle not having an EGR valve. If it did, the EGR
valve would help the reduction of NOx. Since it does not, the reduction of
NOx is mainly up to the Catalytic Converter, and the fuel feedback system.
If your Volvo is running lean it will create high NOx, so it's important to
ensure there are no vacuum leaks or defective fuel control.
Engine cooling problems may cause high NOx also. Remember NOx (Nitrous
Oxides) is created engine temperatures reach over 2500F. For this reason you
want to make sure your vehicle's temperature gauge is always normal and that
your cooling system is working properly.
High compression can cause high NOx also. Carbon build-up within your
engine's combustion chambers will cause increased engine temperature. Keep
in mind this problem is usually seen in vehicles with over 200,000 miles.
The solution to this problem is called an Engine Decarbonizing.
Decarbonizing your engine will remove carbon of your engine's piston heads
and valves. This will lower compression and lower NOx.
