I have an 85 volvo 240 non turbo . I am failing on nox at 2500 ppm on a 1500 limit . I have replaced the catalytic converter , muffler , plugs ,wires, cap and dont know where to go next . This is a non EGR car . Any help would be appreciated .
Try some injector cleaner - use the BG44k. Add to the tank and fill up. Drive the tank till less than 1/4 full - try to do this in one or two days - include plenty of highway driving and some heavy acceleration. After this, immediately change the oil and filter. You might also want to have your fuel pressure checked and there are a couple of engine sensors that could be problems (engine temp for the injection system), but those these don't usually show up as high NOX.