1986 Volvo GLE failed on low speed NOx

Your Volve is a non-egr equipped vehicle, meaning NOx reduction is heavily relies on the Fuel Control system (computer) and the CAT. Since the Catalytic Converter has already been replaced, the two most important things to look at are air/fuel mixture and vacuum leaks. You want to make sure your Volvo is not running lean (not enough fuel getting to the combustion chambers). The second is locating any vacuum leaks. A vacuum leak will also cause a lean fuel condition. Once you've verified your Volve has no vacuum leaks and the engine computer is in proper fuel control you can look into a "high compression" problem which may also be causing high NOx. This can be accomplished through a Compression Test. Your mechanic will be able to check compression on each cylinder and determine whether compression is high and whether it is due to carbon buildup. If carbon buildup is the problem using a product like Sea Foam can help in removing much of the carbon on the piston heads and within the combustion chambers. Prior to applying Sea Foam it is important to make sure the engine is at operating temperature. Find a vacuum line that feeds to all cylinders (the PCV hose or the vacuum brake booster line usually will feed all cylinders). After the correct vacuum line has been located, start your Volvo's engine and disconnect vacuum line. With engine running at idle and at operating temperature slowly pour Sea Foam into vacuum line. The engine will start hesitating while the Sea Foam® is being applied. Be carefull not to stall engine. After 1/3 to ½ can of Sea Foam® has entered the engine, shut the engine off and let it sit (hot soak) for about 10 minutes, then reconnect vacuum line. After 10 minutes you can restart your engine. Insure your are in a well ventilated area. Your Volvo's exhaust will smell strong and be very cloudy for a short time. Begin driving aggressively until the exhaust looks normal. This usually takes up to 5 miles. This process conducts an intense carbon cleaning procedure to your Volvo's engine.