That's the tough one (+) - Saab NG900 & OG9-3 Bulletin Board - Saabnet.com
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That's the tough one (+)
Posted by Vlad (more from Vlad) on Tue, 27 Apr 2004 19:27:53
In Reply to: manual, all stock, NOx failed. thanks!, rider, Tue, 27 Apr 2004 07:50:14
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I am currently having the same problem with my Volvo. CO and HC very low, NOx pretty high. NOx is caused by high combustion temperatures. High combustion temps can be caused by faulty EGR valve, lean mixture, overly advanced timing, high intake air temps. Faulty catalytic converter can be the cause of increased NOx emissions, too.
EGR valves tend to get gummed up with carbon deposits, so that's the primary suspect. You need to check its operation and clean it out well. Most of the time this will fix high NOx.
Most common cause of lean mixture is a vacuum leak. Vacuum leaks are common and easily overlooked. You can find it by using a propane torch with a 2-foot hose. Run the engine and poke the propane hose around the engine: around fuel injectors, air hoses, vacuum lines. If the engine reacts at some point, you have found the location of the vacuum leak.
Overadvanced timing is probably caused by faulty ECU on these cars. The base timing is not adjustable but you should be able to check it with a timing light. If it is off, I don't think there is any way to adjust it other than swapping in a known good ECU.
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