1979-1993 & 94 Conv [Subscribe to Daily Digest] |
Members do not see ads below this line. - Help Keep This Site Online - Signup
Exhaust Leaks and High NOX Emission (Smog) Test
Passing an emissions test can be a trying experience for the 'backyard' mechanic. I'm posting my experience to help fellow classic Saab 900 owners but it is applicable to most any vehicle. Here is a story that could save you major expense and hassle.
Our beloved mid-mileage and reasonably well-maintained '91 Saab 900 Turbo Convertible came due for an emissions test. My pre-test inspections included checking the O2 sensor for proper computer voltage and response to lean/rich inputs and making sure all the vacuum lines and systems were intact and functioning. Given the level of sophistication of the LH injection system and how well it was running, I felt it had a good chance of passing.
FAIL! The NOX numbers, both low and high speed, were extremely high and far from max allowed. All the other numbers were well within the allowed range. Substantial research on the internet and discussions with Saab techs indicated the problem was one or more of the following: high combustion and engine temps due to running lean, a faulty cooling system, dirty intake air sensor measuring low air supply and leaning out the injectors, vacuum leaks causing a lean mixture or a bad Catalytic Converter. The engine was not running lean or hot and had no vacuum leaks so most of these were quickly eliminated. The Catalytic Converter was of unknown age but definitely not new and became the focus of the problem. We replaced the Cat and promptly went back for another test confident it would pass.
FAIL! While the high speed NOX was just within the allowed range, the low speed NOX number was still nearly double the max allowed. Now what? A perfectly running engine with a new Catalytic Converter with excellent readings for everything but low speed NOX? No end of researching every car forum from 4WD truckers to Mercedes seemed to address this problem - especially on a vehicle without an EGR system. I was about ready to eat humble pie, tear up my 'backyard mechanic' credentials and turn it over to the pros with an inevitable and impending exorbitant expense (plus two smog tests and a new Catalytic Converter.) But not yet...
My research had shown a few hints of exhaust leaks possibly distorting emissions numbers but nothing definitive. Plus, I had no reason to suspect the Saab had any leaks and a visual inspection showed nothing. But, I was out of options and desperate. I hooked up the outlet (blower side) of the shop vac to the tailpipe, pressurized the system and sprayed soapy water on all the connections. Sure enough, the downpipe connection to the turbo outlet pipe was leaking. This is downstream of the O2 sensor but upstream of the Catalytic Converter. After loosening and cleaning the clamp bolts, I cinched them down tight to stop the leak. I had my doubts about this being the cause of the high NOX, but it was worth a shot and back to the smog test we went with fairly negative expectations.
PASS! The NOX numbers dropped to near ZERO! Wow! My guess is the new Cat was an unnecessary waste of time and much money. The theory is simple when thought about. The exhaust flow is a series of impulses rather than a steady flow. The impulses alternately pump exhaust gases out and then suck air (and oxygen) back in and the excess oxygen prevents the Cat from functioning properly. At low speed the exhaust flow and pressure is less and the impulses are farther apart allowing proportionally more outside air into the system.
Long story short - NEVER attempt a smog emissions test without first checking for and fixing any exhaust leaks, even small ones, ahead of the catalytic converter.
posted by 99.75.88...
, Wed, 4 May 2011 15:49:25
No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.
 |
 |
 |
 |
 |
