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Originally Posted by Dcraig
I didn't even see the PS, sorry.
Test weight is 4000
SW Version is 310/WP391256
The car just had about an hour canyon drive, stop and go. I took it to a plave without a line, I was the only one. I did turn off the car, for maybe 5 minutes. I didn't know turning the car off was bad.
Does the coolant temp have anything to do with the cat temp? Should I wait for a hot day in the 80's?
Craig
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Coolant temp, within the normal range of operating temperature doesn't have a big effect, but the hotter the better. It's EGT that has the big effect on catalyst temperature. Shutting the engine off prior to the test is a big mistake as this will allow the catalyst to rapidly cool off. As you can see from my test data, even five minutes of normal idling can allow the catalyst to cool significantly. Shutting off your engine, alone, may be the cause of your failure. While waiting, turn on the A/C, open the windows, and keep engine revs at about 1500. You want to place as much load on the engine as possible to keep the catalyst as hot as possible. If the coolant temp climbs up to 100C or above, that's fine. The fan system will keep it from achieving excess temperature. I don't think it makes any difference if the outside temp is 60 or 80 as long as the car is properly conditioned before the test.
High HC with other emissions low can mean an ignition problem. I doesn't take too many misfires to drive up the HC over the limit. On M103 engines high HC can sometimes be improved with a new cap and rotor and new plugs, especially if they have high miles. If your car has a coil on plug ignition system the new cap and rotor doesn't apply, but new plugs might help if the existing plugs are old.
Do you have conventional 30K mile copper core/nickel electrode plugs or does MB specify long life platinum tip plugs for your engine, and how many miles do the plugs have relative to the change interval?
Many think that changing the oil will help emissions. I don't think that really applies unless the oil is beyond the normal service life and/or the engine has high blowby, however, if you are getting close to your next change, go ahead and change it before your retest
Given that you still have some O2 in the exhaust, a hotter catalyst will promote more oxidation reaction. You want to see the O2 at zero, and this will reduce the HC count by 20-30 PPM, but that can only happen if the catalyst is hotter, which gets back to the conditioning issue.
I would go ahead and retest the car. Give it a good warmup drive, go to a drive through test station with a short line and DO NOT shut off the engine, but keep it running while you wait as I stated above.
Duke