1991 420SEL Fails CA emission due to advanced timing
A friend of mine owns a 1991 420SEL which is in mint condition. 137,000 miles, owned since new. He recently took it for a smog check (in California). The car passed the actual emissions test easily. It passed all but one of the visual inspections.
According to the testing person and the sheet of paper they handed to my friend, the ignition timing was not within specifications. They got a reading of 16 BTDC. I believe factory spec is 12 BTDC at idle.
I know that these cars have electronically controlled ignition timing and cannot be adjusted. What could be the cause of the timing going advanced? This may be related, but the car also idles around 1100 RPM. 900 cold, climbing to 1100 when at operating temp. Could the advanced timing be a side effect of the idle speed being high?
He's taking it to a shop tomorrow for diagnosis, but I'd like some input from the great folks on this site. It would be nice to go in knowing all the possibilities.
Thanks in advance.