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#1
08-13-2002, 03:38 AM
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1991 420SEL Fails CA emission due to advanced timing
Hi,
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. Nolan 
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#2
08-13-2002, 11:10 AM
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The increased idle speed could be causing the advance in timing. If you watch the timing mark under a timing light, you'll see it advance as the speed increases. The increased idle speed might be linked to a faulty or sticking idle valve. Easy test would be to simply unplug the electrical connection while at idle, if everything is working correctly, the idle should increase, if the valve is faulty there should be no change (shadetree test mind you).
__________________
Mike Tangas '73 280SEL 4.5 (9/72)- RIP  Only 8,173 units built from 5/71 thru 11/72 '02 CLK320 Cabriolet - wifey's mid-life crisis 2012 VW Jetta Sportwagon TDI...at least its a diesel Non illegitemae carborundum.
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#3
08-25-2002, 01:18 PM
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Solution
Hi All,
Update: The car was taken to an independent shop to confirm my speculation about a faulty idle control valve. $500 later ($400 for the idle control valve), idle is back at 600-650 and timing is back at 12 BTDC as observed by a timing light. Thanks again for the input. Nolan
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#4
08-26-2002, 01:31 AM
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Glad I could point you in the right direction. I take it you've passed your Smog Check now?
__________________
Mike Tangas '73 280SEL 4.5 (9/72)- RIP Only 8,173 units built from 5/71 thru 11/72 '02 CLK320 Cabriolet - wifey's mid-life crisis 2012 VW Jetta Sportwagon TDI...at least its a diesel Non illegitemae carborundum.
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