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Switched to synthetic at 180K, very rough idle at start (check engine light comes on eventually) 900

posted by  someone claiming to be kbodi on  Mon Jan 12 04:49 CST 2009 [RELATED]

So I switched to synthetic in my '95 960. Immediately afterwards, the car started idling very roughly at start, but this would improve as the engine warmed up. Eventually, it got better and I thought the problem had resolved itself, but then the 'check engine' light came on. I checked the OBD codes and then reset it to 1-1-1, and the rough idle came back. The OBD code I got was a 2-2-1, which has the following conditions, cause, and symptoms according to the FAQ:

Fuel trim (lambda control) too lean at part load
Lean; air leak; low fuel pressure; bad AMM
Engine stops when starting from cold; high fuel use

A few years ago I had a very similar issue (rough idle). I also was failing NOx emissions, and the solution was a new (used) AMM. I would think it's the same thing this time, but I'm not sure why it would happen immediately after changing the oil to synthetic. Maybe I nudged something? What else could cause such a rough idle and the 2-2-1 code? The IAC valve? Thanks in advance.

-kbodi





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