Any OBD2 code reader will work. The W202 was the first OBD2 compliant car. All cars had to be compliant by 1996, but that didn't stop a manufacturer from being compliant prior to that. I just use a regular code reader with my 1994 C280. The port is on the underside of the dashboard on the driver's side, about where your right knee would be.

As far as suggestions are concerned, pulling the codes will probably steer you in the right direction. When my car did that it was a problem with the purge valve. It could be something as simple as a broken or disconnected vacuum line somewhere, though.

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Jonathan

2011 Mazda2
2000 E320 4Matic Wagon
1994 C280 (retired)