What year 280CE? Make sure EGR is functioning properly. All new parts won't work if the vacuum signal isn't getting to them. If EGR checks out and you're good on HC and CO, you probably are a bit lean. What do the other numbers look like? Also check ignition timing. Too advanced will give you higher NOx.

I used to work in a shop that tuned older German cars to pass smog. We had an exhaust gas analyzer. It's tough to get a reading on fuel/air mixture without one unless you have a new enough car to have an O2 sensor, which I'm pretty sure your car won't have.

One other less likely possibility is abnormally high compression due to the pistons being carboned up from prolonged light use. Italian tuneup will help with this. Even if it doesn't help, it's still fun to do.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.