If you clear the code does it come back?

Have you checked the fuel pressure regulator vac line for fuel or odor?

I would use copper plugs.

Make sure the engine is well warmed up.

Maybe a can or techron injector cleaner prior to testing.

Dan

I have run into this issue the last several years in Texas. Once my 1990 hits 25 years, I will be exempt from the two-stage tail pipe test.

From my experience, you will want to check the air cleaner housing box, and make sure that the thermostat pin is still working. When it fails, it fails in the open position, and this was the reason I failed my emissions test one year.

You will also want to check your downpipe that connects your exhaust manifold to your convertor. Make sure that the gasket is sealing the connection correctly, and that the downpipe itself doesn't have any cracks. This was an issue I had on another failed test.

Finally, I concur that the Bosch Platinum plugs are not a good choice. I used to run these plugs, however, I have switched back to regular copper plugs. I have also tried the iridiums, and they just run too hot for this engine.

Good luck with your next attempt. Always fun waiting while your car is on the machine, to see if it passes...
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If it needs to be maintained, repaired or replaced on a 1990 240, I've probably done it. '90 240DL, 309,600 miles, will she make it to 400K ?? >>You haven't really worked on a car until you draw blood<< :-}

I think you're on the right path - lean mixture, which drives up NOx.

You've done all the basics, so here are a few other things to look for. Pull the inlet snorkel (big hose between AMM & throttle body) and carefully inspect for holes, especially on the underside (a dark room and a flashlight are helpful). If that checks out OK, then use the carb spray or propane method to check for intake manifold leaks.

Re-tension the timing belt - with the engine off, pull the plug in the TB cover and *lightly* loosen the 14 or 15 mm bolt inside. It has to be loose enough that the tensionser spring can move the idler, but not so loose the belt runs off. Crank the engine a few seconds (ok if it starts, but just for a second or 2) so the belt gets a few full revolutions. Then tighten the idler bolt back up.

One other thing to check if you have a timing light or can borrow one is to see how steady the timing is at idle and at a higher sped like 2000 or so. If the timing mark on the crank pulley jumps around a lot, then the rubber damper in the crank pulley is probably shot and the pulley needs to be replaced. This is a long shot, so check this last.

Just a comment in passing - the sexy platinum plugs probably aren't going to do much good for a B230 engine. Overall experience here on the BB is that conventional plugs work best. That said, their effect on emissions one way or another will be minimal.