There is a lot of misleading information in this thread.

An '87 does not have an EGR system.

This car has high HC and NOx.

High NOx is not caused by richness. It is caused by a lean mixture (unlikely if the system is working properly), too high compression, or a bad catalytic converter.

My guess is cylinder deposits causing high compression. Do an Italian tune-up (full throttle on the highway to maximum revs), or try the "emission-passing magic deposit remover" solutions from the auto store. or pull the head and scrape them off.

High HC can be caused by a rich mixture (unlikely if the system is working), misfiring, a vacuum leak that affects only some cylinders (so that the system tries to compensate and makes the others too rich), or unburnt oil.

My guess is misfiring.

Tackle the basics: new plugs, new plug wires, clean up the cap and rotor, make sure that the on-off ratio is oscillating around about 50% (some say a couple of % under). Make sure that your O2 sensor is ok (there is a checking procedure in the Mercedes CD manual under CIS-E)nand replace it if you are not sure. We are now up to $200, cheaper than going to a repair shop. If your on-off ratio is not oscillating, then it is indicating a fault code, and you had better have a look at the CD manual.

Look in the DIY section for Steve Brotherton's article on evaluating engine controls. It is very thorough on the on-off ratio and EHA.

Your meter does not indicate exactly 70%, because it is not an accurate meter. My Sears meter also is a little off. Don't worry about it; just make a mental adjustment.

Our 190e 2.6 passed aircare yesterday with flying colours, at 370,000 km (about 230,000 miles). It recently needed a new fuel distributor. It had a recent O2 sensor. It has old plugs, cleaned, filed and gapped. It has the original catalytic converters. I just broke down (against my better judgement) and put in a new cap and rotor, replacing the originals. Otherwise, just the basics. Yours has much less miles on it, and is unlikely to have any fuel injection components worn out. Just the basics.

Andrew
1898 190e 2.6