At 192k miles, engine blow-by is probably the culprit.

I know cars. I know late model Mercedes vehicles, so Im guessing here...

1) 1983 with 190k miles - A lot of city miles - very worn car... - a lot of start stops...- if less then 5mile trips are made, it is hard on the CATS. The car has to come upto temperature-This is important in cold weather...

2) The engine blow-by is greater than a 190k mile highway driven car.

I'm assuming if my 96 S320 has a vacuum controlled EGR. Your 500SEC probably has a vacuumed controlled EGR too. If it is vacuum, you can test the whole Exhaust Gas Recirculation (EGR) very easily... This method works on any make or model with vacuum controlled EGR valve.

1) disconnect the vacuum line at the EGR, and plug it with a pen cap

2) Attach several feet of vacuum line to the EGR valve(make sure the line fits snug on the nipple of the egr)

3) With the engine off, suck on the attached vacuum line, and you should feel and hear the EGR vacuum diaphragm actuate

4) Now with the engine on, and idling. If you suck on the line, the engine should slow down, hesitate, and possibly stall. If it does that, the WHOLE EGR system is WORKING!!
If it does not slow down, the EGR passages could be plugged. On most cars a steel pipe taps into the exhaust and channels it to the EGR valve and then channels it to the intake manifold to mix with the fresh intake air. In about 90% to 80% of cases the EGR passages plug up with carbon, next the vacuum to control the EGR valve is not present in about 10% of cases, and lastly the EGR valve fails(now the EGR could be gummed up to with carbon). The Mercedes cars are weird when it comes to the EGR passages. Some engines have the EGR steel pipe, and some have the passages through the engine block. If yours is on the engine block, it is hard to near impossible to unclog. If you have the metal pipe, the passages can be cleaned with a coat hanger on a cordless drill, so it tears-up the carbon, but does not damage the steel pipe- Very much like a kitchen drain clean-out is done.

5) now test the plugged vacuum line. Take the pen cap off. Put your finger on the vacuum line. You should feel little to no vacuum while idling, but while you rapidly accelerate to about 1200rpm and let go the throttle. The vacuum will be greater as the rpms rapidly drop. You may get vacuum when you rapidly accelerate. If this works, the EGR vacuum control is working okay. If this NOT present, you better chase this down as this may be plugged, or you may have a broken vacuum line

The CATS can be tested with a Infrared temperature gun, but this test can only help you with Hydrocarbons numbers. The NOx's are mostly fixed on the engine side.

1) Measure the inlet pipe going into the CAT(s) at about 6" to 10" before each CAT.

2) Measure the temperature on the cat, but measure it from the CAT not the heat shield.

3) Throttle your car to 1500rpm, and hold it for a minute. Measure the CAT's. What you are looking to see in temperature is the the actual cat should be several hundred degrees hotter than the inlet pipe. You will need a helper to hold the throttle. If you want to test this technique. Do a the test on any late model car to test the CAT temperature differential test. If the temperature is the same, than your CATs are probably toast

A thing I would do before the CAT temperature test, or your next smog. I would clean your plugs with some fine sandpaper, yet providing they are not wet.. Next I would clean-up cap and rotor with sandpaper/file/ worn dremel bit on a Dremel. Borrow a dremel, or buy the cheapy $9.00 dollar one from Harbor Freight to clean-up the cap and rotor. Use the pink stones. Be careful when working the rotary tool as it can grind down your cap and rotor. You want the contact points on the cap and rotor to be shinny-It will improve the efficiency of your ignition components.

In some cities, one can find these gas stations that sell about 110 Octane racing gas. I remember when I lived in the Los Angeles area. I saw about six of them.

I'd even get some of the chemicals that help you pass the SMOG tests.

Hope this helps you as these are cheap fixes and test to help you. As the best thing to do, replace the CATS, but that is going to cost you.

Best of luck,

Martin



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