Hello, I am new to the site and posted a few days ago. I have a 1991 560 SEL with 190K miles on it. I bought it as is and running rough and low power.
I have changed the spark plug wires, plugs, rotor and cap are good. I checked the fuel distributor screens, etc. Everything looks goods so far. The car will start up and run smooth for about 20 seconds then start to miss. It even sounds like an old carbuerated car that is loading up. It will nearly stall out, then pick back up, etc.
I have noticed that there is quite a bit of blow by out the oil fill on the left valve cover. This would normally concern me but I took a heat(temperature) gun to the cat and just in front of the cat it reads 298F and just after it reads 160F. I read somewhere in here that is a sure sign of a clogged cat? There is exhaust coming from the exhaust pipe but this is my first mercedes so I am not sure if it is normal or not.
If the cat is clogged couldnt that be causing much of the blow by? The car does smoke a little at startup but then quickly clears up--like valve seals are worn...
Any help would be greatly appreciated...thank you in advance...

Darrell in Houston

 

I pulled the exhaust system off the car. The pre-cat was about 50 clogged. I knocked out both converters and it is free flowing now. This did not fix my problem though. The car will crank and run smooth for about 30 seconds then start missing. You can drive and it will be stumbling, then stomp it and it will stumble a little then take off...Any suggestions?

 

IS THE CAR FITTED WITH THE O2 SENSORS IN THE EXHAUST , i HAD A MERC A CLASS THAT DID EXACTLEY THT RUN LUMPY AFTER START UP THE SENSORS TELL THE BRAIN THE CORRECT FUEL MIXTURE , i REPLACED BOTH O2 SENSORS WHICH FIT INTO THE EXAUST SYSTEM AND IT RUNS SWEET NOW , HOPE THIS HELPS NIGEL IN ENGLAND