Hi.....i have a 1989 Mercedes 420SEC............in good condition, and the only fault at last testing was high HC.......at idle it was around 190, and at 2500 rpm down under 40. Have done an oil change, but still high HC, and there is NO blue smoke as they say at the exhaust on start up, motor runs perfect, and my consultation with Hedins in Ljungby say that the possible cause could?? be top end, valve guides possible worn or dried......and the cost would be between 15 and 20 thou KR..............perhaps a product from Wynns would help solve the problem.......any help out there

Its your catalysator, change it and you problem will be solved!

Are you sure, the CAT has never been mentioned at all...............Mercedes...Hedins i Ljugnby want to open up the top of the engine, and poke around for between 15 and 20 th SEK, then when i check the websites here and in Germany, CATs are not available for this model............strange

lisahund skrev:

Are you sure, the CAT has never been mentioned at all...............Mercedes...Hedins i Ljugnby want to open up the top of the engine, and poke around for between 15 and 20 th SEK, then when i check the websites here and in Germany, CATs are not available for this model............strange

It seems that this modell has been made with CAT and some modells without the CAT. You need to find out which one yours is / check to see if you see the cat under the car.

Lower down you can see both models.

http://sv.wikipedia.org/wiki/Mercedes-Benz_W126

My Merc came from Germany in 1999...........and it does have a CAT.............a huge beast of a thing...........which according to Mercedes here, in Germany and in the UK.......this part is no longer available..as the say NLA. Perhaps one could get one made by exhaust specialists...so as to fit the space between the engine and the rear silencers............and its a big space.............

If the workshop haven't I suggest you take out your car for a journey with high revs to de-carbonize the catalytic cleaner.

May "stuff" the catalytic when driving calm, a little harder driving with arm it so long the oil is good and everything is in operation temperature.

I would recommend you to diagnose the catalytic converter before replacing it. The most convenient way of doing this is measuring the temperature before and after the converter.

If it's working then you would have a higher temperature after the CAT because of the bonding between molecules that's going on inside.

Also remember that even if you are running with a good CAT you can still fail emissions if your engine is running rich. Since this is an older car there is no diagnostic port to plug in and check the o2 sensor readings for a rich/lean condition.
But if the car is running with a catalytic converter then you should also have an o2 sensor and could potentially measure it's output with either and oscilloscope or an multimeter.

This may be getting into the advanced area now but what I'm trying to say is to diagnose your problem first and don't throw money at parts if you are not sure what the problem is. If money is no problem then I would have them do what they wish as long as they fix the problem.

Il leave you with this good video of diagnosing a catalytic converter.

Thanks for all the replies, a new contact put me in touch with a small auto tune workshop in Moheda, a Bosch trained technician, who found the problem in no time at all, 2 slightly faulty plugs, that were not firing every time they should on tick-over, changed all plugs, cleaned up rotor head and a few other things, chainged oil and filter.....passed the test......HC42 and 26 at 2500rpm.........these guys are worth their weight in gold.....3000kr

For information for other who read this thread.

You can't say th CAT is bad based on the values you got at your protocol from "besiktiningen"

Like the thread starter found out there can be bad combustions at low rpm wich increases the HC values.

The only way you can say the cat is bad is when the engine is running at Lambda 1 and the HC and CO is above threashold limits. You will in that case notice the cat efficiency at the CO2 and o2 values. Those values is not included at the "besiktnings"protocol.

it's possbile with some sofisticated garage quipment read out the igntion system condition. You can basicly see charge of the ignition coil, se the burn time over the spark plug, and oscillations in the coil etc. Then you can easily se if one or more spark plugs is worn.