Signs & Symptoms of a Bad Catalytic Converter

[Leon_V]

Hi everyone,

In general, what are the signs & symptoms of a bad catalytic converter? Would a clogged converter and/or old inoperable one cause the "check engine" light to come on for a M119 engine in an E420/400E? Right now, my car is stalling/having trouble accelerating after sitting for a while after having my resonator & exhaust fixed as well as the fuel line. My mechanic thinks it might be these, but I am more willing to bet the root cause will be moisture inside the caps & rotors (even after having changed them 5K ago). We are checking all this tomorrow.

Thank you in advance for everyone's help.

Regards,

Leon

 

[gsxr]

Leon, in general, a failing or failed cat won't cause the CEL to illuminate. The most common catalyst failure is when they become restricted, which reduced high-RPM power noticeably. When severely restricted/plugged, the power may be so reduced that the car won't accelerate. You can use the built-in LED blinker attached to the CAN box, to find out what specific code is causing the CEL.

Stalling / misfiring is likely not related. I agree that moisture in the caps is far more likely, especially if the issue clears up and the car will pull normally to redline after being driven 30-45 mins. If the new (5kmi old) caps don't have the ventilation slots, add them.

Also, if the insulators were not replaced, they should be removed to inspect the back side... if the cam seals have never been replaced, pull the insulators to inspect for oil leakage. Details at link below.

FYI - Essential M119 Ignition "Tune Up" mods [Distributor cap vent / slot] | M119 Engine

Hi all, I have been meaning to address this on my little used 500E for years.... it is a problem I myself recognised about 5 years ago when I first looked at an m119 distributer cap..... Compare to any other distributor cap of the era such as an m104 and you will see there are NO ventilation...

www.500eboard.co

 

[Leon_V]

Leon, in general, a failing or failed cat won't cause the CEL to illuminate. The most common catalyst failure is when they become restricted, which reduced high-RPM power noticeably. When severely restricted/plugged, the power may be so reduced that the car won't accelerate. You can use the built-in LED blinker attached to the CAN box, to find out what specific code is causing the CEL.

Stalling / misfiring is likely not related. I agree that moisture in the caps is far more likely, especially if the issue clears up and the car will pull normally to redline after being driven 30-45 mins. If the new (5kmi old) caps don't have the ventilation slots, add them.

Also, if the insulators were not replaced, they should be removed to inspect the back side... if the cam seals have never been replaced, pull the insulators to inspect for oil leakage. Details at link below.

FYI - Essential M119 Ignition "Tune Up" mods [Distributor cap vent / slot] | M119 Engine

Hi all, I have been meaning to address this on my little used 500E for years.... it is a problem I myself recognised about 5 years ago when I first looked at an m119 distributer cap..... Compare to any other distributor cap of the era such as an m104 and you will see there are NO ventilation...

www.500eboard.co

Thank you for all the information Dave -- it truly is helpful your experiences in servicing and maintaining these cars. In your experience, how often do these cars need new catalytic converters? With them being NLA, do they last longer than 25 yrs+ still functioning optimally?

 

[gsxr]

Catalyst failures are pretty rare IMO, if the cats haven't been physically damaged by smacking into a speed bump, or from a muffler shop gorilla smashing on them. Another potential cause of failure is if the engine is burning a lot of oil (say, 1 quart per 500 miles)... this could potentially foul the honeycomb. Factory cats are high quality and often last the lifetime of the car. I believe Glen's high-miler (~670kmi) is still passing strict CA emissions testing with original cats. With a clean-burning engine, the factory cats never really "wear out"... IMO.

Over ~20 years and probably 20 M119's, the only bad catalysts I've encountered were failed when I bought the car. I believe both were fouled by high oil consumption, and one was physically damaged / rattling internally. I've never had a good catalyst "go bad" over time. Not saying it doesn't happen, but it's gotta be rare as rocking horse poop on an otherwise good engine and undamaged cat housing/pipe.

 

[TerryA]

@Leon_V,
Leon a long time ago I had the CEL come on and had a friend (Guido aka Italian Benz on 500 Ecstasy) at Mercedes Long Beach check it out. It turned out to be a little electronic part in the air scoop behind the driver’s side headlight. It measured the air intake I think. At the time it was a $10.00 part. It took 10 minutes to replace and the light went out.

Dave probably knows the part name I have forgotten it. BTW it won’t help your missing or stalling but it might put out the light.

 

[Blacklisted_Stallone]

@TerryA
That would be the air intake temp sensor

 

[Leon_V]

@Leon_V,
Leon a long time ago I had the CEL come on and had a friend (Guido aka Italian Benz on 500 Ecstasy) at Mercedes Long Beach check it out. It turned out to be a little electronic part in the air scoop behind the driver’s side headlight. It measured the air intake I think. At the time it was a $10.00 part. It took 10 minutes to replace and the light went out.

Dave probably knows the part name I have forgotten it. BTW it won’t help your missing or stalling but it might put out the light.

Thanks, Terry. I do notice on the drivers side the front lights; every now and then there is a drop off water in the headlight lens. And if that's the case -- no doubt moisture can seep in and potentially affect the air temp sensor or distributor caps.

 

[gsxr]

The IAT (intake air temp) sensor would typically trigger a fault code, and a CEL. IAT failures are pretty rare also... I've never had to replace one.