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lou-in-nj 05-29-2004 04:29 PM

HELP: Catalytic Convert Woes? '99 E430
I need an informed opinion about what a dealer's "Service Adviser" (aka
hustler) is telling me. The check engine light went on
on my 1999 E430 with 75,000 miles and they said it was
the Mass airflow sensor and they replaced it. One day
and 40 miles later I was accelerating through a turn
and the check engine light came on again and engines
shook so badly it felt like it was missing 3-4
cylinders. They now tell me my catalytic converter is
shot and the o2 sensor could not get a good reading. I
suggested that I have never heard of an O2 sensor
having such an effect on the fuel delivery system as
to completely cut fuel to a few cylinders, he then
backed away from that diagnosis and said it was back
pressure not allowing enough air/fuel in to some or
many of the cylinders. An aftermarket catalytic convert maker tells me I could possibly cut out the pre-cats and simply replace the main cats if they are even bad. I would be inclined to do this and to save $2500 if there is no conceren for a particular level of back pressure required at that point in the pipe. This vehicle has two O2 sensors per side, one in front of each pre-cat and one behind. Does anyone have an informed opinion regarding the technical viability of this kind of solution? Thanks.
P.S. I can't help but wonder if anyone wacked these units the day before with a rubber mallet ?

Kestas 05-29-2004 07:01 PM

For $2500 I'd be doing a lot more diagnostics.

When the check engine light came on, what code was set?

A bad catalytic converter would limit high end performance such as acceleration and high speed. Converters can sometimes be diagnosed by kicking it. If it rattles inside, then the ceramic monolith is broken up, possibly restricting exhaust, and needs replacement. The converter can be further diagnosed by disconnecting the exhaust between engine and converter and see if the car runs normally. Though I'm not sure what the O2 sensors would do in that situation.

You need to find better service than what your are presently getting. The quote you got was from somebody who has no idea what the problem is, but figures they can try a number of things and may eventually fix it, plus their time would be covered.

If O2 sensors are still suspect, at the very least they could be changed for considerably less than $2500 to put that matter to rest before continuing with diagnostics.

lou-in-nj 05-29-2004 07:32 PM

Converter details
Thanks Kestas,
I infer that the code set was an O2 sensor since the MB "service advisor" said it could not get a good reading that is why my Check Enige light came on. I guess the engine shaking like hell does not set a code (Ha ha. I have to laugh otherwise I would be furious with this guy whose first word to me after having the car for 6 hours were, "I have bad news". This is code for: We're going to rip you off). I have spoken to a myriad of people and most think it is entirely possible a collapsed cat could cause the engine to run poorly and prevent the car from going above 25mph. The guy who says it may be something else also said he usually can beat the dealers repair charges by 50%. OK his price is then $1250. hat is still high in my book. As far as further testing I got under the car today a wacked the exhaust with a rubber mallet and I did here some rattling. So my I am convinced it is a "pre-cat", but what do I do now? Thanks again.-Lou

stevebfl 05-29-2004 07:51 PM

Actually the catalyst could very easily be the right diagnosis. I'm not sure it would be wise to have service writers informed enough to understand the concept well enough to explain it. There probably aren't a lot of techs who do.

As a consumer it is probably easier to spot someone who can't make the story add up than to be able to understand it one's self.

Let me explain. A misfire is deduced by very quickly comparing the rotational acceleration rate of the crankshaft during its travels. It accelerates fastest after a firing and slows generally till the next firing. If now, you have a standing exhaust pulsewave bouncing back off a restrcited cat the acceleration rates are interupted and confused. The control unit misidentifies several cylinders as misfiring and then makes sure of it by turning off the fuel to those cylinders till the key is cycled.

And BTW the I would be real hesitant to put anything but a genuine MB cat on if that is the problem. The OBDII cars can tell the efficiency of the cat and the POS you can get for peanuts only works if tranporting the exhaust is the only criteria.

lou-in-nj 05-30-2004 12:21 AM

Best explaination yet!
Thanks Steve,
This explains everything. It shows you how dangerous a little knowledge (mine) is!Thankfully I knew enough to know the service adviser's story was not adding up as first told. Now all I need to know is where I can get an MB cat to replace 1 or all 4 of themk on my car. Any suggested retailers?? -LOU

stevebfl 05-30-2004 10:27 AM

Because the situation can exist does not mean it does.

Be sure to double check the emissions warrantee on your car. it is either 7/70 or 8/80. Seems I have seen both, may be a year issue.

lou-in-nj 06-04-2004 07:38 PM

Steve was 100% right.
It is unbelievable that the engine computer actually measures crankshaft acceleration after each cylinder fires. But as Steve postulated earlier, which happened to be correct: The "pre" catalytic converter broke apart and clogged the main catalytic converter. The debris was enough to create substantial back pressure and the engine's computer saw all 4 cylinders on the right bank (of my car not the Seine) as not firing because the back pressure prevented the crankshaft from accelerating after each cylinder fired. This must have caused the computer to shut down all of those right bank cylinders. I guess this is done to save working cats from raw gas being thrown down the exhaust pipe and burning up a working catalytic converter. Federal law states that malfunctioning catalytic converters are a warranty replacement item for 8 years or 80,000 miles. Anyway I have 82k on the car so I Know the part is not cheap. There are no aftermarket makers yet. In my case my 1999 E430 will not have aftermarket products available for another 3 years or so. I did allot of research and I found a few aftermarket universal converts that could possibly have been welded in place. However almost everyone in the know said I could wind-up with an equally crummy running engine because the computer is not seeing the precise changes it expects to see before and after the first converter. This time I did not have the time or will to experiment, especially since if I had to go O.E.M. it was going to cost $900+ AFTER all of my experimenting. I hope this helps someone else out.

nbml430 06-04-2004 08:28 PM

Re: Steve was 100% right.
Interesting that this came up. I recently bought an OBDII reader and in the manual, it describes this as one of the requirements of OBDII, that it monitors engine misfire by measuring the fluctuation in rotation speed of the crank. -Norm


Originally posted by lou-in-nj
It is unbelievable that the engine computer actually measures crankshaft acceleration after each cylinder fires......

///alpinepower 06-05-2004 08:04 AM


Originally posted by stevebfl
Because the situation can exist does not mean it does.

Be sure to double check the emissions warrantee on your car. it is either 7/70 or 8/80. Seems I have seen both, may be a year issue.

'emissions warranty'

the cats on ALL post-1995 cars in the US are covered by Federal Law--- if they die before 8 years/80k miles, they must be replaced under warranty. MUST BE! Turbos are also part of the emissions system; same goes for them.

The dealer will charge you if you are not aware of this.

edit: familiarize yourselves with the list on that link--- there are plenty of parts that you should not necessarily be paying for if your car is young

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