Hi everybody,

How do you know if your catalytic converters are bad?

I understand that the core can melt together and block exhaust flow. I also understand that the reactive ingredient can be depleated.

It was suggested that my cats may be clogged, causing my engine light.
I removed the entire exhaust system.
I shook and tapped on the cats to see if any loose pieces would fall out; none.
I took an electric lawn blower and back-flushed the pipes; PLENTY of flow, certainly not clogged.
I looked down into the pipes; the "honeycomb" appears to be very clean, and in great condition.

Any other test I can perform?

Does the "downstream 02 sensor" monitor and report the condition of the cat?

I don't know, they look good to me...Why replace them if they're not clogged?

Charles

 

The two main problems with cats are:
1. Poisoning of the reactive agent (wrong fuel) or
2. Mechanical damage to the honeycomb (numerous causes).

Do you need to change them?
Only if you have a problem which may be caused by the cats. e.g. Failed emmissions test, poor performance, CEL (Check engine light) on due to inability of system to control within limits (which may be due to inefficent cats), Rattle (especially when cold) etc..

Since you do not appear to have a problem and the cats look fine and are rattle free, why do you want to do anything further?

Just put it back together, get a beer, chill out and await the time when you have a real problem.

NB. Cats for your MB are mighty expensive. Once you find out how much you will certainly not wish to change them unless you HAVE to in order to keep the car on the road.

Good luck.
Stuart

 

Hi Stuart,

Thanks for responding. But my check engine light does stay on.

Failed emmisions test
Trouble codes P0170, P0173,

My shop says I need new cats.
$2000 to install, which is fine, but I wanted a second opinion at least.
They are clearly NOT clogged, and do NOT rattle, So I'm somewhat skeptical.

1) Can they flow, have NO rattle, and STILL be bad?

2) Is there a way to check the signal on aftercat 02 sensors to determine if this IS the problem through testing.

Quite frankly, myself and the shop have been trying to get this light to go off for nearly a year now.
Several fuel components were replaced because the test results were close to being out of spec.
There was a small exhaust leak, a tiny drop in vacuum caused by a leaky diaphram control, the plastic intake duct had a cracked mount, and a couple of other marginal controls and filters. So everything is reading right in the middle now, but the engine light will not stay off after reset.

3) sound like cats?

Charles

 

cat can get damaged two ways.

extreme temp due to rich mixture or oil consumption will melt it and fuse together. this results to lack of power because the flow is restricted.

other problem is it breaks apart horizontally. in this one, it will either make clanking noise inside or spitted out on the exhaust which can also block the muffler.

in both cases, you can visually see the catalytic converter or catalyst inside the pipe.

if it is in good honeybee comb arrangement, then it's fine. color is light khaki to grayish.

 

Thanks guys.

I found something on my repairCD. It says that the afterKAT O2 sensor has to show a 25% minimun drop of 02 from the reading of the preKAT 02 sensor. Now this makes sense finally.
So if the KATs are used-up the alarm will go stay on. So I can measure the outputs signals and get an idea if they are actually absorbing oxygen. Right?

Oh, I don't mind buying them whatever, I just don't need an extra set of pipes laying around here if it doesn't fix the problem.

Charles

 

Hi Charles.
Changing the cat is UNLIKELY to be the solution.

You state the CEL is on and you have P0170, P0173 etc. codes. Exactly which codes since this definition will assist in determining what is wrong. The fault generates a code which in turn causes the light to come on dependent upon the code definition.

P0170 is Fuel Trim Bank 1.
P0173 is Fuel Trim Bank 2.

These codes give the signature of a faulty Mass Air Flow meter (MAF).

When the ignition is turned on the engine computer carries out a series of tests similar to a PC booting. One of these tests does not give the correct reading and this is why the CEL is coming on even without starting the engine after a code reset.
Fuel trim is the amount of correction required by the ECU to maintain correct output (closed loop) from the engine management system. The output from the O2 sensors causes the ECU to makes adjustments to the duration of the fuelling every fraction of a second (referred to as Short Term Fuel Trim). These corrections are averaged over a set period of time to give the Long Term Fuel Trim. The STFT and LTFT can be read from the ECU memory with an appropriate scanner. Your LTFT is outside of limit. Not sure exactly what the limit is (about 30) but if all is in good order should be in single figures (eg 6)

Note that the codes say the problem is on both banks therefore the fault needs to effect both banks. Therefore unlikely to be anything cylinder specific. Plug, injector, valves, air leak etc. The MAF is suspect No. 1

Have you already changed the MAF? If so was it a new OE one? If not how do you know it was good? The resistance of the MAF changes over time/mileage and as it does it calls for a compensation by impacting the STFT = LTFT.

The next input to the Fuel trim (FT) is the O2 sensor (faulty signal value) but why would the O2 sensors on both banks fail. A good scanner will also enable you to acess live O2 sensor data from the ECU. You need to ensure that the output values are what you would expect with lower values from the rear most sensors and that cycling time etc. looks OK. ie regular and many times a sec.

I believe you cats are OK unless you have run on leaded fuel. Even VERY small amounts of lead can kill the operation of the cat. Could you have filled the car with leaded fuel for the mower ever?

regards, Stuart

 

Hi Stuart and Sleepwalker,


Scanner:
I have the little code reader, and I'm shopping for my own good scanner. What scanner would you recommend for me? My info came from the shop.

Fuel:
Car has only had premium fuel. Eberything runs on premium, even the tractors.

MAF Sensor:
The MAF was NOT replaced because it passed these tests.
a)signal: 0.9-1.1V with ingition on, result 1.1
b)signal: 1.3-1.7 at idle result 1.4
c)800-4000 RPMs: >1.3 result 1.4-2.8
d)supply 4.7-5.2 Volt result 5.0
e)5volt to ground result 5.0
f)supply 11.-14 Volt result 12.0

I have NO problem buying a new one tomorrow.
I always use authentic MB parts, sealed in a MB box.
If it isn't the problem, then I'll use it for a test part for the future, so no harm done.
At least it will narrow down the problem solving by replacing No1 suspect.

Q= Can I simply install the new MAF and connect the plug, or does it have to be calibrated/etc. at the dealer?


Charles

 

just replace it.

O2 sensor values are fine.

 

I'll have it in the morning.
Wow, it would be great to have it running properly again.

I'll not work on it today though. We had a storm last night; the sky is cleared this morning and the animals are making so much noise in the trees I can't hear myself sneeze. Even though the cars have an air-conditioned garage I'll have to check them for snakes before I open the lid.

Uh, I'll explain.
Last year a 4metre snake refused to leave the 190E. Everytime we pulled him out, he just sifted right back to the car. I guess he likes Mercedes too. It took two men to drag him out; one had to hold the head. The snake got angry and out of control, and we had to shoot him in the head four times. It was sad, because snakes are generally friendly here, as long as they are smaller than you, and help control the small animal population, but the situation is considered bad savvy. An old woman believed that this act would result in bad things, and made a charm which I thought was superstitious, but later rodents actually chewed into the car and did approximately $4000 of damage to the seats, wires, and rubber hoses in about 30 days. Animal damage and tribal charms aren't exactly coved in the Mercedes ISO work instructions, but it happens all the time here. LOL

Anyway, thanks for eberything, have a great weekend, and I'll report how the MAF sensor works.

Charles

 

Hi Charles,

Just plug in the new MAF. Ensure it is the correct one. Different engines use different MAFS even though they may look like one another.

It would be nice to know what the LTFT value is. A good scanner will give this and avoid unnecessary expenditure. A good scanner will also provide flash memory data = the conditions prevailing that caused the light to come on.

No need to clear the CEL light with a reset since the fault appears to be one that is detected at startup. If there isn't a fault it actually resets itself.

However, as a matter I usually do this since should my diagnosis be wrong you at least know that the fault is repeatable.

Where are you based. I'm not very partial to snakes!!

Good luck,

Stuart