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Discussion Starter #1
Hey gang,

Well after cutting out the rear cats and welding pipe in, the mis-fires are gone and the mpg went back to normal.

However the mpg have now plummeted to dismal and I'm at a bit of loss.

I have no stored or pending codes.

I checked the cats with an IR gun and they seem fine as in way hotter in the rear than front.

I had the Autel hooked up and was lookimg at the 4 o2 sensors and the front were ranging all over which AFAIK is normal B1S2 was quite low and steady which is normal? However B2S2 was ranging all over.

Could this cause high gas consumption?

Even with no code set, is this sensor bad? What are typical o2 sensor voltages?

Thanks peeps!


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Here's my GUESS.. by cutting out the back cat's.. you've dropped the back pressure, exhaust gases are blowing by the front 2 o2's.. The front two are now upping the mix to get the fuel trim back to zero.. About 10 years ago, I had a Saturn that was playing around with the o2 sensor, trying to get it to run lean.. Made up an emulator to control the mix, got it up to 50mpg on the highway.. Now I can hear you screening, "that will burn up the valves and rings"..Well, after putting 300k on that car.. I drove it to Naples to pick up a truck that my father was giving me.. The Saturn went to junk yard and the ford truck I drove back home.. In retrospect, it should have been the Ford going to the junk yard and keeping my beloved Saturn.. I guy at the yard, was a little perplex, why I was junking my Saturn when it was running so fine..
PS, you can make up an exhaust gas analyzer.. An 02 sensor on a stick.. shove it up your pipe as you "tune" your car..
 

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If you want to run without the back cats, (1) you are going to have to restrict the exhaust, ie.. smaller pipe.. OR (2) trick the ECU with an emulator.. Or (3) reset the trims with a MB Star scanner ?.. and I'm not sure about #3 because I don't have one.. Anybody know if you can "tune" the trims with a Star?
 

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Here's my GUESS.. by cutting out the back cat's.. you've dropped the back pressure, exhaust gases are blowing by the front 2 o2's.. The front two are now upping the mix to get the fuel trim back to zero.. About 10 years ago, I had a Saturn that was playing around with the o2 sensor, trying to get it to run lean.. Made up an emulator to control the mix, got it up to 50mpg on the highway.. Now I can hear you screening, "that will burn up the valves and rings"..Well, after putting 300k on that car.. I drove it to Naples to pick up a truck that my father was giving me.. The Saturn went to junk yard and the ford truck I drove back home.. In retrospect, it should have been the Ford going to the junk yard and keeping my beloved Saturn.. I guy at the yard, was a little perplex, why I was junking my Saturn when it was running so fine..
PS, you can make up an exhaust gas analyzer.. An 02 sensor on a stick.. shove it up your pipe as you "tune" your car..
imo, I don't believe the cat delete is reducing the MLs mileage Carl is experiencing.
Read here as a recent example - https://www.benzworld.org/threads/our-ml500-exhaust-project-upgraded-muffler-2nd-cat-delete-exotic-v-8-rumble.2991819/

I have done exactly what David has done in his post. We've also done this (recently) in my daughters ML430 (cat delete) that we recently picked up for her. No lower mileage issues.
Perhaps David can comment on his mileage to-date.

It's another issue. As to what that issue is, it's up for grabs at this point. A good start would be to take some live readings while on the move.
 

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Yes, like said it's a GUESS.. But if he's running rich, and if the trims are near zero.. I would assume the O2's aren't reading the exhaust accurately What would could cause the O2s not to get a good sample?.. You say cutting out the back cat is not a problem..OK, that's not the problem.. What is a little off is that high ramp readings on #2 back O2, It's as though he hasn't got a front #2 cat either..

Carl, .. You need to put up what the short and long term trims are doing..

PS.. that's why I like an independent O2 up the pipe.. With the looping control mix.. You can't tell if it's the chicken or the egg..
 

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Yes, like said it's a GUESS.. But if he's running rich, and if the trims are near zero.. I would assume the O2's aren't reading the exhaust accurately What would could cause the O2s not to get a good sample?.. You say cutting out the back cat is not a problem..OK, that's not the problem.. What is a little off is that high ramp readings on #2 back O2, It's as though he hasn't got a front #2 cat either..

Carl, .. You need to put up what the short and long term trims are doing..
bobby, I realize it was a guess. :)
My post to is to have you both see what the (recent) w163 cat delete (and muffler swap) resulted in.

Agreed, he has to obtain live readings.

EDIT:
I do recall reading a thread (cannot find it due to the brilliant website upgrade...) where the o2 sensor up front was on the verge of failing (and not registering on a scan as failing or faulty). It was replaced and "readings and running resumed business as usual".
 

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EDIT:
I do recall reading a thread (cannot find it due to the brilliant website upgrade...) where the o2 sensor up front was on the verge of failing (and not registering on a scan as failing or faulty). It was replaced and "readings and running resumed business as usual".
[/QUOTE]

Yep,.. that might be the easiest..and the fastest way to get the O2s out of the way as the culprit..If after replacing the O2 and he still has a problem.. That's when you start chasing your tail..
 

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I have no cat issues but, was reading this and thinking why cut out the cat? Then I went to an online Benz part site and the rebuilt cats for a 500 are discontinued. So off to google mostly cut in cats, or ebay which looks like used cats ( which the local pick a part says because of federal law they can not sell, if that is BS or not I don't know but they cut them all out). So if one does not want a cut in what do you replace it with? Anyone have a source?

I found a online dealer but the ML cat they have is listed as v6 only, not a bad price on them.

 

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Discussion Starter #9
Hey Guys and thank you for chiming in!

I will scan it and get the long and short trims soon and post them up.

I like your "guess" Bobby735 and it does sound reasonable, however to many others have done this with great success. As we all know, nothing is off the table until it is fixed.

I found this video by DiagnoseDan on Youtube and am going to dig into his lesson on reading the o2 sensors as I believe they are original and may be due as suggested. For a short 14 minutes, this guy packs a lot of info in!!

Wallyo, I remember reading a post a year or so ago on here where a member used magnaflow cats. I'm not sure if they were for front or rear or both, just that they were highflow and had no negative impacts on driveablity nor did they set any codes.

Updates coming later today!!
 

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Hey guys:

I was wondering that myself:

Khomer2:

"I do recall reading a thread (cannot find it due to the brilliant website upgrade...) where the o2 sensor up front was on the verge of failing (and not registering on a scan as failing or faulty). It was replaced and "readings and running resumed business as usual".

Has anyone done a 'pre-emptive' R & R of the front 4 O2 Sensors on a m112 or m113 and found a benefit, even if no codes were seen ?

I guess we can watch that YouTube Carl posted, then scan for o2 values in live data, and that would help us avoid buying sensors "Just in Case".

FWIW, my ML500 and the ewagon, both of which have the rear cat delete & muffler upgrade are running perfect, smooth and decent MPG.

The thing is I love the sound & increased power so much, my foot is now heavier, so MPG real world is about the same. But I suppose that's an increase in efficiency as I am going MUCH more quickly for same consumption, right ?

Take Care guys - David in Texas
 

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Discussion Starter #11
Okay fellas,
Here are the trim numbers...
That is the long and short of it....sorry couldnt resist.
Seems to be okay, right?



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Does anyone who has cut the rear cat out live in a state that emissions tests?
My state does not require it, but I live in one of the few counties that does by plug in data read, not by sniffing.

Anyway any issue passing with one cat gone?
 

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Okay fellas,
Here are the trim numbers...
That is the long and short of it....sorry couldnt resist.
Seems to be okay, right?



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Sorry, I don't see where it says what bank it is on your scanner, so I'll use pix #1 (top) and pix #2 (bottom).. Pix #1 short trim negative % looks good..Long term doesn't look like it's been set yet.. Pix #2 short term, ..The ECU thinks your running lean, it's dumping more fuel to correct lean condition.. Long term looks like it hasn't set yet.. Replace O2s on #2, cross you fingers and hope 2's short term drop to a neg%.. Again, A whole lot of guessing going on here.. Good luck..

PS a perfect trim #... neg. 0.3%
 

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Discussion Starter #14
Thanks Bobby,
Okay, ill look for data that dictates which bank, im sure this scanner has it...

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Okay so the first picture shows how the data is arranged in descending order and the second is showing the live numbers and #2 hitting negative.


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Discussion Starter #17
I also found this which is short term in the same live data set WOW 99.2%!


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You want to know where all your gas is going.. 99.2% WOW!!! What in the heck is going on there?.. Your O2s must be crapping out.. Does your scanner have a graph display?.. See if your O2's are dropping out at the same time that 99% is popping up..
 

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Show the graphic dispay if you are able...

Post #17..... says it. Take it for a long drive and post these readings again.

Im hedging to swap out your sensors, unless of course, you can adjust the trims manually (which I have never done). Possibly with a Star system??? (I'm not sure...)
 

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You want to know where all your gas is going.. 99.2% WOW!!! What in the heck is going on there?.. Your O2s must be crapping out.. Does your scanner have a graph display?.. See if your O2's are dropping out at the same time that 99% is popping up..
That value is for oxygen sensor trim. 100% being perfect. 99% is just fine.
 
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