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2000 E 320 Wagon + 2002 ML 500 SUV
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Discussion Starter #1
...and if so is a pair 2 rear (post Cat) on each side?

Or 2 fronts?

Or a front & rear on either side ?

Reason I ask is because since I replaced the Right - passenger side - post Cat sensor (after my Catalytic + sensor were stolen roadside last month), my MPG has risen about 3-4 mpg.

Naturally I'm thrilled with the better MPG but wondering if the system is unbalanced as I didn't do the Drivers side Rear post Cat sensor, also ?

Assuming I COULD have a bank of cylinders performing better than the opposite one.

Make sense ?

I was baffled but it's a legit rise I've tracked it for about 1000 miles / 3 tanks full.

It's getting 19.5 on Premium
18.5 on regular mixed Hwy driving.

Thanks for any insights y'all have.!

David in Texas

I know
 

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1999 E320, 2000 E320, 2003 E320 Wagon, 2005 C230K SS, 2010 Accord LX w Eibach & Koni FSD's
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I had to replace the right side cats on my 1999 E320 because the aft one was rattling bad. I also noticed a very significant increase in gas mileage. I also replaced the front O2 sensor while the exhaust was out as it is very difficult to reach otherwise.

I don't know if the increase was due to better flow thru the new cats or a new O2 sensor. Could be both? My city mileage went from 16-17 to 22, highway from 22 to 27. An extra 5 mpg means an extra 90 miles per fill up. That's like 4 free gallons of gas and @ 3.19 = $12.76

This is on winter gas too! So mileage should increase even more with the summer blend :)
 

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2000 E 320 Wagon + 2002 ML 500 SUV
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Discussion Starter #3
Great story.
Nice mpg bump.

So, you have 2 new cats and just 1 new front O2 Sensor.

It's hard to know best course of action. My gut says to replace the Rear O2 sensor on 'other side' so system is balanced at least.

On ML, like e320, Fronts look harder to access.

Cheers - David
 

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2003 ml500, 2004 ml350
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Great story.
Nice mpg bump.

So, you have 2 new cats and just 1 new front O2 Sensor.

It's hard to know best course of action. My gut says to replace the Rear O2 sensor on 'other side' so system is balanced at least.

On ML, like e320, Fronts look harder to access.

Cheers - David
David I was wondering this too and asked it at some point during your ordeal. Like tires, breaks, shocks, etc; are cats best replaced in pairs when of course it come to a dual exhaust/dual cats? Would one cylinder bank have more back pressure then the other.
 

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Discussion Starter #5
On the Cats, not sure.

As you know I did a "Cat Transplant" of Rear w210 (only partially clogged) Cat to Front Right on our w163 due to theft of existing one.

It was virtually the same size as original & correct shape for that tight area squished between frame rail & torsion bar.

Way better fit vs the "stock" universal MagnaFlow the exhaust shop offered me, which could never fit.

Its working great. It did take about 3-500 miles for engine pressure & heat to blow it fully clean.

I think due to the expense of Cats they tend to be replaced singly.
Versus o2 sensors that are much cheaper.

I imagine you want it all balanced.

My concern with O2 imbalance was would one bank of cylinders operate at a lower efficiency due to older, presumably less effective exhaust gas management, affecting fuel trim ?

Thanks - David
 
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