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Old 11-25-2008, 07:18 AM
Registered User
Join Date: Apr 2006
Location: Richmond, VA
Posts: 36
W210 Poor Mileage After Cat Change

Hi, Folks,

The dealership in town changed out both catalytic converters on my 2002 E320 last week under warranty, (no questions asked, 79,600 miles). Since that time, my in-town fuel mileage has dropped from about 20 mpg to 10 - 11 mpg. The exhaust sounds louder under part-throttle operation. The car seems to have a harder time starting, though it runs fine after a few seconds.

I've contacted the service adviser at the dealership, but haven't heard back from him. Prior to speaking with him, I was wondering if anyone has an idea regarding the cause of the new symptoms I'm describing.

Thanks, in advance, for your help.


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Old 11-25-2008, 08:06 AM
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
See if they plugged the 02 sensors back in....
A Dalton
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Old 11-25-2008, 12:09 PM
Senior Member
Join Date: Sep 2002
Posts: 3,043
Originally Posted by shailey View Post
.....I've contacted the service adviser at the dealership, but haven't heard back from him.....
If you don't receive a return call by the end of the next business day (at the very latest), call back. If the SA won't/can't take your call, ask to speak with the Service Manager.

One of the biggest no-no's an advisor can commit is not returning a clients' call.
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Old 11-25-2008, 01:54 PM
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I'll bet they forgot the 02 sensors.
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Old 11-25-2008, 02:05 PM
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Join Date: Dec 2004
Location: Nashua, NH
Posts: 3,956
He didn't mention getting a Check Engine Light but I would assume not having the O2 sensors connected would trip the OBDII system and throw a code/warning light. Not that it matters, the dealership needs to fix whatever they did wrong and obviously they did something wrong.
Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz
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Old 11-26-2008, 09:06 PM
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Join Date: Jul 2008
Posts: 1,236
That's a very good point - I would expect you are correct, it should throw a code.

I guess we could sit here all day and guess the problem, but he really needs to just take it back to the dealer.

Maybe they just forgot to take the plastic cap off the sensor
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Old 11-27-2008, 11:01 PM
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Join Date: Jul 2008
Posts: 121
First things that come to mind...
1) Correct spark plug wire routing and making sure each one is COMPLETELY pushed onto plugs and terminals at both ends
2) Correct spark plug gaps on each plug
3) Correct park plug heat range
4) Any of the new plugs get cracked during install
5) Spark plugs installed tightly
6) Maybe a vacuum line got disconnected while working in there
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Old 11-29-2008, 10:28 AM
crashone's Avatar
1983 300SD, 4 speeed
Join Date: Jul 2006
Location: Raleighish, NC
Posts: 664
Hey!! I did not know you had to remove the spark plugs to change out the cats!! Hummmm learned something new.
If it ain't broke take it apart and find out why.

1983 300SD, 4 speed
1994 C280
1987 300TD wagon
1996 HD Road King

Ride in Peace Eric Peterson, Harley of Macon
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Old 12-10-2008, 09:31 AM
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Join Date: Apr 2006
Location: Richmond, VA
Posts: 36
Angry An update from the frustated owner......

OK, here's the latest update from the dealership:

After nearly two weeks in the shop, the service adviser says that they can find no problem with my E320. No codes, O2 sensors installed and operating, etc. I'm picking it up today. I have been instructed to run two full tanks of gas through the car, carefully recording the actual number of gallons used and the mileage driven.

You know, I have earned three technical degrees, (a BS and two MS's), from top US universities and I find the insinuation that I can't calculate fuel mileage vaguely insulting. I brought this car back to the dealership because of a fuel mileage problem that coincided with an emmisions system repair. I expect them to do more that just check for codes.

I will perform the experiment that the dealer designed to the letter keep the forum updated with the test results.

A question for the forum: If there really is twice as much fuel being injected as is stoichiometrically required, what other damage is the engine likely to incur?


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Old 12-10-2008, 10:17 AM
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Join Date: Feb 2000
Posts: 2,580
Your mileage is now 1/2 after their work. The dealer cannot(or maybe will not) resolve the problem.

Their suggestion is nothing more than a stall tactic...hoping you'll go away.

Time to contact Mercedes-Benz USA Corp. HQ.
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
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Old 12-10-2008, 10:28 AM
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Join Date: Jun 2000
Location: Plano, TX
Posts: 2,558
Not like it's difficult to calculate the fuel economy of this car - it includes both trip and long-term fuel economy numbers in the instrument display. The trip display resets anytime the car has not been driven for ~3 hours. The long-term display is manually reset using the "r" button on instrument panel, same as the trip meter.

I suggest resetting the display so you can show them without question what are the economy numbers.

- JimY
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Old 12-10-2008, 11:42 AM
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Location: Florida / N.H.
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I highly doubt that... twice the fuel would give a radically changed fuel mix ratio .. and that should pop trim codes all over the place and run like crap...
It take very little change in S to effect fuel consumption, b/c you are changing the power range. B/C you get 1/2 milage does not equate to 2x fuel being injected in relation to S.

But , you may want to ask them for adaptation numbers .
A quick and simple First check for you would be to take the vac line off the fuel pressure regulator and start the car..see if any fuel drips out that port on the regulator...that would be a very possible as they are a known fault .
There should be none and there should also be no fuel evidence of petro in that vac line. If yes , then that is the problem and the reg has to be changed ...[ simple DIYer job]

A Dalton

Last edited by Arthur Dalton; 12-10-2008 at 01:56 PM.
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