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.

Steve

See if they plugged the 02 sensors back in....

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.

I'll bet they forgot the 02 sensors.

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.

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 :rolleyes:

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

Hey!! I did not know you had to remove the spark plugs to change out the cats!! Hummmm learned something new.;)

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?

FYI,

Steve

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.

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

<If there really is twice as much fuel being injected as is stoichiometrically required>

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]