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Hi - I have purchased a used 2008 GL320 CDI from CarMax with 17K mi on the odometer. I am getting very bad mileage even with a VERY light foot on the pedal. My mileage is currently 12 city / 17 hwy. I don't expect to get factory claims of 17 city / 21 hwy but my current mileage is worse than a GL450. Anyone have any idea what would be causing this other than dirty air filter, nonaligned tires, low tire pressure all of which check out fine? I have the option of returning the car to CarMax by 5/17. Unless I can fix the mileage problem I plan on returning the car and replacing it with a GL450. The CarFax report for the car shows that the car was taken out of the country for a short period of time and I worry that there is something materially wrong with the car that my local Mercedes dealership isnt picking up on (the car passed a prepurchase inspection at the MB dealer and the dealer doesnt have a clue why mileage is so low). Thanks - David
 

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Oh yes that does not seem normal at all. With 320 CDI mileage should be eve better than the current bluetec models and yours sounds worse than gasoline. When you first start the car in the morning does your trip computer "from start" begin at a high number above 20 mpg? What do you mean taken out of the country? Where?
 

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i'm quite happy with mine

I get about upper 25 mpg. And if I take it really easy, I get about 650 miles per tank.

It always puts a smile on my face when I see half tank and already have 320 miles.

And Diesel here is cheaper than regular.

I would check your tire pressure and ask the dealer if there are any software updates or reset proceedures.
 

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Thanks. Why should CDI mileage be better than Bluetec? Yes, my odometer resets to 24.8 at start and then quickly drops as I start to drive. The CarFax report shows that the car was taken from Miami to Costa Rica for less than a year. I test drove my car against a GL450 today back to back. The GL450 average 10.3 mi/gallon in hilly city driving and my car averaged 12.9 mi/gallon on the same exact route. I then did a back to back test against a 2009 GL320 Bluetec on a different route with more highway driving. The Bluetec got 20.3 mi/gallon and the CDI got 19.7 mi/gallon. This makes me think that my particular CDI is getting slightly lower mileage than it should but that none of these three cars are getting the mileage they should get. I just don't see how anyone can get more than 21 mi/gallon highway on either a CDI or Bluetec as I test drove with a very light foot today. Any thoughts here? As an aside, the new Bluetec drove much much smoother than my CDI and had considerably less deisel rattle. Thanks - David
 

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The 2009 Bluetec and the 2007-8 CDI are the EXACT SAME engine.

All that's different is the urea injection system in the exhaust.

This could of course have required changing the "tuning" of the engine a bit, but all the mechanical components are the same and there's no reason one would be noisier than the other.

The fuel in Latin America was not likely ULSD and this car could have clogged EGR, intake manifold, etc. as a result. This is something NOT likely to covered by warranty if this is the case.

Contamination/clogging of diesel particulate filter in the exhaust system is another possibility and this could occur from bad fuel as well as incorrect motor oil. Finding correct motor oil south of the border also seems less likely. Again something MBUSA unlikely to fix for free.

I'd say let CARMAX keep this one.
 

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I would think that any clogged or plugged pipes and filters, especially in the exhaust, would cause a code in the computer. Apparently it does not or M-B would have noted it.
Another bothersome thing is your comment that the diesel noise is louder in the CDI than the Bluetec. Something wrong there, My CDI is so quiet when idling I can barely here the engine from 10' away.
I agree with Kent, If you have the opportunity to take it back, DO IT!:eek:
 

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What is crazy is that I just logged in to post that my mileage is getting worse as the car ages. We just drove a 900 mile trip and "only" got 21 mpg. We usually would post 25 to 27 mpg on the same trip. I also noticed that the 320 cdi is consuming a lot oil. The oil lamp has come on 3 times since the last service. I wonder if they are related? The car has 55K on the clock.

As for the original post, those MPGs are bad. I would not trust the computer MPG reading because it is an estimated fuel flow, not actual mpg. I would fill it up and keep track of the miles, then calculate it.
 

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Problem solved. I drove my car back to back with a friend's GL450. My car is getting at least 30% better mileage as advertised. The extreme hills around my house just skew the MPG. I also drove my car back to back with a new Bluetec and the engine noise isn't noticeably different. Thanks for all of the replies though..
 
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David Boudreau
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