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Old 07-25-2007, 12:41 PM
Mike Miller's Avatar
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Location: Huntington Beach, CA
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'92 R129 500 SL, Check Engine Light and Gas Mileage

Maybe I am loosing it, but a strange thing happened to me last weekend. I drove to San Diego and got about 17-18 mpg for the 100 mile trip at 75-80 mph. While in San Diego, my Check Engine light came on. I figured I would drive back to Huntington Beach and have my indy check it out. Sunday I filled up in SD and checked the starting mileage. At about 75 miles on the return I began to think something was wrong with my gas gauge. When I got to HB I topped off the tank, to be sure of what I saw. Low and behold I got 25 mpg coming back at a steady 75-80 mph speed!!!

I am ready to reset the light and keep driving to see if it repeats!

Have any of you experienced similar changes?? Can any one of our technical guys explain that big a jump? I appears the car was running very lean, BUT....The car ran perfectly, no hestitation or cooling issues.
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'92 MB 500 SL
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Old 07-27-2007, 01:03 PM
Mike Miller's Avatar
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OK, let me rephrase this to see if anyone wants to comment. Forget the MPG differences.

What would cause A) a Check Engine Light, and B) the engine to run lean?
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'92 MB 500 SL
'01 BMW 525i Sport
'98 Harley Davidson RoadKing
'74 BMW 3.0 CSi - Gone to a good home
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  #3  
Old 07-27-2007, 07:56 PM
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I got around 28 MPG driving through New Mexico at a steady 80 MPH a few years ago. That car was a 300SL with manual transmission.

A check engine light would be caused by a fault in the fuel injection controller. Codes for this are easily read with an LED from Radio Shack.

A lean mixture could be caused by a mass air flow sensor badly adjusted. See Steve Brotherton's article: http://www.peachparts.com/Wikka/EngineControls.

Note that a lean-running engine will generally get worse fuel economy than one that is running correctly when driving on the highway.
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