'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!!!:eek:

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.

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?

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.