1999 430 ML with 66K. Is it likely that an air leak from a fuel tank cap could cause the CEL to go on and the fault not be picked up by the computer codes. Would it be smart to replace the cap if the CEL goes on every two or three days after I disconnect the battery to reset the computer? It cost me $165 for Mercedes to tell me that the codes say I have an air or vaccuum leak but they are not sure where. Crank shaft sensor and air mass sensor changed last month. CEL still goes on.

You could change the cap, but that may or may not solve the problem.

It sounds as though an MB dealership told you that you have a problem, but cannot locate it. If that's the case, go find a good independent in your area. There is a site at mercedesshop where you can scan for comments made about shops in your region.

If you took the car to an independent repair shop, take it to a dealer or perhaps a much better independent shop.

Bottom line - the shop you dealt with shouldn't be fixing cars.

first you should check your warranty booklet for emission related coverage.
remember emission related items I think for 1999 are eight years or eighty-thouand miles. First you have to determine if it is emission related.

I would also contact the Mercedes regional representative about the $165 diagnostic bill. Did they tell you further diagnosis would cost more?

Because that is a damn scam to charge that kind of money to tell you they cannot find the problem.

Your cap is probably too new to have deteriorated enough to set the codes off...and if the cap was at fault, it would have done so immediately upon driving (after battery disconnect). The fuel system is pressurized, so the computer would have caught that early.

As far as the diagnostic charge, it's on par with what MB dealerships charge for the same service down here...but it's waived if service is performed.

Too expensive though for not turning up a solution...I suggest an indie..mine owns a 99 ML430, so I don't have a problem letting him service my ML...

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car