Mercedes Benz repair: ml320, mass air flow sensor, mass air flow

Question
Two years ago I purchase a 1998 Mercedes ML320 in mint condition with 50,000 miles. After 20,000 miles the check engine light went on and I took the vehicle to a Mercedes dealer in Doylestown, Pennsylvania. They did an engine electrical testing and diagnosis on the vehicle. The service manager stated that they traced out fault codes to determine a malfunctioning 02 sensor. They replaced the sensor located right next to the Cad-Converter with a factory part. The total bill was unbelievable $428.
Three days later the check engine light went back on and the vehicle started acting funny. When running the engine cold, first gear would stay on longer before shifting. The engine now has a deeper sound. When driving and then trying to accelerating from 35 to 50, the rpms would take off but the engine would do very little until seconds later. The gas mileage prior to service was 19 miles per gallon city, 24 highway, now it is 13.5 to 15 mile per gallon all the time.
I scheduled an appointment and took the vehicle back to the Mercedes dealer. Upon arriving at the dealership, they stated that they would have to charge me $170 to perform an engine electrical testing and diagnosis if the problem was not from the part that they replaced in my vehicle. I then decided to go a good friend‘s import repair shop.
My friend performed the same test for nothing and found “Bank” 1 & 2 were lean. He then took the vehicle out for test run and then performed a second test which determined cylinders 2, 4 & 6 were missing, but nothing on the bank 1 & 2. He determined that all 4 sensors were running at factory specs, but the cylinders were still missing. What is going on and what should we do.  

Answer
A I recall, your ML320 has four O2 sensors, personally I replace all or none, which could explain the cost.

Getting back to your current issue.

Best advice:
Replace the mass air flow sensor, spark plugs, spark plug wires and air filter.
Clear codes and test, 99% chance all issues will be gone.
Sorry, you will need ALL of these parts, no skipping or one part at a time repair.

The valve cover gaskets, serpent belt and idler pulley need careful inspection at your current miles.

Simply call or e-mail Phil your VIN# and parts list for assistance.
Toll Free 1-888-333-4642 or philip@mercedesshop.com