I have a 2003 MDX in which the mileage has dropped from 16-18 mpg to 13-14 mpg in city driving during the past 12 months.The vehicle has 55,000 miles on it. The air filter has been recently replaced as have the plugs (they were totally thrashed with the electrodes almost gone) with no subsequent improvement in mileage. The car does idle slightly roughly. I normally run 89 octane midgrade but have switched to 91 premium but have seen no change in mileage. In discussions with my local Acura dealer they claim that there is nothing that they can do to trouble-shoot this issue unless there is an error code on the engine contol module. I haven't had any error codes.

My check list to solve the problem is as follows:

1. replace oxygen sensors
2. replace EGR valve
3. check valve clearances and adjust if necessary
4. replace catalytic muffler on the assumption that it is plugged up

Does anyone have any suggestions for how to diagnose this problem without doing all these items and spending a boatload of money?

Any references to repairshops in the Orange County area of CA that have experience in diagnosing this problem?