I took my 2012 Accord to the dealer for scheduled maintenance (replace transmission fluid). Afterwords, I noticed that my average MPG was lower by about 3MPG. Since MPG can vary, I ran the car for a while and recorded the MPG after each fill-up and the lower MPG values continued. There was no CEL indication and the car seemed to run fine except for the lower MPG. Thinking that I might have a fuel injector problem, I started to look under the hood. After about 5 minutes I noticed one of the metal clips on the Engine Air Cleaner (clamshell) box was hanging down and not fastened. I attempted to fasten it and it wouldn't clip to the box. Then I noticed that the box where the clip goes had an air gap and that the box was not properly secured. Honda apparently has a fail-safe design where the upper clamshell has to be properly engaged (there are tangs on the back edge of the box) in order for the two halves to proper join together and seal. If not put together the side metal clamp cannot be clipped on although the front clamp can be clipped. I removed the clips, and put in a brand new air filter and reassembled the top clamshell in the correct way so that all metal clips could be attached. I figured this would likely correct the problem with lower MPG, but it didn't.

When I discovered this problem, I went back to the paper work on the last scheduled maintenance. Part of their maintenance is a multipoint check where the tech is supposed to examine the engine air filter. Apparently the tech did not secure the upper box although he should have notice the problem since he could not attach the side clamp. At the time I corrected the problem, I had put 2700 miles on the car (since above described scheduled maintenance). After I corrected the problem, I monitored the MPGs to see if they went back to normal, but still the average was about 3 MPGs lower than what I had before the "incident." I keep accurate mileage records and loaded this data into a speadsheet so I could graph MPG over time. I can clearly see the drop in the graph beginning with the point when the maintenance was done.

Sorry for the long explanation, but this is needed for my question which is: What is the likelihood that my engine suffered damage from a compromised Engine Air Filter? I am assuming that the 1/2 inch air gap along the rear sides of the filter box allowed unfiltered air to enter the engine for a period of about 2700 miles. The car is driven on highways and not dirt roads and stored in a clean garage. Since I corrected this problem, I have driven the car for another 3500 miles and the MPG average is still 3 MPGs below what it used to be before this incident (29.6 MPG before 26.3 MPG after). I have brought this issue up with the service department of the dealership where I have had the car serviced. They think it is no big deal and that since the car has about 60,000 total miles, I should expect the MPGs to be lower. They want to charge me $150.00 to have the car checked out. I think they are responsible for this problem and they should be on the hook for analyzing the car fixing it. It does have an extended Lifetime Powertrain Warranty.

What is the likelihood that the engine has suffered damage?

What should I do to get the engine checked. Can this be done by an outside mechanic or do I have to take it to a Honda dealer for service? Mile last MPG reading was 23.4MPG and it looks like the values are trending down.

 

Having unfiltered air can ruin your engine eventually (piston rings). I don't know if you ran your car long enough with the amount of unfiltered air getting into the engine for it to damage the rings. I would presume not. I would get some cleaner like red line SL1 and through it in the tank. As your car getting older parts start to wear. Your MPGs will not stay the same due to many factors of part wear and cleaning.

 

A few week/few thousand miles will not have done any damage unless you drove through a dust storm.

 

Have a cabin in Connecticut, there has about 1% of the ambient dust of the inter-mountain west, to judge from need to clear dust after an absence, so you are totally good to go as far as engine wear is concerned. My son, in the inter-mountain west, had an unsecured air filter for ~10000 miles on a Toyota and it runs just as good as ever to my observation..