I have a 99 Accord EX, 2 door, V6, The little flapper in the fuel filler neck is gone causing an evaporative code. Do I need to drop the tank to replace the filler neck or can I bolt it in without all that work?

 

On pretty much all the Hondas, you don't need to drop the tank. You may have to take a wheel well cover off and/or other type of cover off to expose the fill pipe then start unbolting and unclamping the fill pipe from the filler pipe gasket.

In this other thread, a guy replaced it on his Honda Pilot, similar part; but, different part number.
Honda Pilot - Honda Pilot Forums - View Single Post - Fuel Smell and CEL

For your car, it looks like it is Honda part number: 17662-S04-003

 

I looked for a procedure on All Data but couldnt find one. A guy at Autozone, for whatever their worth, told me the tank needed to be dropped? He said I could use a sawsall and cut the trunk interior sheetmetal up and get at that way?

One thing he did say is the lack of presurisation in tank could cause a reduction in gas mileage? and a bad idle? I have had an onging idle problem at normal operating temp but not when cold. I did everything from cleaning the egr port in the plenum to adjusting the valves, and the usual wires, plugs, cap rotor and bleed the cooling system stuff. After changing the O2 sensor and the ignition switch the idle probelm was gone!

My question is could the lack of pressure in the fuel tank cause and ilde issue and could it cause a "detectable" reduction in fuel economy?

Hopefully, I wont have to drop the tank; thats a huge job, you have to drop the exhaust and the subframe.

PS, These cars are known for idling issues, I wonder if this gas flapper could be root of some of these problems? I do know the new 02 sensor really made it idle better in closed loop and the wave form is much crisper.

 

Your car has a fuel tank pressure sensor near the EVAP canister under the car (before the fuel tank), the ECU use that to help determine if there was a EVAP system leak such as a loose or faulty gas cap. From my understanding EVAP system leaks should not affect the car's performance. A leak at the fuel filler pipe gasket area would be an EVAP leak. The EVAP system is to prevent gas vapors from escaping into the atmosphere from the fuel tank system. Your first post stated there was an evap code.

The reduction in fuel economy and idle may be due to something else.

In the parts diagram I attached previously, part #21 is the fuel filler pipe cover. I believe you have to remove that cover to expose the fuel pipe and you should be able to replace the fuel filler gasket without the need to drop the fuel tank.

 

I discovered that the "flapper valve" doesnt have positive pressure; it's only purpose is preventage spillage in rollovers so it may not be the root of my problem. I noticed some springs and pieces of the old gas cap in the filler pipe. I bought an aftermarket cap for $7.00 but the code came back on. I was told I cant use an aftermarket cap only an OEM Honda cap.

 

What was the actual evap trouble code?