I install a Tmax and my mileage went down to about 26 per gal. I try everything lean out the cruse range lean out the whole map until I heard some pinging. This did not work I contact thunder max and they could not get the mpgs up finely I gave up and after 4 months I put the old Harley ECM back in I notice that I still get’s bad gas mileage about 29 per gal. Could it be passable that the ECM lost its map (which was a stage one down load) because it sat so long I kind off doubt this but you never know. Or could something else be wrong with my bike like spark plugs or spark plug wires. My 07 road king has about 8000 miles and has a stage one ac and rushes 2.0 slip ons. My bike seams to run fine it does idle ruff but I think all Harleys idle ruff

 

Putting the stock ECU back in was a good start to find the problem. Even if you lost the stage one download because the stock map runs at 14.7, you should still get good fuel mileage. You would just experience signs of running leaner. It sounds like you have another problem going on. Its one of those things you have to troubleshoot one thing at a time. Just curious when you put the stock ECU back in did you switch back to your stock narrow band sensors?

 

Sound like you have other issues besides the ECM. Did you check the tmax diagnostics or link up to comfirm the afr for front and rear? Some times an exhaust leak will fool the o2 sensors to keep adding fuel to reach the target afr. This will cause it to run rich. Don't think that would explain the stock narrow band though. Injector(s) could be faulty, as well as possibility of an intake leak could be looked at.
Ron

 

Yes I put the stock narrow band sensors back in, when the tmax was in I did check to see if the sensors were working and they seem to be working real good. I think you are right I most likely have an exhaust leak I will check both exhaust and intake to see if they are leaking.