Not very good gas mileage, please help
 

My 2005 Civic VTEC with an automatic transmission is, for some reason not getting as good of gas mileage as it should. I have done a few things to it in part to improve mileage. It has a power steering delete, AC removed temporarily, new timing belt, minor weight reduction, almost new LRR tires filled to 40 PSI on alloy rims, all LED lighting except interior and reverse lights which will get swapped too, recently I fully rebuilt the engine, high flow header, cat and muffler, D16Y8 intake manifold and fuel injectors, custom air intake system, and a couple other minor things. It only gets around 26-28 MPG. I checked all the sensors and it has a new thermostat. It runs very well, but not very efficiently. The only things that I can think of that might be contributing is a crack in the exhaust header, the EVAP purge valve is stuck closed and it is missing the front wheel well liners, but I will install those soon. Any ideas? If you have any questions or things to check, please let me know. Thanks a lot.

UPDATE: The last 2 times I filled the tank after I installed the wheel well liners and JB Welded the crack in the header I got 31.5 and 33.7 MPG on my last 2 fill ups. The first tank was a decent amount city driving, and the second tank was more highway driving. I might be able to get the mileage even higher if I fix the EVAP purge solenoid, fix the problem with the o2 sensor intermittently going dead and possibly reconnect the EGR, although I do not want to cause more carbon buildup in the intake and cylinders by pulling in dirty exhaust, so I probably won't reconnect the EGR unless I can keep the carbon out of the intake somehow.

City driving? Highway driving? normal automatic or CVT?

You're about right for mostly city driving in a normal automatic trans.

Welcome to the forum. It's possible that rebuilding your engine is part of your problem mileage wise. If you tightened everything up in the rebuild, it may take some time to break in again, just like in a new car. Also, the cracked header may be effecting the O2 sensor readings. May not be enough to through a code but enough to reduce mileage. Just brainstorming with you. Did your mileage drop after all that you've done or is the quest for better mileage something recent?
Automatics are harder in general to hypermile due to the limited driving techniques at your disposal. I found it worse when I did a lot of in town driving. Wondering if your commute might be fighting against you.

It is a normal 4 speed automatic, and I do mostly highway driving.

Should I unplug the oxygen sensor and see if the mileage improves? If I understand correctly, the oxygen sensor just trims the air fuel ratio if there is a problem, and it theoretically effects nothing unless it detects that the engine is running rich or lean. This car used to get around 33-36 MPG, but around the time the old engine started failing, the mileage dropped, probably because of increased friction and reduced compression, but when I installed the rebuilt engine, the mileage did not improve again. I also noticed that when I let off the throttle on a flat road, the car starts to slow down fairly quickly, but that might just be me.

No, that will not help you.

The upstream O2 sensor provides data that helps the map in the ECU regulate fuel/air mix. That would be way important for your current problem. And if there is oxygen infiltration via a crack, it will throw the data off.

You have a D16Y8 manifold on a D17 engine. Did you install that before or after you started noticing a fuel economy drop?

I installed it after the fuel economy drop, and it did not seem to have any measurable effect on the already not very good gas mileage, but it did improve throttle response a lot, and best of all it is not plastic, so it won't crack.

Check for dragging brakes. When you finish a drive, put a finger on each brake disc or drum. If one is hot, that brake is dragging. If they are all hot, they are being used too much.

Maybe a bearing ?

Handbrake ?

If not something like that, an o2 sensor that is bad would drag down fuel economy. So replacing that could help.

Details about your driving patterns? How fast do you drive? Any traffic? Head winds? Do you have any fuel economy instrumentation?

Food for thought, I get about 50mpg at 80mph, and 100mpg at 55mph.