My ef sedan 1991 is getting around 15 MPG. What should I do to improve the fuel economy? Thanks in advanced

 

What trim level is your Civic? DX, LX, or EX?

If DX or LX, it could be the injector seals being made of 31 year old rubber. Even if they are not cracked, the are likely hard as a rock and shrunken. When this happens, they can allow unmetered fuel to get past them and cause a very rich condition.

Pull your plugs - how do they look?

 

yeah it is the Lx trim. I’ll look into getting the injector seals replaced. Spark plugs and spark plug wires are still pretty new

 

is it original? or are you running an engine swap?

 

Good that they are new. Still, you should pull them and see how they look. Do they all look the same? If one is sooty/wet and the others aren't, it could point to an issue with a specific cylinder. I had one of these engines lose part of the exhaust valve head on one cylinder and it ran very poorly (compression on only 3 cylinders) and got bad gas mileage until it was fixed.

 

When my speedometer cable was unhooked I got ~19mpg. I also had a fuel injector hang open causing some bad mileage. I don't think it was 15mpg bad, but still enough to notice. I couldn't figure it out until I went on a road trip, it got worse and ended up partially melting my cat. Pulling plugs will tell you a lot like @DaX said.