Bad fuel efficiency - 98 Eclipse?

I am reposting this question because I don't haven't gotten any tips to try yet... any help is appreciated...





I drove my 98 Eclipse from PA to GA, which is 900 miles and had to stop for re-fueling once. When I got home, I still had 1/4 tank left over. I averaged about 28-30 MPG. Now I average about 20 most times, and it's inconsistent. We've done the following:



Oil change

Air Filter Change

Spark Plug Change

New Fuel Filter

Cleaned out the EGR Valve

Checked the fuel pump/regular/tank

Changed from 87 to 93 octane

Used a bottle of STP octane booster

Used two bottles of Lucas Fuel Injector

Tires filled to 33 PSI



I do not race the car at all. I keep a moderate pace on the highway, around 70 mph (speed limit is 65). Usually on the street the speed limit is 45 and I go about 50. I very rarely go over 4,000 rpm.



I'm still at a loss... What else can we try? GA is an emissions state, so I was considering boring out the cat, but then it won't pass. Is this my only option?



Thanks for any suggestions...Bad fuel efficiency - 98 Eclipse?Go back to 87 octane gas. There was a Technical Service Bulletin stating that 93 octane gas was NOT RECOMMENDED for the 420a motor, it causes decreased fuel mileage and hard starting.



What is the state of your ignition wires. Stock please.

Air filter? Paper, not a silly one like a K%26amp;N.

Spark Plugs? If they are anything but a copper cored cheap NGK, replace them with NGK.

O2 Sensors? Replace every 125,000 miles.

Oxygenated fuel? Then you're screwed.
Bad fuel efficiency - 98 Eclipse?
Go back to the 87 octane. Higher octane does not give a noticable difference in either fuel economy or performance. Then get a fuel pressure test. The fuel pressure regulator may be bad.



Here are some Fuel Pressure Regulator Failure Symptoms %26gt;%26gt;%26gt;%26gt;

Engine runs very rich - black smoke from the exhaust

Engine starts but stalls shortly after startup

Noticeable decrease in fuel mileage

Engine stumbles or sputters

Engine will not start

Engine runs rough

Noisy fuel pump



Hope this helps and good luck
Bad fuel efficiency - 98 Eclipse?
the front o2 sensor is a biggie. Also if you put in the wrong spark plugs for this type of vehicle (aka anything platinum or multielectrode tips, etc.) you can loose lots of mpg's.