Not very good gas mileage, please help

[EcoCivic]

Originally Posted by Ecky

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.

I mainly drive to and from work, which is almost all highway driving. The last tank I went on a road trip and averaged around 70 MPH, in some heavy rain if that matters. Not much traffic, but when I am stopped for a short period of time, I shift to neutral. I shut the engine when sitting for a long time. Not sure about headwinds. I have a Scangauge 2 for fuel economy instrumentation and monitoring temp, voltage etc.

[California98Civic]

Filters too. Check fuel and air filters for clogging. ...but I like the O2 sensor for air infiltration from that crack screwing with the ECU.

Maintain and repair the most obvious first... then move on to less obvious... you'll find it.

[ar5boosted]

oh, biggest "simple" killer of fuel efficiency is a inlet manifold leak, possibly around the injectors or throttle-body somewhere. This would explain everything. Spray with hydrocarbon-spray till you find a vacuum-leak.

[EcoCivic]

Originally Posted by California98Civic

Filters too. Check fuel and air filters for clogging. ...but I like the O2 sensor for air infiltration from that crack screwing with the ECU.

Maintain and repair the most obvious first... then move on to less obvious... you'll find it.

The air filter is clean, and the fuel filter is in the tank and is supposedly not a maintenance item, but I changed it at around 200K miles, and cold starting and throttle response improved, and I think mileage improved a little. I think you are onto something with the crack in the header pulling air into the exhaust, o2 sensor sees the exhaust as lean, computer adds more fuel to compensate, engine runs rich. Good suggestions.

[EcoCivic]

Originally Posted by ar5boosted

oh, biggest "simple" killer of fuel efficiency is a inlet manifold leak, possibly around the injectors or throttle-body somewhere. This would explain everything. Spray with hydrocarbon-spray till you find a vacuum-leak.

I checked for leaks with starting fluid about 20 times, I did not find anything. A leak would definitely kill gas mileage though.

[California98Civic]

When you get it figured out, come back and tell us what did it. We'll be interested. Better yet, become a regular on the forum. You are welcome here, of course. You seem to know your car really well, too. I had no idea they moved the fuel filter into the tank after the 6th generation. That just makes things unnecessarily hard to check.

[EcoCivic]

Originally Posted by California98Civic

When you get it figured out, come back and tell us what did it. We'll be interested. Better yet, become a regular on the forum. You are welcome here, of course. You seem to know your car really well, too. I had no idea they moved the fuel filter into the tank after the 6th generation. That just makes things unnecessarily hard to check.

Yea, I know my baby pretty well, after all I put 2 engines in it. I will definitely update this post when I figure out what is wrong. I agree with you, it is stupid to put the fuel filter inside the tank. No matter what they say, it will clog over time. The filter is actually built into the top of the fuel pump housing, and even an aftermarket one was $40 on Amazon. And it is a real PITA to change. I actually thought about somehow removing the filter from the housing and just installing an inline one under the car, but they seem to make it impossible to remove the filter from the housing. If anyone figures out a way to get the filter out of the fuel pump housing, please let me know.