DumpdEJ6

So here's the deal. I have a 1998 Civic EX Autotragic that I use as my daily. The pertinent points:

50/50 Highway/City driving ---24 MPGs
Never at WOT, I drive normally.
A/C on medium/low most of the time

Things I've changed:

NGK Spark plugs
NGK Plug wires
New cap and rotor
Air filter
Fuel filter
Seafoam treatment
Adjusted timing to 15 BTDC
Checked for exhaust leaks----none
New Delphi upstream o2 sensor (mostly out of desperation, the voltage readings from the old one were ranging within spec. Interestingly enough though, I DID get 4 MPG more with the new sensor)
Alignment
Compression tested around 180 across the board
Checked for vacuum leaks, none to be found

I used to get a code for Fuel Trim Bank 1 Too Lean, but it comes and goes, which leads me to believe that its right on the cusp of throwing the code most of the time. It hasn't come on probably 2-300 miles.

Anyone have any idea why I'm not getting closer to 30MPG like I should be? The only thing I can think of is my A/C usage...but can it really make a 6-7 MPG difference? And what's up with the Too Lean code?

NotARaCist

iTrader: (1)

Yes, using your AC can make a 6-7 MPG difference with an older compressor, and it doesn't matter how hard you have it blowing. If it's on, it's on, period. You said seafoam treatment, but exactly which areas did you treat? The proper "full" way is third bottle in fresh tank of gas, third bottle in oil just before oil change, and third bottle pulled in through a vacuum line. Did you do all three? How many miles are on the motor? Did you replace the fuel filter with an OEM unit? Have you tested your fuel pressure? When was your PCV last replaced? Have you tested your MAP sensor? Is the intake stock?

rovafa2002

Check your thermostat, you could be impressed on how a stucked open thermostat would hurt gas mileage. Also you might need an ultrasonic fuel injectors cleaning. Other things that will help...
- Cleaning IACV
- Clean your IAT sensor
- Cleaning your MAP sensor
- Cleaning EGR (i dont know if the US version has one or not)

DumpdEJ6

It hasn't been due for an oil change yet so I have only done the 1/3 in the manifold and when I changed the fuel filter I filled it up with seafoam instead of gas and ran it through the injectors.

About 170k on the motor.

Fuel filter was not an OEM unit but I believe a Fram filter.

Have not tested fuel pressure.

PCV age is unknown, that is connected to the vacuum line running down through the intake runners and to the back of the block, correct? I might take that out and run some brake cleaner through it while shaking it. A thorough cleaning of it would render it back to stock specs, right?

Haven't tested the MAP sensor because I didn't get a code for it, what is involved?

And yes, the intake is stock, the entire car is stock. Probably should have mentioned that.

DumpdEJ6

The engine warms up quickly at cold start-up, I'm reasonably sure the thermostat is functioning correctly.

I haven't cleaned any of those...brake cleaner and a shop rag will clean most of those, correct? It does idle perfectly right now, right around 750 RPM. No erratic or "hunting" idle.

oneheadlight

no... brake cleaner is too strong to use for those sensors.. go get some electronic cleaner or MAF sensor cleaner (I personally like the CRC brand).

DumpdEJ6

Ah, ok. I do have a can of the CRC MAF cleaner in the trunk that I used on a customers car. I didn't think the MAP sensor was as sensitive as the resistors in a MAF. I'll do that tomorrow, as well as clean the other things NAR mentioned.

NotARaCist

iTrader: (1)

You can't really clean a PCV. Just replace it - they're less than $5, and should be replaced once a year (and no one ever does). Test your fuel pressure, and you can try cleaning the MAP, but the sensitive bits are the electronics. Testing it involves a multimeter. If you have one, I can pull up the FSM page for you. You should also test your fuel pressure, to rule out pump and FPR issues. You'll need a certain type of fuel pressure gauge, which you can buy for fairly cheap, or usually rent from your local parts store.

Just because a code isn't being thrown doesn't mean that a piece isn't failing - it just means that it hasn't totally failed yet.