Low MPG, looking to improve, help?

[TwoGlassEyes]

Brand new to the forum, hi everyone.

Here's my problem, followed by what I've tried so far:

I'm getting 37 mpg in town, 34 or less on the highway in my 1994 Civic VX. In town, the mpg stay the same whether I run AC or not (and I live in Southern Arizona so the AC is on a lot).

Starting from a stop always seems a tad rough; very easy to stall if you've never driven it before (I've seen a couple of people do it who should know better, if that makes any sense).

Especially when the AC is on, the car gets a little jerky (sort of like it might stall) when I try to maintain speed, especially at higher speeds. I feel like I have to be very slowly accelerating all the time to avoid this, then let the clutch out to drop speed before repeating the cycle. The ride is still smooth; I only notice because I drive it all the time and can feel it.

I replaced the O2 sensor about a year ago to pass emissions, but did not see an improvement in FE (though it made a dramatic change to my emissions).

I replaced the spark plugs, spark plug wires, cap and rotor; so far, no noticeable difference in FE.

The tires are NOT what I want, but I can't convince my wife to let me swap them out since they're still in good working order. They're 175/70/13; I know it's supposed to be 165/70/13. I'm sure this contributes to the low FE, I just don't know how much.

Also, I don't know when the car goes to lean burn mode, if at all; I'm not sure what indicators I should look for to see if this is happening, etc. This is my first Honda, my first VX, I've had it for about 2 years now and haven't been able to look into this issue when I've wanted to (my wife's Corolla takes up most of my limited garage time).

Thanks for reading. Any suggestions are welcome.

[theholycow]

The tires probably aren't making a huge difference. Are you calculating the odometer error into your fuel economy numbers?

[bowtieguy]

no codes i presume? how's your idle? should be well below 1k w/ a/c off.

could be a number of things. try cleaning your IACV, and throttle body as well as replacing your PCV valve as a cheap start. maybe change the fuel filter and add a quality fuel system cleaner.

are the tune-up parts all quality parts?

[TwoGlassEyes]

Originally Posted by theholycow

The tires probably aren't making a huge difference. Are you calculating the odometer error into your fuel economy numbers?

No; is there a math formula or an online calculator to figure that out?

[TwoGlassEyes]

Originally Posted by bowtieguy

no codes i presume? how's your idle? should be well below 1k w/ a/c off.

could be a number of things. try cleaning your IACV, and throttle body as well as replacing your PCV valve as a cheap start. maybe change the fuel filter and add a quality fuel system cleaner.

are the tune-up parts all quality parts?

No codes. Idle seems fine, it's usually below 1k with AC off but I haven't really taken notice recently.

Thanks for the tips on IACV, throttle body and PCV valve, I'll look into that. I did the fuel filter recently and have used fuel system cleaners several times though not recently. The tune-up parts were quality--NGK wires, Bosch cap and rotor. On that note, when I replaced those, I blew my coil--I guess the increased resistance from the new parts were too much for it.

[theholycow]

Originally Posted by TwoGlassEyes

No; is there a math formula or an online calculator to figure that out?

Yes. There is a tire size calculator linked in my sig, and more of them in my meta-sig. I just went to one and fed in your numbers. The result: 2.5%. Add 2.5% to your past MPG numbers to find out your real MPG, and in the future add 2.5% to your distance traveled before you divide by gallons used. The quick way is to multiply by 1.025 (rather than multiplying by .025 then adding it back in).

It doesn't sound like much but I have a 3% error myself and it does make a noticeable difference.

[TwoGlassEyes]

Thanks, I'll remember to use that from now on.

[kamesama980]

check the cam timing. If it's off you can get a nice bump to low end torque/economy and kill mid/high real easy.

[hondaguy72]

Which spark plugs are you running? Anything other than the stock plug will hurt FE.