Hi guys,
Have a 1999 EX.

Basically, I'm curious as to how many miles average are on a full tank? I'm getting roughly 300 (about 150 per half tank). I'm not too sure if this is the common ground for miles on this motor? I usually do about 60-70 percent highway driving, and the rest on the streets.

Also, what should my car be idling at if everything is set? I typically idle at around 1500 RPMs.

Thanks a lot.

 

That doesn't sound to bad. The most I ever got was all highway at 355.

When warm my car idles at a little under 700rpm. Is that your warm idle or cold?

is your car stock? meaning stock wheels etc?

 

Seems about the same.
This is at warm idle. It stays almost constantly at 1500 RPM, sometimes it dips below to 1000.
My car is stock except for a AEM intake and 15 inch rota grids.

Could there be something wrong in regards to that idle? I do have a check engine light on, which leads to that idle surging problem many have going on. Cleaned the IACV out already, which solved the problem for almost a week, and once again the light came back on.

 

The CEL is for the IACV?

Remember, larger wheels, 15's will have a negative impact on MPG's.

 

I thought this was the case.

Well, I checked the CEL with a code reader and got a reading of P0505, which I looked into and saw was related to the IACV being dirty.
I take it this check engine light can hamper my MPG also?

Would that be explaining my higher idle?

 

I don't think the IACV could affect MPG.


I think it would have more effect on the idle.

 

If you are still getting an IACV code it means its faulty, not dirty. Replace it. Then set your ildle to manufacturers specs.

 

A dirty/non-functioning IACV will definitely affect your mpg... if you are idling at 1500 rpm then you are using way more fuel than you would be if your idle was at 700 or so (make sense). This would be like cruising in 4th gear at a higher rpm instead of 5th gear, the higher rpm will almost always use more fuel (the only exception is lugging an engine at low rpm, you will actually use more fuel than if you downshifted). Get that IACV functioning correctly and you will probably see a small increase in MPG.

 

Also, as posted above, one of the factors limiting your mpg is your tire size. Larger wheel/tire combo's =higher rolling resistance. Higher rolling reistance = increase in load placed upon engine, which in turn = decrease in mpg.

 

The other thing you didn't tell us is what speed/rpm you are driving down the highway at? For instance, in my civic vx if I cruise flat ground at 60 mph down the highway I have seen as high as 59 mpg. Now if I bump that up to 75-80 my mileage will go down to around 43 mpg or so... makes a huge difference! I know your not talking about a vx, and your mpg drop would be a smaller amount, but the same principle applies.

 

All city driving 31mpg

d16y7 with y8 header and y8 transmission. 170,000 miles on the engine.

 

really? it could also be a vacuum leak somewhere correct?

and I usually cruise it at 65-70 in 5th gear.

I am going to attempt to adjust my idle speed. I want to expel all my options before I go replacing the IACV.

 

I just ran from Wyoming to So. Cali and back, 1800 miles round trip and each tank varied just a touch, but trip down average 34.7 mpg, trip home 36.6 mpg, average speed 75-80mph.
1999 Civic Lx 100% stock D16Y7, auto trans, studded winter tires 222,000 miles

 

if it is surging and throwing codes, adjusting the idle probably wont fix the problem. Try it out. If you are still getting codes, check the iacv according to chiltons/haynes manuals and see if it is within spec...

 

here's this thing. How often will the idle having that surging problem?

Really I rarely if ever see it. Usually its after long periods of driving, but even then I see if for a bit, restart the car and its gone.

I cleaned the IACV, noticed the surging wasn't happening as often as it had been, and was perfect. Creeped up again a couple days ago.