Low MPGs, no obvious reason.

[darcane]

Since buying my Civic, I've only had five tanks that averaged less than 40mpg. Four have been in the last few months, and I'm not quite sure why.



I'm getting no engine error codes. I have noticed that the engine idle varies a bit, sometimes it's at it's normal 600rpm, other times it ranges up to about 1200rpm but otherwise the car drives normally. It may be a little down on power, but with a light foot and low rpms, it's hard to tell.

Most, if not all, of the decrease is during city driving and stop-n-go. This morning, the freeway was pretty clear and I managed to get better than I ever have before on this stretch, 65.8mpg:

I suspect part of this is due to the low rolling resistance tires that I put on in November.

I can't claim the decrease is due to cold weather, since we have had an inordinately warm winter.

Also, last summer I had the head rebuilt, so there shouldn't be any problems with valves, headgasket, EGR, etc. I recently double checked the valve lash and it was within specs.

Any advice on what to check would be appreciated. I have a suspicion, but want to get other opinions before I buy expensive parts...

[2000mc]

do you know what your fuel trims are running?
When you fillup, does your gauge agree with the actual fill as well as it did previously?

[user removed]

02 sensor getting tired?

regards
mech

[darcane]

Originally Posted by 2000mc

do you know what your fuel trims are running?
When you fillup, does your gauge agree with the actual fill as well as it did previously?

Fuel trim isn't one of the gauges I have been displaying, but I can shuffle them around to show it.

Pump and gauge match up pretty close, since the gauge doesn't understand lean burn, it's always a little off but not much. My reported economy in the Garage is strictly off fuel used according to the pump versus odometer reading from the dash.

[darcane]

Originally Posted by Old Mechanic

02 sensor getting tired?

regards
mech

Primary O2 sensor is less than 2 years old. Should be good.
http://ecomodder.com/forum/showthrea...ust-25576.html

[Baltothewolf]

Kinda the reason I sold my HX. Thinking back it was probably the compression that was the problem, I was running ~150-165 on all cylinders. Anyway, I couldn't break 40mpg consistently, thus the move to the insight. 60mpg at 70-75mph isn't bad.

[ksa8907]

You're not the only one, ive lost roughly 25% fe with winter... but i dont suppose it gets quite as cold where you live.

I assume you've checked the obvious dragging brakes, alignment, etc. any chance the timing belt/chain was installed a tooth off? Or perhaps is stretched?

You could be getting poor ignition from a bad coil. My car isn't doing so hot in that department, 1 or 2 coils are intermittently not firing properly but its not setting codes. Still drives ok but slight loss in power and sounds like octane knock around town. Too busy and cold to fix it right now.

The irregular idle could be a dirty throttle body or idle air controller.

[spacemanspif]

I had a wandering idle in my Saturn but it was "searching" any time it was idling in closed loop, turned out to be the fuel pressure regulator was leaking gas into the vacuum line. If your gas pedal feels a little sticky your TB could be dirty, which could also mean your IAC could need cleaning like KSA said.

Could be a lot of things, what is status of tune up stuff? Plugs, fuel filter, etc.

[2000mc]

do you have a vacuum modulated fuel pressure regulator? leaking regulator would explain poor actual mileage, while electronics report best ever

[2000mc]

jinx