3.0i excessive fuel use at idle or obsessive driver?
 

Is my motor sucking down an abnormal amount of gas at idle? I have no codes, a relatively new ICV ( ~ 20k miles) and my highway mileage is about 20-22 @ 60-70mph over a full tank. Everything else seems to be operating within expectations.

I track my mileage via odometer vs. gas pump dispensed volume. For some time, I have noticed that if I allow my 3.0i to idle for any period of time north of 30 seconds, the MPG data on the on board computer starts dropping. I can lose 2/10 of a MPG idling for 45 seconds with no AC.

Normally, I wouldn't trust the OBC, but my calculations consistently support decreases it reports due to idling. If I average 20mpg through most of a tank (per computer) and then idle for a while and drop to 18, odometer calculation will bear out most of that drop (I believe my computer normally over-reports about ~1.0mpg to begin with).

Is this normal slop in the OBC mileage reporting/calculation or am I actually losing a lot of gas at idle?

the longer the car idles then the engine gets hot and injects more fuel to help cool it

So I am not crazy.

If this is true, then is the idle fueling table referenced to the IAT sensor? Is that table editable? None of my other cars have idle fueling tables referenced to anything. They just idle.

Or can this be combated with a lower temp thermostat, reprogramming the aux fan to come on sooner, or some combination thereof? Perhaps I am just doomed to have idling drive me berserk?

why would you ever idle?
any idling more than 7 seconds and you might as well shut off and restart. maybe that is why we get a reports 24.5mpg....

and this is with the oem roof racks on all the time and about 35% miles with cargo box on it.

That's not really germane to my question, and barely a complete thought.

From someone that has probably not spent a Summer in Houston! No way would I sit in my car without the a/c running at the moment; in fact we often start it for a couple of minutes before we get in it.

Which in turn makes me think that the a/c compressor switching on and off would also make a difference to the fuel economy at idle. (Either way fuel consumption is 0mpg at idle, with or without a loaded roof rack. ;))

I need to look at how the mpg is calculated, in one of my old Toyotas (back in the U.K.) it was filtered over a set time or distance, so you could sit and watch the fuel consumption go up whilst idling. Have not read my manual much yet.

i apologize in advance for helping with your ignorance, but my comments are very germane (thank you for the word of the day).

you asked if you are losing alot of gas at idle. I took this to mean that idling was decreasing your MPG. the answer is clearly yes, and I offered an easy way to fix that. Your harder than necessary acceleration and inconsistent driving habits also are relevant and decrease the MPG.

If you didn't want objective answers and opinions, why did you post?

We could all agree with you and stroke your ego, but then you wouldn't learn as much, would you?

Back on the original track:

Your reported mileage is averaged out over a period of time, as others have said. I take your use of the work idling to mean vehicle stopped with the engine idling, and not just idling while moving slowly in heavy traffic. Obviously, your mpg when idling and the vehicle stopped is zero, so you are just seeing a mathematical average. If you don't reset the average mileage for 10,000 km or so, then it will not change when you are stopped for two minutes. If you reset the average mileage with every tank, then it will obviously move much faster because you are working against a smaller number of accumulated miles in the calculation.

All of this discussion of engine temperature, idle valves, idle fuel tables, etc, is rather obsessive-compulsive, in my opinion. Here is a simple test. When you idle, then accelerate, do you get a cloud of black smoke from overfueling? If not, then why would you think there is something wrong with your fuel consumption at idle? If the engine idle speed is fine (fuel quantity), and it isn't too rich (fuel ratio), why is there a suggestion that there is too much fuel? I don't understand where it would go.

Backhanded apologies and wiseass invective concerning my driving habits and motivations, about which I have not posted and you have no grasp of, are neither helpful nor appreciated. I asked a question about fuel consumption of my engine at idle, by itself. Why I idle, how I accelerate or drive in general and your opinions on those things have zero bearing on my question. Seriously; reread my post and ask yourself if you added anything of value.

That is what I am trying to see. I reset it every tank, because a drop in mileage would indicate mechanical problems, as you know. The problem is I could have 450 miles on a tank and the computer says I average 21 mpg, and 45 seconds of waiting at a highway interchange and it will go down to 20 or worse. I just find it difficult to believe that short amount of time can weigh so heavily on mpg of a whole tank. But I suppose it is possible.

I like to understand how things work.

My idle is not the smoothest, and sometimes stumbles even without the AC on, which either means it is getting too much gas, or not enough air. I could have a leaky injector, which would probably only manifest at idle. Your test would probably catch that. But if it is leaking it is not egregious, as there is no smoke.

I was thinking that to protect the engine, the factory sets the idle fuel table somewhat rich. If everyone's car did what mine did, that would confirm to me such a situation, unless it was tuned back and then would not idle at all..in which case the I6 would just be a massively inefficient idler, and that would be the end.

If I could tune out some fuel from the idle table and still have it idle, I could save some gas and live happily ever after. OCD, perhaps. But if I could cut out half of the gas the car appears to waste at idle, I bet I could power my other car for 6 months for nothing.

In conclusion, I was trying to determine if my idle fuel consumption was A) a malfunction or B) the status quo.

Bloody well right he's never lived in Houston.