Accurate way to measure idling fuel consumption
 

Because the amounts are so small, I want to be sure and measure the fuel consumption of my car while idling accurately. How much of a run do you guys think I need on my SG2 to be accurate? Any tips on testing procedure would be useful too. Thanks!

You only need to run your car until it's warmed up to get a baseline idle fuel consumption. Five minutes is probably enough, but you can go to ten to be sure the car is warmed up. My Nissan drinks .22 gph, but my Scion only drinks .15-.17 gph. When they're cold, .35-.5 gph isn't unusual.

If I'm getting 60 mpg at 30 mph avg, I'd use a total of .5 gallon to go 30 miles in an hour. In a typical low speed scenario like that, I'm pulsing 1/3 of the time, and FASing the other 2/3. But if I were to coast NICE-on, I'd add another .2 gallons X 2/3 hour, or .133 gallons to my fuel consumption. That would bring my 60 mpg down to 30 miles/.633 gal, or 47.4 mpg. That's a significant cut in mileage, and is one of the reasons my wife's AT car only gets 40 mpg on the same country road route that I can get 60 mpg on with either of my MT autos.

My initial estimate is around .19 gph, but I'm not sure how accurate that is (pulled it from my coast down data). I'll rerun things the next time I'm out and the engine is warmed up anyway. Probably not critical for my P&G calcs anyway, but it'd be good to know.

Wrong. For 5 mph P&G ranges it matters a lot. I'll rerun my program after I get the idle info.

Much easier to do while not driving! I used my stopwatch to time how long my SG2 took to get to .03 gallons. Sitting there wasting fuel was hard! Took 530 seconds, so that's 0.20 gph, which is pretty good agreement with data from my coast down.

You might not be aware there's a gph function on the SGII, so you didn't have to monitor current trip fuel consumption and calculate it over time.

Yeah, I noticed that yesterday afternoon when trying to get IAT to show up. Didn't know it was in there. :o It shows either .21 or .22 when coasting. Slightly different than the idling calculations, and since they're both reliant on the SG2, I wasn't positive which to go with. I just called it .21 and ran my program.

Time for me to re-read my SG2 manual.

a bit OT, but i found this on another thread... https://www.milesgallon.com/calculate...t_per_mile.php

i put $1 for selling price because it wouldn't take zero and i put miles on the car when i got it for miles when new. couldn't understand that one. oh, and i put 10 years for ownership.

anyway, it says i get 14 cents/mile. but it does not account for maintanence. even @ similar miles driven and similar vehicles, there must be a huge variance in cost regarding maintaining a vehicle.

I use the Edmunds TCO numbers and then divide by the mileage to compute $ / mile. I made a spreadsheet so I can adjust their values to be more in line with my needs.

Re: Accurate way to measure idling fuel consumption
 

Is it normal for my 2010 Kia forte 2.0L 4spd automatic to consume 0.3+ gph at idle when warmed up? Also, is there any way to lower the idle speed on newer cars?

Re: Accurate way to measure idling fuel consumption
 

That sounds abnormally high to me. My 2.5L SE-R consumed .22 gph at idle, and my 1.5L xB sips about .14 gph.

Re: Accurate way to measure idling fuel consumption
 

I don't know much about these new engines, is there any way to make it idle lower? Idle seems high, around 900 RPM.

Re: Accurate way to measure idling fuel consumption
 

Custom tuning is the proper way to lower the idle. It would involve using a special computer cable and special software to edit the car's computer programming.

If available for your car it is probably going to cost a couple hundred dollars and beware that you'll be asking someone who usually deals in tuning to increase racing performance to do the opposite of what he usually does.

It may be possible to do an awful hack on the Idle Air Controller or electronic throttle, but the computer will probably find a way to compensate and will almost certainly throw a code/set the Check Engine Light.

Re: Accurate way to measure idling fuel consumption
 

I might be able to talk the dealership into looking at it, since it's brand new with no modifications. I think the idle is too high, shouldn't it be closer to 750?

Re: Accurate way to measure idling fuel consumption
 

You'll need to check with other 2010 Forte Koup 2.0 automatic owners to find out what their idle RPM is. If you ask the dealer they'll just tell you it's normal. Even if you go in telling them that similar cars idle lower they'll probably still tell you it's normal...but being confident will help.

Re: Accurate way to measure idling fuel consumption
 

I wish I had a throttle cable like the old days, things were so simple back then.

Re: Accurate way to measure idling fuel consumption
 

I'd agree if I wasn't struggling with a carburetor that makes no sense.

Re: Accurate way to measure idling fuel consumption
 

I enjoyed working on my 1991 Toyota MR2. No Carb, but equally retarded AFM (air flow meter).

Re: Accurate way to measure idling fuel consumption
 

OMG!! I figured it out guys, I put my car in N when idling and the GPH went to 0.18! That's a huge difference from 0.30-0.34 GPH when in D!

I even tried putting it in N during coasting and my iMPG went from 120 to 180-200 MPG! This is so awesome lol.

Re: Accurate way to measure idling fuel consumption
 

Good work!

When putting it back in D, rev-match to reduce the extra wear on your transmission. Know from experience how fast your engine will go once you put it in D, and rev the engine to that speed just before shifting.

Re: Accurate way to measure idling fuel consumption
 

I switched from N to D many times on a 60 mile trip yesterday, I never noticed any noises or vibrations. I will try to rev match it anyway, I don't want to damage my transmission. It's a tiptronic transmission, don't know if that makes a difference.

Re: Accurate way to measure idling fuel consumption
 

The tiptronic stuff is just a different interface for a normal automatic transmission. Shifting between N and D wouldn't cause any symptoms, it would just cause an abnormal amount of wear (same kind of wear as always, just more of it).

Re: Accurate way to measure idling fuel consumption
 

Good sleuthing. I've managed >40 mpg on a nearly 1000 mile drive to Las Vegas & back in my wife's Hyundai Elantra w/AT and 2.0L engine. P&G between 50 and 65 mph on levels and downhills. Climb hills at 55 mph. That's how I did it.

Re: Accurate way to measure idling fuel consumption
 

Just filled up at the only non-ethanol gas station in my county, which happened to actually be cheaper. Using my new found techniques of using N at stops and coasting while in N so far I'm averaging 45 MPG. That's what the ultragauge says, however, when I manually calculated my MPG on the last tank the ultragauge said 33 and my calculation was 33.35.

Re: Accurate way to measure idling fuel consumption
 

Wow! 45 mpg is terrific. I averaged 43.7 mpg over 3600 miles in a rental Kia Rio this Spring, but it's got a much thriftier 1.6L engine. If I throw out the two tanks in SD where I was fighting 40+ mph crosswinds, my average rose to 45 mpg.rr

Re: Accurate way to measure idling fuel consumption
 

Nice Work. Very impressive!!!

Re: Accurate way to measure idling fuel consumption
 

Thanks, but now I'm scared to continue doing this. See, I don't think automatics like to be Pulse & Glided such as what I was doing today. I haven't noticed any noises or vibrations, but I read my owners manual and it states do not flat tow over 25MPH for more than 25 miles.

What do you guys think?

Re: Accurate way to measure idling fuel consumption
 

That precaution is for towing/rolling with the engine off. As long as you're gliding with the engine on, your automatic transmission is being lubricated, and there's no risk of damaging it.

Re: Accurate way to measure idling fuel consumption
 

Oh, thank goodness! I'm glad you guys are here to answer all my questions. Sorry to the OP for stealing his thread!

Re: Accurate way to measure idling fuel consumption
 

Some vehicles don't cool the transmission as well at idle as they do when the engine is at speed. My wife's Isuzu is like that, and is a bad candidate for pulse & glide. The dash will likely light up a warning if the transmission is overheating.