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Discussion Starter #1 (Edited)
Second try, this time in the correct subforum ...

I'm seeing a difference of between 10 and 20 mpg between what the FCD shows and what the pumps show. To head off some of the common gotchas: I know that the FCD doesn't always report the same in different Insights, and also that gas pumps are inconsistent and that you have to try and remove that variable as much as possible. I log my MPGs fairly religiously (offline) and always ensure I'm using the Trip buttons correctly and not accidentally using the Lifetime mpg, etc etc.

Having said all that, I still think I have some sort of problem due to the large difference between my reported and calculated MPGs. I've read some other threads regarding differences between FCD and manually calculated MPG on this site, and in those threads, other users report maybe a 5-10% difference. But what I'm seeing is FCD calculations on the order of, say, 60-63mpg with manual calculations approx. 45-50mpg - a much larger difference on the order 20-30%.

I'm concerned that I might have a fuel leak somewhere between the tank and the injectors, but there are no obvious signs such as a gasoline smell etc. I'm going to get the car up on jacks and take a look underneath/under the hood the first chance I get, but until then... anyone else had a similar experience? Thanks!

EDIT: Car is a 2000 manual, RE92s on stock wheels inflated to 45psi. No MILs. I run a Calpod and typically have the IMA battery (functional but ageing, approx. 9 yrs old) disabled when cruising at constant speed and enabled when accelerating hard or when ascending hills.
 

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You don't accidentally have a UK model with Imperial mpgs ? ;)

All I can say is keep a fuel log like on Fuelly and note your indicated mpg and compare to the mpg calculated from the odometer and pump volume.
You can only tell for sure if you have the data to prove it.

And, are you sure the volume as indicated by the gas pump is accurate?
It should be, it would be illegal to overindicate the volume provided. And very lucrative for the pump owner.

I actually encountered the opposite case. One pump on a gas station seriously underreported the volume. After the second fillup (confirming to me that the pump had that 'feature') I informed the employees of the station about it, but they just shrugged. I was a loyal customer until it finally got fixed months later ;)
 

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Discussion Starter #3
I don't use Fuelly but I do keep my own logs of fuel economy - this is a fairly consistent behavior irrespective of fuel station or pump. What I'm hoping folks can suggest is any problems that might cause this behavior if it doesn't turn out to be just inaccuracies in the volumes provided by the gas pump meters, the FCD or both. The discrepancy in my case seems to be somewhat larger than the discrepancy Insight owners typically report from those causes, which is what's causing me concern.
 

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Your location ('United States') gives no clues about the weather. Anchorage or Guam makes a difference :)

When it is really hot the gas evaporation from the tank may be a major factor. It should be captured in the overflow canister, from which it gets sucked into the air intake.
This will enrich the air with fuel, helping combustion, while the ECU does only register fuel consumption by measuring what went through the injector pump.
So the ECU sees better than real fuel consumption. After all, not all gas you tanked went through the injectors.

This is the case with all cars using injector quantity so all of them will be optimistic on FE. A 5% positive economy reading is my average here in cool-mild Dutch weather; but during a heat spell 8 to 10% is not uncommon.

Methinks 60 mpg is exceptionally good for a 2nd gen Insight. I am an avid hypermiler, but everything needs to go my way to get a per tank economy that high - and only in summer, trailing the semis at 50 mph.
lifetime FE over 0.1 GigaMeter or 66 KiloMile

 

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Discussion Starter #5
Methinks 60 mpg is exceptionally good for a 2nd gen Insight.
Yes, me too. Which is what made me realize I posted this in the 2nd Gen MPG forum by accident :rofl2: Thanks for your help anyway! Now time to go put this where it should have been... apparently my diligence in economy logging doesn't extend to checking the generation of Insight subforum I'm posting in :D
 

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It's all a bunch of percentages adding up, some possibly by design*.

One thing I noted via GPS reading was a consistent ~3% discrepancy between satellite reading of speed vs. dash indicated speed in the car [using stock size re92]
*Possibly a design feature 'within tolerances' to soften corporate gas figures?:confused:
fwiw: my '00 as acquired initially ran on 175/65r13 tires until the re92's were acquired. GPS and dash display agreed. Now, with stock 165/65, indicated speed is ~3% under gps velocity.
 

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My discrepancy has been FCD mpg only +1-2% over fill-up mpg calculations, for last 5 years or so. So OP's discrepancy is huge. Not sure what would cause that, though...
 

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My discrepancy has been FCD mpg only +1-2% over fill-up mpg calculations, for last 5 years or so. So OP's discrepancy is huge. Not sure what would cause that, though...
Since there is no info about the car, tires, inflation psi, transmission, any MILs, maybe he's got a load of moonshine in the back :rolleyes: it is just about impossible to help.

My speedometer with RE-92s was exactly the same as my GPS reading last time I checked it. My mpg when calculated by hand vs the car display is slightly different but I just don't worry about it since it's so high either way.
 

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Discussion Starter #9 (Edited)
Well, olrowdy01, I don't wish to render diagnosis impossible, so: It's a 2000 MT. RE92s at 45psi. No MILs. The IMA battery is old (9 yrs) but functional, albeit with a robust regen schedule. I typically run it disabled via a Calpod for cruising and leave it enabled for accelerating from a stop or when ascending hills.
 

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Well, olrowdy01, I don't wish to render diagnosis impossible, so: It's a 2000 MT. RE92s at 45psi. No MILs. The IMA battery is old (9 yrs) but functional, albeit with a robust regen schedule. I typically run it disabled via a Calpod for cruising and leave it enabled for accelerating from a stop or when ascending hills.
Have you checked the speedometer vs a GPS?

Do you try to put as much gas as you can get in the tank at fill ups?
By chance do you fill up way before the tank is near empty?
It's known that the Insight sometimes shows inflated (like 150+ mpg) right after a fill up for a short time.

A common fuel line rust spot on salted road Insights is under the hood where the fuel line is attached to the alum bar from one shock mount to the other.
 

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Seems like I've seen a thread with the same question before, quite a while ago...

I don't see what difference the 'car' variables would make - for instance, if tires are inflated to 45 psi or 20 psi - as the question is about the discrepancy between the FCD vs. hand calculations and both would be affected equally by, say, different tire pressures...

OP says manual calculations are lower, so, that would mean you've got 'more gas' in your manual calculations, or fewer miles. On the flip side, it could mean your FCD calculation has 'less gas' or 'more miles'... So, what are the ways that these things could happen?

-The 'more gas' in the manual calculation could come from a faulty pump that's telling you you're filling up with say 10 gallons but it's really only 8 gallons. But I see that's not the kind of answer we're needing at this point...

Seems like if we worked through the possible answers to these 'more/less gas' questions we might get somewhere...

[later edit: I had to change things a couple times because I got things backwards...]

-I think we can strike the 'miles' aspect as that should be the same for both the manual calculation and the value that goes into the FCD's calculation, assuming OP is using the trip readout for miles. The only thing in play seems like the 'more and less gas' aspect...

If you fill-up with 8 gallons and the pump tells you you've pumped 10 gallons, then the manual calculation comes out 20% less than actual -- because you use 10 gallons in the denominator of, say, 480 miles / 10 gallons = 48 mpg, rather than the actual 480 miles / 8 gallons = 60 mpg...

If you fill-up with 10 gallons and the pump tells you 10 gallons, BUT, you lose 2 gallons out a leak hole, then you calculate the same result as above - 480 miles / 10 gallons = 48 mpg. The FCD, on the other hand, should read correctly - as it measures the amount of fuel used, 8 gallons in this case, and uses that measurement in its mpg calculation...

If the gas pump is working correctly - 10 gallons is 10 gallons - and there is no leak, perhaps the measurement of fuel used is in error? Your low manual calculations are correct, but the FCD is wrong - because for whatever reason it's measuring fuel use as being too low - 20% less than it should be... Is that possible? Or rather, how would that be possible? Seems like you might need to get a hold of an OBDIIC&C, dial-in the injector duration parameter and maybe a couple others, try to cross reference your values with someone else?

Given that you've got an MT, high tire pressure, etc., sounds like the FCD is right and the manual calculations are wrong. Seems like it's gotta be a bevy of evil gas stations where you fill-up or you're losing gas between the tank and the injectors...
 

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Discussion Starter #12
Seems like it's gotta be a bevy of evil gas stations where you fill-up or you're losing gas between the tank and the injectors...
Well, upon checking the mount point under the hood for the fuel lines, I think we may have found the problem... Time for a more thorough check of my fuel system, I think, to see what else is on the verge of corroding out. (And, wouldn't you know it, today I caught the faint whiff of gasoline outside the car for the first time, too.)
 

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Install a pressure gauge on the inlet pressure line. Turning on the key, it should register the pressure building until it is at it's max. Turn off the key and it should hold pressure.

HTH

Willie
 

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Well, upon checking the mount point under the hood for the fuel lines, I think we may have found the problem... Time for a more thorough check of my fuel system....
Ouch, that looks terrible. There's a thread around here that details replacement of the fuel lines. Might want to search around for that...
 

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+1 on Willie's pressure test.

Those fuel lines look pretty close to a bad leak. Another common spot is at the base of the fire-wall where the line bends back under the car.

Ol Rowdy asked the question I had in mind. During fill up, do you add extra fuel after the pump nozzle shuts off ? Also, do you use the same pump each time ?

I set the pump nozzle on the lowest setting for hands free filling (some gas stations remove such devices). After reading a couple threads about overfilling causing bogus fcd read-outs and messing with the evap canister, I no longer add extra fuel after nozzle shut off.

Calculations on all my G1's are within 1/10's of an mpg compared to the fcd for the tank.
 

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Ditto on what MD said about method..
On the recent trip to HYBRIDFEST 2017, the Caravan kept good figures on the fill ups and the diffference between FCD and ACTUAL. Sometimes there was a "Hugh" difference.

I was very constistant on the same procedure on fill ups. Of course different pumps at the gas stops.
I'll try to post those later on.

Willie
IIRR I got the best MPG in the group every time.
 

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I'm pretty consistent on fill-ups and calcs, but not terribly so. I go to different stations off and on, for instance. I still only average less than 2% difference. OP's discrepancy is on an order magnitude difference - 20% plus - that makes it extremely unlikely that pumps and pumping are the explanation...
 

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I can confirm that the harder you drive the car the more off the FCD becomes. If you do some quick accelerations it appears that they don't register in the average MPG. My assumption is that it only takes an average every 10 seconds or so, therefore some quick accelerations are in between that time and get completely missed by the FCD readings.

How's your driving style?
 

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As promised, here are the FCD/ACTUAL mpg per tank in order of fill ups on the "CARAVAN" trip to Madison and back to Ca., through 10 tanks of gas and over 4700 miles.

THE "FILLUPS" were dictated by whoever was at the lowest reading on the "FUEL GAUGE".

MILES FCD ACTUAL
372.9 56.8 55.8
446.0 61.2 58.7
531.1 62.4 71.7
507.6 67.9 58.4
405.9 64.6 72.4
404.4 62.2 54.6
482.9 69.0 68.9
450.8 56.4 55.6
466.7 65.0 62.2
625.8 62.7 61.9


I was always consistant on the method of filling the tank.
Slow fill and stop, no topping off.

ENJOY!
 

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My assumption is that it only takes an average every 10 seconds or so, therefore some quick accelerations are in between that time and get completely missed by the FCD readings.
I disagree. The FCD only updates periodically, but the car constantly calculates. If you "double click" the FCD button, it will update immediately. It couldn't do this if it was only calculating according to a set schedule.

Sam
 
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