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Discussion Starter #1
Hi all,

bit of a weird one... just trying to chase down some fuel economy gremlins.
I'm very confused about what i'm seeing in terms of economy but what i see from my wideband and dyno AFR's.

Basically, i'm getting around 200-220km to a tank, not crazy driving, just normal driving. Fuel is E85 but even with a higher consumption fuel, it's still really thirsty.

The weird thing is, on two completely different dynos (one 2 hub dynapack, 1 awd mainline) and the wideband setup in my car, the AFR's look fine, don't show as ever getting really rich and are even fairly lean in some parts (as we've been hunting economy)

So what i don't get is how AFR's can look ok, not show super rich yet somehow end up with economy that SHOULD result from having really rich AFR's????? Where else exactly can the fuel be going if the AFR's look ok???

Could it just be leaking somewhere perhaps? not that i would probably notice on garage floor etc given fuel evaporates quite readily, but garage never smells fuely (E85 has a very distinctive smell) and same for when i'm on the road, no major fuel smell.

I'm just really confused about where my fuel is going! :confused::confused::confused:
 

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When you say you have checked AFRs on the dyno - are you talking about under full load, or at cruise? What happens if you log (or watch) AFRs during one of your normal "not crazy" driving, and what kind of AFRs are you talking about? The first thing that comes to mind if you do indeed have steady consistent and sensible AFRs is that you must have a heavy foot ;)

Another option I have seen on cars with mixtures reading as sensible (especially if they run a closed loop lambda setup) is if you have an exhaust leak the extra air mixing in the exhaust makes O2 sensor read leaner mixtures than you are really getting. If the ECU adjusts (or it was tuned!!) to suit the mixtures read in those conditions then you will get worse fuel economy than you need, from slightly to quite a lot worse - depending on the nature of the leak.

That definitely sounds off though - even on E85.
 

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Had a really thirsty gtr when I just bought it, and this was due to a well know UK tuner doing a maintenance on the car just before I bought it and pinching the fuel pressure line while doing so and I only noticed when she was up on a lift and running...
Might be worth to check everything once over ;)
 

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Discussion Starter #4
yes, checked afr's in cruise conditions, both on road and dyno, looks fine. It's definitely not lead food related, grandma driving a couple tanks back to bank and 220km was about the most i could get.

i was thinking maybe an exhaust leak somewhere... if ex manifolds were cracked could this be an issue?

freakazoid3: not sure if i follow, was your fuel line being pinched somewhere??
 

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Yeah crack exhaust manifold(s) could cause it - do you have some kind of closed loop fuel setup on your car?
 

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Discussion Starter #6
if it's the manifolds, i'm going to crack it! pun intended! would i need to do one of those smoke test things to hunt exhaust leaks?

um... i'm not sure? we did have the wideband setup to trim afr's in cruise conditions if that's what you mean? but i'm pretty sure that's turned off at the moment...

i also have a flex-fuel setup?
 

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Yeah that's what I meant - hmm if it is still guzzling it like anything and the AFRs look sensible then the odds are its not an exhaust leak, unless the leak has been there since it was tuned
 

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Talk lambda instead of afr. It only becomes confusing when using a different type of fuel.
What is your lambda value while cruising?
 

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I was just sticking with OPs terms, and a lot of ECU report in AFR even if it isn't a very accurate term so thought using lambda would cause more confusion despite it being technically more accurate. Part of the reason I was asking about target AFR is also the fact that some people seem to understand that stoich changes for different fuels but don't realise the calibration of what is reporting the AFR needs to be updated to suit... have seen someone trying to tune an E85 car to 7.2:1 on full throttle with the dyno set to convert lambda to petrol AFR
 

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Is the car making the right power for the spec, or is it not?

Just wondering whether too much ignition retard could be a factor, though admittedly it would have to be pretty extreme I'd have thought.

Just a (probably quite crap) guess, nothing more (if your lambda turns out to be correct).

Cheers,
Kingsley.
 
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140 miles to a tank of E85 x that by 25% to get an approx miles per tank for Petrol = 180 miles.

Yes I would say the car is a little un-economical but whats the spec of the car? big high lift cams set up with plenty of overlap if so the motor will never be very frugal on fuel I feel.

Stoichimetric Ratio's

E85 9.8

E100 9.0

Gasoline 14.7

Lambda reading is the way to cross reference readings between fuels thou much easier to understand.

If the car still has closed loop lambda set up it will be easy to see if the map is pulling or adding fuel to make the closed loop Lambda figure with a fuel KM's of data-logging.

One thing we have seen from tuning with E85/E100 or even methanol is the lambda sensors don't last long when being used to measure rich lambda figures when tuning they seem to detoriate very quickly we have found.

Has the car had the Lambda sensor re-calibrated?
 
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