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Poor MPG / High(er) than normal fuel consumption

 
Old 02-21-2011, 11:51 AM
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RallyDogRacing
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Default Poor MPG / High(er) than normal fuel consumption

All - I wonder if my memory of my old C4 is a bit rosier than perhaps it should be. This new-to-me C2 Targa seems to be hoovering fuel more than it should. Combine that sensation with an occasional 'she's rich' smell stopped at a traffic light makes me think my fuel is out of whack. Overall with non-agressive driving I'm getting somewhere between 16 and 18mpg per tank. 60% highway speeds & duty-cycle. Low-end power seems not quite what it should be, but once the revs are up she really sings. Which is another symptom I think of being overly rich and until the revs and engine-load are up she's just bogging a bit.

I've got a new 02 ready to toss in there, but I seem to recall that there are other inputs that have more authority over the fueling than the 02 sensor. Does anyone have the authoritative list, and the check-out procedures to exonnerate each one so I'm not just blindly throwing parts at her? As I recall from long ago the order of precidence is:
1. Air Meter - flap-door-contraption
2. CHTs - cyl head temp
3. OT - oil temp
4. 02 sensor

BTW: I am learning to love the Targa experience. 4 days of top-off driving down here in TX already...
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Old 02-21-2011, 11:59 AM
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When I did my 02 I did get noticably better mileage but lost some of that upper end grunt. Porsche ran them lean for fuel economy and for emmisions. One of the main things a perfomance chip will do is remap the fuel ratio.

When the O2 goes bad it open loops.

Althought it will never get great mileage. It was originaly rated 18 city 20 highway.
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Old 02-21-2011, 12:56 PM
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Unkle
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I got about 28 mpg on the way to Liverpool recently, i thought that was pretty good. So the 18mpg does seem off
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Old 02-21-2011, 01:21 PM
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I'm lucky to get 20 consistently
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Old 02-21-2011, 01:58 PM
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I am lucky to get 20 also.

28?
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Old 02-21-2011, 02:08 PM
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There's 4.55 litres in an imperial gallon vs. 3.78 litres in an US gallon, which may explain why our fellow Rennlister from across the pond achieves 28 mpg or around 23 miles per US gallon.
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Old 02-21-2011, 02:08 PM
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Hmm - so maybe my consumption is more in line with the other aged USA cars. I suppose the question becomes, are they all suffering from older and perhaps nearly out of spec fuel-trim related components or not?

I guess I will be the lab-rat and make a change to see how it manifest in power or MPG. First up will be the 02 sensor replace. But even before that I have to dive into the front-suspension and see what mistakes the PO did in there. New Bilstein and some stock-looking-but-new Weltmeister coil springs installed. Alignment rack has LF @ -1.7deg, and RF @ -0.7deg on camber, and the shop couldn't find any adjustment... I got worried when they started calling me asking for concentric control-arm bolts. So I came and brought it back home. Spec says 0.00deg on front camber so clearly something ain't right. I can't see any evidence of any non-factory sheetmetal in the front-end so I doubt she's been bent.
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Old 02-21-2011, 02:17 PM
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Yeah sorry just checked it was about 26-27mpg not 28mpg. 430miles on a tank approx

Or is my maths that bad...
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Old 02-21-2011, 03:01 PM
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Unkle, what gallons do you refer to ? Imperial or US ?
I live in the netherlands and we have litres, I have a calculated 20mpg consistently,
26-27 looks to me impossible, Porsche gives 20-22 mpg !
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Old 02-21-2011, 03:55 PM
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Euro/uk gallons.

I did mention it was a run to Liverpool, so all motorway and back but compared to you guys it seems high. I get 20-22mpg driving normal and locally around town, but then i am in the country within 1mile from my house so i don't do a great deal of urban driving its mainly B roads or motorway.

On the inside back page of my handbook is a page giving the figures, your book must be different to mine.

Here's a pic, i do thinks its optimistic with the quoted 56mph figures though but then i have never tried driving at 56mph on a motorway.

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Old 02-21-2011, 04:05 PM
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yeah on my c4 on combo I get around 24mpg and driving on long trip highway I got just over 27mpg
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Old 02-21-2011, 04:12 PM
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its not just me then

I think it depends how you drive, leave a big gap to the car in front and never brake unless you life depends on it really helps. My other car did 63mpg on a European trip in 2010 but its no where near the same fun

I will be interested to see if you get any improvments RallyDogRacing
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Old 02-21-2011, 04:21 PM
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I have a rather heavy foot, but that's the fun with a Porsche !
But with the gasoline price of 1,65 per litre. (thats around 6,5 usd per gallon)
it is hurting now.
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Old 02-21-2011, 04:27 PM
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I get between 18 to 20 around town depending how much I put my foot in it. 17MPG if I am really doing heavy traffic in Portland, but usually on the freeway going to Salem or up to Seattle I get around 24MPG.
Basically the MPG I expect from this era of high performance automobile.
I have noticed that I get better mileage if I let her shift the gears instead of me (yes I have a Tip) -D▓

@ Rarebear- I thought $3.62/gl was getting a bit much but I can see your way over what us yanks are dealing with here.
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Old 02-21-2011, 04:36 PM
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Dirty air filters, dragging brakes, too much toe in or out, high ambient air temperatures, etc. etc, and a heavy foot in the city, all contribute to less fuel mileage. Growing up under the Dominion, I think in Imperial and on a regulary travelled 150km highway jont with the cruise set on 100 kmh, I've recorded 28 mpg many times, but once I start getting a bit heavy on the foot in the city I see my mileage easily go below 20 mpg.
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