[s-cars] Interesting state re: fuel consumption
munrof at sympatico.ca
Sun May 11 04:19:06 PDT 2008
Both Jonathan and Teddy postulated that the coils begin misfiring as they age, reducing fuel mileage. That is what I initially thought as well but there is nothing subtle about a random misfire and I'm usually pretty sensitive to that and pick it up right away. I wonder if as the coil deteriorates it takes longer to build and collapse the primary field and it's firing a bit late, or the weaker spark is igniting the mixture a bit late? That you wouldn't pick up by the seat of your pants and retarded timing does reduce power and fuel mileage. I can't recall whether the Motronic pushes the timing until it detects knock and then backs off or runs standard timing and backs off if it detects knock; I suspect it's the latter. It's interesting that both Jonathan and I noticed an improvement in mileage after replacing aging coils, but then we both have '97s, so maybe it's a special feature Audi built in for the last of the line :o). I do notice the '97 seems to have more power than my old '94, but maybe it's been chipped - the carpet has been cut but I've been too lazy to pull the ECU to check.
From: J. Khang [mailto:jhlk99 at yahoo.com]
Sent: May 10, 2008 9:28 AM
To: s-car-list at audifans.com; Fred Munro
Subject: Re: Interesting state re: fuel consumption
"Fred Munro" <munrof at sympatico.ca> wrote:
> Even stranger economy point on my S6. No.5 coil finally bit the big one and
> after I replaced it, I took a 400 km road trip to Toronto. Fuel economy was
> 8 litres/100 km going down (~30 mpg) and exactly the same coming back. I've
> never gotten fuel economy like that on either of my S cars. It seems to be
> holding up in my daily commute as well. Can't figure that one out - must be
> something wrong :). Kind of nice at these fuel prices though.
> Fred Munro
> '97 S6
This is actually not surprising because the coil packs are deteriorating over time and the available firing voltage is dropping. I expect that the spark kernel is getting weaker and a new coil pack will make a difference by reducing misfires.
I had two coil packs fail recently and replaced all five because I didn't want to mess about with them again. Like you, I have observed improved fuel consumption. The engine also runs slightly smoother and there is more power.
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