[s-cars] Interesting state re: fuel consumption

[Paul Gailus]

I suspect that it only takes a small amount of misfiring to cause a noticeable increase in fuel consumption. Each misfire not only ejects unburned fuel but it also increases the oxygen content of the exhaust significantly. The lambda control loop will then react by increasing the duty cycle of the injectors to cause a richer than stoichiometric mixture.

I replaced the coils on my '93 S4 a few years back because the diodes in series with the secondary winding had degraded based on I/V measurements. These diodes are there to prevent a "spark on make" when the POS first turns on. The new coils appeared to eliminate some infrequent engine "hick-ups" at light throttle. But I can't say for sure it was the coils because I made some other changes as well.

Paul
93 S4
95 S6

-----Original Message-----
>From: Fred Munro <munrof at sympatico.ca>
>Sent: May 11, 2008 6:19 AM
>To: "J. Khang" <jhlk99 at yahoo.com>, s-car-list at audifans.com, tedebearp at yahoo.com
>Subject: Re: [s-cars] Interesting state re: fuel consumption
>
>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.
>
>Fred Munro
>'97 S6
>
>-----Original Message-----
>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.
>Jonathan
>'97 S6
>
>
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