V8 troubles

[John Cody Forbes]

I had one on my A8 once. I had parked the car outside my fathers shop for a 
weekend while waiting on a new starter. When I installed the starter the car 
fired up and ran, then I went to wash my hands while letting it idle. While 
gone I heard the car sputter and die. It wouldn't restart, fuel gauge showed 
a completely full tank. Checked spark, which was good, then checked fuel 
pressure which was not. Went round trying to figure out why for an hour 
before finally just dumping in a gallon of fuel out of desperation. Fired 
right up. Turns out that on modern quattros the fuel tank uses two level 
sensors with inverse resistance curves... at full tank they together have a 
high resistance which lowers as the tank empties, but then at half tank it 
starts going back up. As I understand it, the instrument cluster remembers 
the last reading, direction, and position and if the crossover happens while 
the engine is running the gauge continues down. If it happens while the 
engine is not running the car assumes that fuel was added and the gauge goes 
up. Somebody has siphoned my tank and the car thought it had gotten filled 
up. As soon as I actually filled it up the gauge returned to working 
normally and I've never had a problem since.

-Cody

-----Original Message----- 
From: Tony Hoffman
Sent: Tuesday, May 28, 2013 9:08 AM
To: 2Doxies
Cc: quattro at audifans.com
Subject: Re: V8 troubles

Back in about 1991, when I was still a newbie to mechanics, I was working
at an independent shop. We got this Dasher station wagon in, 1.6L CIS FI.
Wouldn't run. Guage showed half tank, pump ran, no fuel to injectors. After
spending about 3 hours chasing my tail, my boss came over to the car. It
was up on the lift. He tapped on the tank with the head of a screwdriver,
said "sounds empty to me". Moral of the story, I don't believe the guage
anymore, either. Lesson learned!

Tony

On Sun, May 26, 2013 at 2:24 PM, 2Doxies <doubldz at aol.com> wrote:

> Oh boy. It's hard to admit this and I have to say this has never happened
> before....
>
> Mike, you were right, I was out of gas! This is mortifying. I could blame
> it on being blond but I don't want to offend anyone on the list ;) I could
> blame it on the electronics in the dash not being 100% on the money most 
> of
> the time. At best it's unreliable. Usually it at least tells me when I 
> have
> 2 miles left in the gas tank. This time, it didn't or I didn't notice, my
> bad.
>
> That said, I'm greatly relieved that it was an $8 fix (the can of gas) and
> I could do it myself, LOL.
>
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