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RPMs fluctuate bobbing between 0 and 1000 (recent battery change??)

A8 / S8 (D2 Platform) Discussion Discussion forum for the D2 Audi A8 and S8 produced from 1994-2002

RPMs fluctuate bobbing between 0 and 1000 (recent battery change??)

Old 03-01-2011, 11:57 AM
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That's the funny thing.
At idle, it used to fluctuate, but since putting it all back together with new breather tubes, it still hasn't fluctuated again. It holds steady rpms now, but the CAT light still comes on at times, and sometimes flashes, and sometimes goes away after driving for a while.

The high-pitched whistling noise, that seems to be coming from the passenger-side (U.S.) top of the engine, makes me think that air is getting in there. I've heard that spraying a mist of brake cleaner near the leak will cause engine idle speed to decrease, if there is a leak. I have not tried that yet.

Out of curiosity, I have observed that the 'manifold tuning valve' does not actuate still, even at very high rpms (even above 5500, and it still won't actuate the variable intake.)

I obviously need to still make sure I haven't missed any vacuum lines / reservoirs, check valves, manifold gaskets, throttle body gaskets, injector o-rings, and the such, and it looks like it wouldn't hurt to try cleaning the MAF sensor.

I just wonder if brake cleaner really is the same stuff as MAF cleaner, as they said at the parts store here.

Even then, I was shocked today to find that, while driving I can empty the gas tank all the way down to the bottom of "the red" and it runs fine, but when I go to start it, it won't stay running. If I fill the tank back up, even just adding 10 liters (less than 3 gallons) and it starts, and keeps running, but once I drive around, and burn that off, and stop and turn the engine off, the problem comes back. There's no problem getting it started, but it won't stay running, until I add another 10 liters (two five-liter gas-cans full,) and restart.
The gauge is still down below the last quarter-tank marker, well into the red, but it may just be an intermittent problem, as it's only been a few days worth of experimenting. I wonder if it's possible there's some sort of vapor-lock, or if the fuel-pressure regulator is still not getting the vacuum it's supposed to, and that is somehow making it more difficult for the fuel system to pull the fuel from the very bottom of the tank.
Who knows?
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