midnightgolfer

1997 Euro A8 Quattro, 3.7L AEW engine.

Does this sound familiar to anyone else?

The tachometer bounces slowly between 0 rpms and 1000, making a cycle about every 1 to 2.5 seconds.
It does it in Park, Neutral, and with foot on brake, in Reverse and Drive as well, and usually it also does it when simply idling without touching any pedal at all.

It will sometimes dip from higher RPMs to down around 300-600 RPMs when in gear and rolling.

On occasion, it dips so low that the "idiot light" that looks like a battery comes on for a split seconds. Even rarer occasion, it stalls, and dies, with other idiot lights, that all go away when the key is turned again to restart the engine.

Sometimes it seems like the rpms decrease suddenly, when I first just even slightly depress throttle, but recovers quickly if I keep pushing.

It seems it use to react differently before, when I could 'feather' touch the gas pedal with my foot to accelerate very slowly, and let it off slowly to decelerate smoothly. Now, the fluctuations in rpm seem to be larger than my own input, especially in slow traffic, and roundabouts where I like to drive a comfortably and fluidly as possible.

(Don't get me wrong, when the time is right, I like to just floor it and feel the power and exhilaration, but in slow traffic, it seems like it should be easier at responding to my foot, and my own responses should not get "lost" in whatever it "thinks" it's doing.)

I'm hoping this can be something easy to reset, and I am currently awaiting my VagCom order and a Bentley manual.

I should also mention, even though I doubt it's relevant, that I just recently replaced the battery myself, and a few things that used to work fine, have also started behaving differently. The rear windows won't roll up or down, not from driver's buttons, nor their own. The courtesy lights in the doors sometimes work, sometimes don't, now. And probably most significant, it no longer responds to being placed in Tiptronic mode.
Before, when the stick was tilted over in Drive, it would (as expected) allow for me to have more control over what gear the transmission used, and if the display in the dashboard were turned on, it would show the number of which gear was 'selected.'

Any ideas?
Isn't a '97 too old for these type of things to reset when replacing a worn out battery?

(Just in case it matters, it also had previously replaced the Fuel Pump Relay, the pump itself, and the Fuel Filter, and now no longer dies on it's own between 1/2 and 1/4 tank) Half and quarter tank.

tozoM8

That's a lot of problems. I would check for vacuum leaks, they bounce the iddle like that. Check the ground cable from batt to the car and the frame to the motor.

midnightgolfer

I'm starting to look for vacuum leaks.

Would a split in the gasket inside the oil filler cap cause a leak like this?

Sorry to be such a newb.

silverd2

The leaks you're lookin for would be in the intake circuit...esp anything that would allow unmetered air to get to the throttle valve.
Oil filler gasket would be under pressure and just allow a little oil vapor to escape...should have no effect on intake.

midnightgolfer

Thanks you guys.

But, I think I made something worse, and I think I should have waited until I got my Bentley manual and the Vagcom cable. The battery connections were clean and tight. The vacuum connections and hoses seemed like they were all good, and I could not detect any hissing sounds that would indicate where a leak might be coming from.

But, now it stalls and dies at practically every stop I come up to after a good stretch of prior acceleration, and it's worse when the engine has warmed up. The engine dies even if I put the transmission in Neutral right before it seems like it's going to stall.

I can even it it to stall out when parked. There's still a fluctuation in the rpms, and by just barely tapping the accelerator pedal at just the right time, I can get the rpms to peak and then drop all the way to zero, by timing the tap I give it with my foot, with the natural frequency that the engine seems to be following otherwise.

TerminA8r

Hard to diagnose with out vag-com, but I wonder if your fuel pump is dying.



Remove fuel line and check for fuel pressure with ignition on. No pressure, bad fuel pump.

silverd2

Codes, codes, codes...soon as you get em, post em. Even a MAF gone south could act this way.
Curious to see what you find.

midnightgolfer

Okay, so I was sloppy.
And also, my Ross Tech order, including the manual, is stuck in customs in Madrid, (which has happened to everything I've ordered this year from outside the EU, and can take up to a month to resolve.)

So meanwhile I decided to take a better look at the vacuum lines, (right after the fuel evaporated, that had sprayed everywhere while checking the fuel pressure, which seems fine.)

I got brave enough to actually pull the narrow hose off of the fuel pressure regulator itself, labelled "A" in picture, and felt with my finger if it 'sucked air' at all. It didn't produce any suction, at any RPMs.

So, now I'm convinced there's a serious vacuum problem, but it must be somewhere I can't easily get at.

The larger hose that exits/enters the intake air tube, labelled "B" in the picture, right before it enters the throttle? body, is way too soft, almost like it's rotten. It collapses in on itself, and there is a sound that I now think may be from a serious air leak, down underneath where I cannot see.

So I'll probably need a trip to vagcat.com and then the dealer, and then wait for the Bentley manual so I can figure out how to replace it.

midnightgolfer

Unless I get antsy again, and just decide to use generic hose stock from the hardware store, and see how it goes.

TerminA8r

Go to the auto store and buy 15 Feet of 5/32" Vacuum Hose.
They are due for replacement anyways.

http://www.audipages.com/Tech_Articl...sereplace.html