Tstealth32

Been having this problem for awhile. While driving, the RPM needle will bounce about 100-200 rpms however the engine is not changing rpms. Sometimes more noticeable at a lower speed, 34-45 mph. It can also be duplicated while parked and raising the rpms to 2000-2500 rpms.

I've used vcds to plot the rpms, and it's rock steady +-20rpms.
I checked the torque converter using vcds and it's acting as it should, and wasn't slipping.

Could the cluster be bad?

2008 A8 Base. 143,000 miles.

Jack88

Check the fluid level in your transmission. It's a bit of a pain, but I have seen this symptom on a car that turned out to have a slow leak from the pan gasket. Any issues shifting?

Tstealth32

Transmission is silky smooth. however; a couple mornings this past winter (32 degrees F), just during the first block, when I let off the gas, the car was jerky. After that, it was smooth. I'll check the fluid level​.

Looks like I just get the transmission fluid up to temp, then fill it from underneath?

Jack88

That's about it. There was a thread that covered it a while ago on here, if I recall. The car I remember this from didn't have any obvious leak, it was pretty subtle.

Mister Bally

I experienced this just before the crankshaft position sensor (RPM sensor) failed leaving me in a no-start situation with a hot engine.

Tstealth32

I have replaced the RPM sensor, thinking that was the problem. It did not fix my issue. But at least I have a new one in there! I also replaced the low pressure fuel pump, that didn't help.

ltooz_a6_a8_q7

So, the problem Is not a real one? Just the tachometer is displaying incorrectly? I would find the tach wiring diagram, find out where it get the rpm from, find an old tach meter or a cheap tach, hook it up, then you would know if the rom generator is flaky or the tach instead of throwing parts at it, you may screw up the engine in the process of replacing unnecessary parts.
Cheers,
Louis

MP4.2+6.0

I think he already basically knows RPM is not fluctuating in reality. OP says w/ VCDS it was only +/- 20 RPM. To me that is already better data already than trying to wire in something else.

Net, seems like the conclusion is back to flaky tach gauge. Especially w/ RPM sensor changed out--assuming that OP means the crank position sensor. Presumably that is what VCDS is reading, let alone the ECU that dives smooth running (old not) to begin with.

Jack88

When I experienced a 100-200 rpm fluctuation in a D3, you couldn't feel or hear the change. It wasn't the gauge, as topping off the transmission fluid after replacing the leaky gasket(and filter because why not,) solved the issue. I figured it was something about slightly low fluid levels interfered with the operation of the torque converter.

MP4.2+6.0

There was a subtle RPM fluctuation of about this range, if you watch it carefully for the torque converter lock up. If you have gear display showing (via the VCDS mod) while in D, you can induce it with a little more throttle but short of a shift down from 6-5. Thus it stays in 6th per the display but seems to step out of lock up and RPM's move up with torque converter inherent slippage. The six speed is also unusual in having lock up in most all the gears (maybe not first?), instead of just the final one. Still separate though from whatever VCDS is showing for RPM if that holds steady. The lock up function and input and output tranny speeds is probably in VCDS measuring blocks somewhere.