Hello all,

I'm having a problem with an 2000 Audi A4 (Automatic).

For a few months the tach has been jumping up and down by about 500 RPM. I doesn't do it at all in neutral/park. And it really only does it when the A/C compressor kicks on. If I shut off the A/C the tach stabilizes.

I'm a marine mechanic, so I know my way around an engine. I got three codes...

-Intake resonance switchover solenoid control circuit fault.
-Long Term Fuel Trim Multiplicative System, Bank1 System too Lean
-Bank 1 O2 sensor too lean or low voltage

So, I changed out the bank 1 02 sensor, all the plugs, and I found/replaced a slightly cracked intake vacuum hose.

It now seems like the trouble isn't as bad. But when the car is running, and I turn the A/C on... the tach just bounces up and down (from like 900-1000rpm).

One way I found to get the problem to stop, even with the A/C on, is to put it in neutral. That said, the problem is obviously directly affected by load. It is magnified by load (A/C compressor, putting the car in drive).

It drives great on the highway, regardless of A/C.

So, I think I'm going to start checking the coils... there is one for each plug. But if it was a coil, wouldn't the problem be consistent?

I don't want to just replace things, I want to find the cause.

Local mechanics have been NO help... I got fed up and just thought I'd pay someone to deal with the problem. One claimed there was nothing wrong with the car. The other tried to tell me he fixed it, but no dice.

Anyone ever seen something like this? What would you do?
Thanks!
Daniel