Idle and tach issues
 

OK, so I have been reading for a week, but no-one seems to have this exact issue.

For the last month, my previously perfectly running 84 unmodified Carrera has:

1. fluctuating tach needle (not corresponding to revs) - checked alternator, 14.2V at revs, 13.5 at idle, 12.0 off. Assumed bad battery. Replaced battery, ran excellent for 1 week. Now tach blipping again - battery voltage with car off 12.8, other volts same.

2. After car warm - car now surges at idle. Revs steadily from 800 to 1200 rpm, drops back to 800, goes to 1200, does not seem to want to die, just revs constantly. If I turn it off, even for one second, and back on again, it idles completely normally again. Start up cold is normal. Cleaned ICV, O2 sensor works.

So here are the questions:
- could the aternator still be bad?
- are these 2 things related at all?
- Why did the new battery seem to fix it, when the voltages with the car running now seem to be normal?

I an trying a good charge from the wall charger to see if this fixes it again, in which case it would seem to be alternator, correct? But I have not seen alternator and idle linked much in my searches.

Next stop would be either alternator shop or idle mixture shop (unless I can do it myself in less than an hour - work is nuts right now) so which do people think is best bang for buck?

Sounds like your alternator is fine. A low battery should not effect your idle. I think you have something else going on.

An idle that hunts up and down can be signs of it running rich or having a vacuum leak. I wouldn't adjust the idle until you find out what has changed. Something is either richer than it was before, or you have developed an air leak. Either should be diagnosed and repaired.

I agree that the alternator would seem to be fine, but...

- why did it go away with the new battery for a whole week?
- why does it go away if I turn it off and restart?

The second seems electrical - I don't know enugh about DME, but for example with a new car, the computers reset, etc. when you switch it off.

Thx for the input, tho.