RPM drops when coming to a stop

I did clean the throttle plate, the battery connections look brand new, but I went to a couple of Auto zone places and they told me that the alternator had failed, one time a guy told me the alternator took him 3 test until it come up good, because it was very slow increasing amp or voltage not sure.

Dealer told me that everything is checking good, I guess they only get it tested with the diagnostic Software.

Thanks for the answer btw

Carlos

I have the car at the Stealer, also my RpM needle when acceleration before changing gear it was making a quick jump and then doing the gear change.

The guys there think it can be a bad Tach, they think about the dropping of the idle to be normal, I don't think thats normal, but also don't think it can be related to a bad tach as I can hear the engine note changing everytime rpm drops as if I was turning my A/C on and was getting more engine load.

I asked them about the charging system but they keep saying everything is within specs.

I've know a faulty battery or alternator will do crazy stuff, but what about if its starting to fail, but still working, does this sound like something electric is failing?

Thanks again

Thank you for your fast response,

sorry to bother you
I was told that the jump on the needle when driving was because of a faulty tach.

Regarding the drop of rpms I was there today to show the technician what I was feeling, the car feels rough, as soon as the rpms increases it feels smooth but also the engine note get rough when I put load like, steering wheel, heated seats or windows, technician did noticed that.

I did pointed out. The first time I noticed the electric was one day I left the ignition on and the a/c running for 15-20 sec and I got a light for battery discharged.

Then I went to NTB checked the battery and told me it was ok but at 60% charge. It was low
autozone tested. I was told the alternator was taking a lot to put charge and it was slowly building up not constant
different auto zone, told me the starter
oreilly auto parts: I was told also alternator had failed the test
oreilly different. Also battery

dealer just told me they checked and everything pass as normal.

Thanks again

sorry if I've been taking your time

Carlos

I'll do that as soon as I get my car back from Porsche,

but they always tell me vacuum leaks are imposible as this will cause a CEL I know hard to believe.

Thanks again for your help