Since I just acquired a new to me Super V8 and it has been years since I had one, I am going through that OCD phase over every little noise.

I parked my car and before I turned it off, I noticed the RPMs bumping up and down roughly 25-50rpms every 20-30 seconds. It is slight, but I do notice the tachometer bouncing around a bit and can hear it in the cabin.

Normal? Vacuum leak? Paranoid here.

 

I dont have an answer for you but, I have had the same paranoid feeling from time to time since I've owned my SV8. It doesnt help your issue, but your not alone.

 

I should amend my post that it is really around 650 it idles and bounces a little between 600-650. I just started it up again and it seems to be moving just a little.

 

Without hearing or seeing your car, I'll take a stab at this. From what l've seen, RPM variations at idle are normal with any car. Typically it has to do with accessories switching on and off (i.e. an AC compressor when the AC or defrost is on, an alternator, etc), which will vary the engine load and cause it to idle slightly different, whether that be manifest in an audibly rougher idle or a variation in idle speed, the latter caused by ECM programming to modulate the increased strain on the engine. Ofcourse, how much the idle speed varies is based on vehicle type and conditions.

 

1st, as ChillyPhilly notes, check your climate controls and make sure your A/C is off. The vehicle has auto climate controls, A/C is still active even in the cold months. When it kicks in, your idle will go up.

That said, you mention the idle is hunting every 20 -30 seconds. No codes?

2nd thing to check, block the supply to the PCV. If the engine smooths out, replace the PCV.

3rd, put a vacuum gauge on it and test for leaks. These are likely culprits when your car isn't throwing codes.

As I was the previous owner, I didn't experience this. One of the first things I did when I bought her was clean the intake and injectors as part of my normal maintenance routine so it's unlikely they're the culprit.

 

What year car?

Have a look at the fuel trims.