Hey I have a 2000 Blazer ZR2, 95K miles. I was missing around today and after driving I home I started reving the engine up. If I rev it past 2500 rpm and then completly let of the throttle the engine will drop down below 500 rpm and almost stall, lights dimming and flicking and then it will idle normal again. This happens everytime, I revved it up several times back to back and it still did. Also the speedo would start to turn and maybe reach 10-15 mphs. Is this suppose to happen???

So for the engine I probably just need a tune up right???

and for the speedo is there a sensor that goes bad???

 

Check your battery connections and any grounds you can find.
Just a thought....

 

The dip in rpm when coming down from a high rpm is nothing unusual. Some vehicles do it worse than others. What is basically happening is that when you let off of the throttle, the momentum of the engine keeps the rpms up a bit. In this amount of time, the PCM sees that the engine should be idling at 600-700rpm, but isn't and adjusts the IAC (idle air control) valve in, shutting the passage down in an attempt to lower the idle. Now when the idle gets back near its set point of 600-700rpm, the IAC is in further than it should be for proper idle speed and the RPMs dip lower than the set point. The lights dim because the rated output of the alternator at idle speed is pretty low. Drop the rpm down further and the alternator cannot keep up with the demand of the various systems in the vehicle and the lights are the most voltage sensitive as far as their output goes.

On the speedo... It does this while in park and just rev'ing the engine? Shouldn't really do that, but if it operates normally and the check engine light isn't on, then I wouldn't worry about it.

 

okay. I didn't really think it was to serious. Yes the speedo works fine and is accurate at 80mph... but could this be effectiving my abs at all. It kicks on at low speed, I know its a common problem but I have 2 new hubs and the abs still kicks in.

 

I know my speedo moves when I rev up the truck in neutral and whatnot.

My dads old '93 suburban did it too...

If my Lumina could have revved past 3000RPM in neutral or park, it probably would have too...