Greetings.

I'm new here. I just bought a '96 Z24 with 160k on it, as a replacement for my wrangler, due to a longer commute to work, and high gas-prices. I'm nursing it back to health, as the previous owner was a 19yr old who put up with a lot of problems instead of spending any time fixing them. (even ridiculously easy ones)

So my question is this:

With the vehicle stopped, I can rev my engine up to 3k or so, and the speedometer will jump up to around 10 or 15mph. Weird. I thought it was simply instrumentation, and wasn't too worried about it having any effect on the drive-ability of the car. . . Then a few days later, doing the same thing, I was slowly revving the engine higher and higher, and when it got to around 3500rpms, it did the speedo-needle jump, but at the same instant, revved itself about a grand higher without my pressing the gas-pedal any further.

I've got one of those USB OBDII-scanners so I plugged it in, and watched my throttle-position sensor, and repeated said test, and when the rpms shot up by themselves, and the speedo-needle jumped, I didn't notice the TPS data change in any noticeable way. I expected it to jump from 35% to 50% for a split second or something. It's very possible that the sensor did malfunction, and the computer saw the spike in TPS voltage, but the scanner was too slow to capture it.

So, I suppose it's two questions..

Why would my speedometer jump while stopped, and
Is there a different issue that would cause both the speedo-jump, and the unexpected engine rev.

Thanks in advance for any help you guys can give.

-Dave