I was driving home from work tonight and my vette (96) started to surge, if that's the right word. The tacometer was going down fast then jumping back up, it did this a few times. This is probably a dumb question but does anyone know what causes this?

 

Vacuum leaks are almost always the culprit.

A vacuum leak will make even a lawn mower surge.

You HAVE TO find vacuum leaks. There'sat least half a dozen vette functions that are triggered by vacuum.

https://www.corvetteforums.com/m_37283/tm.htm

Vacuum leak on a TPI can trip up EGR code. LT1, 4, 5, are 'port fuel' injection also, so vacuum leak on these could also trigger EGR code.

Do you have loping idle?

If you have a vacuum leak, which again, can cause the loping idle, then in order to find the leak, you have to crank up the idle to around 1200rpm, so you get a steady idle (unless it's already steady).

Then use a piece of hose to your ear, and with the other end, move it around the component joints - anywhere there's a gasket - even the injectors. Listen for a hiss.

Another way is with a propane hand-held. Crank up the idle, open the propane valve, and feed the propane around the component joints. If you get a idle surge, you got the leak.

 

I tried the above to fine the source of a vacuum leak. Come to find out, I had a broken valve spring.

 

Thanks again cwb, I'll look into it soon. Just wondering why it doesn't do it all the time though. The last time my car acted like that was back in December I believe. Thanks again.