I have a friend that has an 02 civic si and we discovered a problem today. Whenever you were driving at high rpms (didn't matter the gear), it would start revving up and down really quickly and shaking the whole car. It didn't happen if you were still pressing hard on the gas, but only if you were trying to cruise at a high rpm. It almost felt like when you redline, but only at like 4-5k rpm (if you pressed hard on the gas then you could still take it up to redline). Any ideas?

 

When was the last time it got a tune-up or a valve lash adjustment? Valves need to be adjusted every 60K miles or so. When was the last time the EGR, IACV and throttlebody were cleaned? They need to be cleaned periodically too. Check for codes first. If no codes inspect the spark plugs, make sure the injectors are working and test the fuel pressure (which should be like 40-45psi).

 

I know it just had it's 100,000 mile tuneup, so new timing belt and spark plugs. I'm not sure about the valves, but I know it's not the IACV or throttle body.

The battery connector was pretty corroded, so we cleaned that and it seems to have fixed it. Does that seem like that could've been the problem?

 

It very possibly could.