I have a 99 civic dx hatchback, i just installed a new intake, but when the car is in idle it keeps reving up and down. is that normal?

 

You probably have an intake air or vacuum leak. Start by checking the hoses and connections. Did you install new gaskets on the IM or throttle body? If not, the gaskets could also be leaking. If you disconnected any coolant lines, then also bleed the cooling system of possible trapped air.

 

i looked at the intake and found a hole about 1' on the back of the intake pipe. what goes in it?

 

Post a clear photo to get productive feedback.

 

this is a picture of the hole i found

 

That hole may be where the intake air temperature (IAT) sensor should plug into. Would the hole let unfiltered air enter the throttle body? Is the CEL ON?

 

no, the cel is not on, where is the iat conected to and what does it look like?

 

Look between the firewall and throttle body for a loose connector attached to the wire harness. It used to plug into the right side (standing in front of engine) of the stock air filter box.

 

is it this one?

 

Yes, that looks like the IAT sensor.