just put in used fuel pump into my 2000 camaro in which my old fuel pump was working but sending unit was not showing fuel on the gauge now the used fuel pump that i just installed is fluctuating at times from full to half empty and then full again can anyone tell me if the sending unit is bad or the control module

 

does it do it when you are still on a level flat surface ?

 

Yes it does and now it says it is full