limp mode wont do anything to an engine,its there to PREVENT damage to an engine.Limp mode is the computers way of running in "safe" mode,it basically sees a problem (hence a faulty sensor,mis-fire leading to a thrown code,faulty wiring,etc) and prevents the motor from reving past 3500. Like said,if your computer is running in limp mode,your going to be throwing a code. Find out what code your throwing and we can go from there. if you need to know how to check and get the code,PM me.