My PT will nut go over 40 MPH . engine will not go over 3700 rpm

My first guess is you may have an exhaust restriction. When power is limited along with engine speed, it is usually because the engine cannot breathe properly. Either too little air intake for the fuel or too little exhaust flow. All a guess, and testing is required. You will want to check for fuel trims and a few other areas of the data list, along with any diagnostic trouble codes. If your check engine light came on, there are codes -- so perhaps a better diagnostic tool is needed.

Think rev limiter!