There are few reasons some simple some not. Check your gas cap make sure it is tight and not leaking. Check to see there are no vacume leaks under the hood. If these check out then you need to hook up a code reader to the OBD port and see what "P" code number reads.
If you do not have a scan tool any autozone part store will hook up a scan tool for free.
There is a CEL, Check Engine Light, on the 1989 Thunderbird.
Take the car to an major auto parts store and they will check your check engine codes.
have u had the check engine light checked for the code? if not take it to auto-zone parts house and they check it free. also check for a vacuum leak
Check the oil pump housing gasket
Poor acceleration can be tagged to one or more of the following1. Closed loop (DTC) mode "is there a check engine light on?"2. Restricted exhaust "cataylitic converter plugged"3. Vacuum leaks "check ALL lines, especially on the thunderbirds"4. Fuel injectors worn or leaking.5. Low compression "have a wet/dry test performed" to determine overall engine health.6. TEENAGERS DRIVING YOUR BIRD TO THE GROUND ?7. Restricted Airflow "check air filter, replace as needed"
ok where is the knocking coming from, brakes, engine ? only happens during acceleration ? if it is truly an engine knock, check oil immediately, but maybe to late, again need more info.
If it's ONLY during acceleration I'd check the CV joints first.
usaully its one of the four sensors. mines always go out right before the smog check. fairly easy to replace.
If your check engine light is flashing on and off with the engine running an engine cylinder misfire has been detected ( avoid heavy acceleration or deceleration while driving until repaired or you can damage the catalytic converter )
spark plug dead or it fell off.....
Check the timing first. Then check the egr ooperation. Then check the compression wet and dry. Run some engine cleaner through it.
it is a strongpossibility you plugs could be your only prob but also you should check you air fuel ratios and injectors plugs and vaccums are the easiest mto cgheck