adding to what ecomike suggested, the 1988 4.0l fuel injecton straight six is controlled by what is called the RENIX(renault and bendix) fuel management system. typically, the 87-90 xj's will be refered to as the renix 4.0
with renix system, there are no codes to read(unless you have an expensive hard to find scanner) when diagnosing a problem with the sensors. that being said, to test the TPS you will need a voltage meter and a testing procedure from the factory service manual. you may be able to find the testing procedure somewhere on this site.
the iac is on the front left side of the throttle body and can be removed and cleaned with throttle body or carb cleaner, while your at it, the throat of the throttle body should also be cleaned.
the tps is on the front right side of the throttle body. do not clean the tps with any type of cleaner. the tps needs to be tested following the fsm procedure.