wont keep a steady idle idles up then down then up then down and then dies on 1990 Mercury Cougar

i have an 88 mercury cougar that wont hold a idle. it idles up then down then up then down and dies......have replaced fuel filter, throttle sensor and a temp sensor what else may be causing the problem? anyone with ideas, pls help!!
Thank you

Asked by for the 1990 Mercury Cougar
Check the idle air control valve... Also, look for vacuum leak or a broken air box or an air intake leak. Check base fuel pressure.
we replaced the air control valve.... how do we check for base fuel pressure????
You need a fuel pressure gauge and need to tap into the port on the fuel rail. This is more involved than most back yard mechanics get into, honestly. You could always take it to someone that can perform a test.
thank you for the input but we ave discovered it to be the MAP sensor and throttle sensor combo of the two were causing the prob.. but thank you so much for the advice!
Good job, glad it's resolved!