Cadillac Repair: Rough Idle/driving on 89 Sedan Deville, intake leaks, vacuum hoses

Question
I have an 89 Sedan DeVille 4.5 liter FWD with 138,000 miles.  It has intermittent rough idle with bad timing when driving. This is guaranteed to occur when engine is cold and improves with engine warming up although it recurs intermittently after warmed up.  I have secured all vacuum hoses and double checked for air intake leaks. If the above problem is not occuring, the car drives smoothely and gets its optimal gas mileage.  I have noticed that when it acts up, I can sometimes temporarily eradicate the problem by turning the engine off and back on again, or disconnecting the battery for 2 minutes and then reconnecting and restarting it.  After driving highway speed for a while the problem seems to go away temporarily....and it drives great.  I wonder if there is a bad sensor to the computer or some short that changes the computer intermittently.  Cadillac dealers don't seem to want to try to diagnose it unless I leave it with them for 3 days, etc.  Does this problem sound familiar?  

Answer
You stated: with bad timing when driving.


Just HOW did you determine this ?.


Yes, i have seen this before, But there are so many possible faults that there is no way to tell EXACTLY which one you have without putting a scanner on the vehicle and performing numerous diagnostic tests and procedures in order to determine which component is at fault.

It could be ignition related. Fuel related, Emissions related, Timing related, any number of sensor intermittent faults and all these would need to be manually tested before a diagnosis can be made.


I could give you a MAYBE parts list as long as your arm but that gets very expensive in guessing at parts replacing in hopes of hitting the correct fault.