Cadillac Repair: erratic idle and overheating, intake manifold gasket, vacuum leak

Question
I have a 94 SLS.  My wife has been complaining the last couple weeks that the car wants to go by itself sometimes.  Friday night I drove it and dropped her off at the mall.  I waited outside with the car running and the idle would go back and forth between 1100 and 1400 rpm.  After a few minutes of just sitting there, the darm thing started to get hot.  Reached a temp of 240.  It had always stayed around 208.  Had to shut it off.  We then left and every time we had to sit at a stoplight it would start getting hot again.  Could this by a plugged catalytic converter?  No check engine light and no codes were dropped.  Fan was working okay. No maintenance was done before this started happening.  What gives?
Thanks,
Brian

Answer
Hi Brian, An 1100 to 1400 idle is too high and can cause overheating especially if a cooling fan doesn't work. I believe that around 225 degrees F is when the cooling fans turn on slow speed and by 240 they should be on high speed. A plugged converter wouldn't cause this. Maybe the battery was disconnected or went dead and then the idle wasn't relearned correctly. There could be a vacuum leak to the intake manifold gasket, injector(s) going bad, sticking idle speed or control motor, tps out of adjustment or engine misfireing so the computor boosts the idle speed to keep it running to name a few things. Scanning the computor to see if all the sensors are with-in range would be a good place to start. Then cleaning the throttlebody carbon and relearning the idle. Making sure the cruise linkage is adjusted propperly and allowing the throttle to close completely.  Some things you could check and others a shop will need to do so money isn't spent needlessly. Once the idle returns to normal and if the engine has the correct amount of coolant in it and the radiator cap is holding 15psi then the engine shouldn't overheat anymore. Hope that helps. Bill