1999 Cadillac Deville

Engine Mechanical problem
1999 Cadillac Deville V8 Front Wheel Drive Automatic 111000 miles

I have a 1999 Cadillac Deville D' Elegance. It stalls every now and then at a red light or while parked. I noticed when the RPM is above 600 while idling it's fine. Once it drops under 600 the car stalls. It turns right back on with no problem. My question is would this have something to do with the throttle sensor or idle air control valve? I took it to a mechanic and they have no clue. They keep changing one sensor and when that doesn't work they put the old sensor back in and change another sensor.
December 26, 2008.

Have them hook up a scan tool to the engine and monitor the IAC in live data mode, look for IAC movement (called steps) when engine is about to stall, there should be a large movement to prevent stalling. If not then IAC or wiring to it may need replaced.

It may be necessary to remove TB and clean it.

Other possibilities are that the EGR is leaking, which would cause stalling, remove and clean it and make sure the PCM is not commanding it " ON" during idle.

Dec 27, 2008.
They hooked it up to the machine and they said the machine doesn't give them the exact problem. It only gives them an idea of what could be wrong. Another problem that I noticed with the car is the anti freeze keeps disappearing. I have to put a gallon in every two weeks.

Dec 27, 2008.
If you are losing antifreeze, it is either leaking it or burning it, which may be the cause of the stalling. If headgasket, then the O2 sensors will have to be replaced and possibly the CAT.

Have a compression test performed to isolate the problem, if OK, then look for leaks. If compression is bad, then headgasket or possible intake gasket.
If headgasket then it will need to be changed and if a bolt is broken or threads in head are stripped, you will need a Timesert repair. Don't let them talk you into a heli-coil or anything else, Timesert is the only repair for a Northstar.

Dec 28, 2008.