Replaced intake and oil is mixing with water on a

Are the cooling fans coming on?

Sounds like a vacuum leak Check all of the vacuum connections

Check oil by draining. Trans tank on radiator bottom may be oily slick you see if oil pan drained clean. Then trans is probably contaminated with antifreeze and that's what is mixing.

Crack in head passages could link oil and water, if you verify antifreeze and oil mixed in oil pan.

YES AND THEY CRACK AND LEAK,THEIR ABOUT 280 AT PARTS STORE.

The Grand Marquis's have a know problem that causing overheating. The intake manifold cracks around the water inlet tubes on the intake manifold causing it to loose anti-freeze which in turn causes overheating. Mercury has a beefed up replacement intake manifold that cures the problem. It was a subject of an unannounced recall but they would replace it unless the problem occurs. The recall was for 7 years which the 1999 model wouldn't qualify for now. It happened to me and cost about $450.00 (parts and labor) to fix it. I used a local repair shop to do the work. If, after the engine is hot, you hear water/steam coming from the top of the manifold, you have the problem

There are two. One is the sending unit for the temperature gauge. It is located in the intake manifold runner just to the left of the alternator. (Left, meaning your left as you are facing the front of the engine -Passenger side of the engine)
The other is the sending unit for the computer which is located in the left cylinder head under the intake manifold. (In this case, Left means the engine left bank or driver's side of the engine.) The intake manifold must be removed to get to it.

Hey you have a head gasket failure, stop driving and get it fixed. I had a 1999 grand am and years down the road I had water in the oil, mechanic said it was the intake gasket, 1600 dollars later and I sold the car to escape the problems with grand ams, the water is a milky color in the oil, this means there is a breach near the pistons and your colant line and thats where you water and oil mix sometimes cause you engine to overheat and sometimes not able to drive.

So you took the heads off as well? If you took the heads off and let sit over night condensation built up on pistons just wipe clean and install head gaskets and tighten to specs with new bolts. But if you took heads off and antifreeze come pouring out you have a bad head gasket change both sides. The reason it has not leaked into the oil pan yet, cause it has not gotten bad enough to do so. Believe its a mess.

about $1200 US, this requires the dash be pulled out, about 12 hours of labor.

Here is the most common cause of this stall, it is the idle speed control air-bypass valve and throttle valve, they get full of gunk over the miles and cause idle issues (stalls) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake