2000 Buick Lesabre

Engine Performance problem
2000 Buick Lesabre 6 cyl Two Wheel Drive Automatic 91000 miles

I have a V6 Buick 3800 engine. I do not have a Scan tool. It started missing during normal driving. I checked under the hood and saw that the coolant tank was low. I replaced the upper intake manifold due to the miles, loss of coolant, and history with this engine type and the intake manifold leak. Miss in engine still present. I replaced spark plugs, miss still present. I checked spark plug wires, #3 was bad. Replaced wires, miss is gone. Vehicle drives normally. However, while trying to figure out what was wrong, I encountered this problem. While in Park, if you try to rev the engine past 4000 rpms, it surges up and down, seems to me to be not getting enough fuel or some sort of sensor problem. If you drive it, no surge occurs at 4000 rpms and it revs up in rpms normally. Since it drives okay, I am inclined to think it is sensor related due on only occurring while in park. Any ideas, I really do not have the money to buy a fancy scan tool, and not sure it it will tell me what sensor. FYI, fuel pump was replaced a couple of years ago due to bad fuel level indicator, so I do not think it is the fuel pump. Could be fuel filter or injector, but since it works fine in normal driving, leaning toward a sensor.
July 21, 2009.

Engines are made to operate under load at 4000 RPMs, not in Park, with no load. If it runs well in Drive at 4,000 RPMs, who cares what it does in Park at 4000 RPMs?
The engine is probably telling you that you're confusing it by revving it in Park, and warning you to stop it before you damage it.

Can you give me a logical reason to rev it to 4000 RPMs in Park? If it's to charge the battery, it's much better to charge the battery in the manner the car was designed, which is while it's running on the road, in Drive.

Oct 15, 2009.