Grand Marquis, sound from the car

[ksaman2004]

I have 97 grand marquis, 77000 mile on it. the problem is:
after shifting into the third speed, I start to hear a sound almost coming from the engine when I accellrate the car and temperature of the car is well above the half marker on the temperature gauge.

this sound is like knocking.
I have showed the car to many mechanics, two of them told me that the car developed carbon deposits on the valves and need to be decarboned. One told me to rebuild the gear and anohter told me to unload the car computer!

Any body has any suggestion about this problem? and if it is related to the engine overheating?

[Samurai0]

You must do alot of short haul driving (around town).

If your drive @ 65mph for about 2 hours straight on an open highway, that should be enough to 'burn the carbon' out.

It works.