Vacuum leaks or fuel pressure, or ECM.
When you press the accelerator you allow more air into the engine. The same extra air can get into the engine through leaks in the Vacuum system. Sometimes using a spray bottle of water and spraying the seams of the Intake manifold on a running engine can reveal the opening. Also spray any Vacuum lines and the Brake booster. A leak will change the RPM.
A high fuel pressure can cause an increase in RPM.
ECM- has a lot to due with timing and fuel demands.
Binding Linkage. Disconnect the throttle linkage and see if the RPM drops. If it does not, shut off the engine and work the Air horn Butterfly and check for smooth operation. It is possible that a mounting screw on the Butterfly shaft has slipped and the flap is binding or dirty.
SOURCE: 1997 Mercury Sable GS
Two possible sensors could be affecting it like the TPS or MAF sensor. The Throttle Position Sensor (TPS) might have a bad spot where the ECM won't get signal properly and causes the RPM to fluctuate
SOURCE: Car won't idle without revving when cold and dies when in drive
This Vehicle needs an Idle air control/Idle speed motor. Good Luck
SOURCE: 1996 Mercury Sable Wagon Overheating
sometimes the radiator is just too plugged to respond to flushing. (happens a lot with a marginal radiator because when flushing, the solvent simply goes around the plugged area, and cannot build up enough force to break it free). From what you have written, this may be the case. Is the electric fan coming on and off at the right times? if not this also can give you similar problems. You have done the right things but may not have gotten the desired result. Hope this info helps you a bit.
good luck
SOURCE: my check engine light came
Take it back and have them properly do,
what they should have done the first time
SOURCE: I started having a problem about a month ago. My
Check the number 8 plug wire to be sure it hasn't corroded or burnt through at some point. It wouldn't hurt to replace that wire with a new one. If yours is the 4.6 liter you also may have a bad fuel injector on that cylinder causing it to either burn lean or over fuel causing the misfire.
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