Increased speed without pushing on gas pedal - 2003 Mercury Mountaineer

Could be two issues. First could be the cruise control. I would try disconnecting the link to the throttle from the cruise and retest. If there is still an issue then your problem can be a throttle position sensor or the onboard computer. This is a hard one to diagnose without seeing the vehicle itself.

Is the car equipped with the Automatic Transmission?
If so, read on.
Otherwise skip to the end and answer some questions.

POSSIBLE SCENARIO:
I have observed a condition where my car surges slightly when the torque-converter clutch (TCC) cycles between lock and unlock when driving on an uphill grade.
First some basics and history that will explain why the TCC is used.

Engine, Torque Converter, TCC, and Transmission relationship--
The TCC allows for a solid connection between the engine and transmission which allows the input to the transmission to rotate at the same speed as the engine.
Without a TCC, there is slippage between the engine and automatic transmission. The slippage is greatest at low engine RPM. That is what allows the engine to run with the automatic transmission in gear, like when you first shift into gear or stop at a stop sign. When the throttle pedal is depressed, the engine RPM begins to increase and the torque converter begins to slip less and less the more the engine RPM increases. The car moves. But even at cruising speeds the torque converter slips slightly. Engine RPM is greater than transmission input RPM, which is realized as slight decrease in fuel efficiency.
When acceleration is complete and a constant speed is being maintained, the engine power output is reduced to the point where the TCC can engage and eliminate any slippage between the engine and transmission. If the car has a tachometer the engagement of the TCC can be verified when a slight reduction in engine RPM observed without a corresponding change in vehicle speed.
One method used to test the operation of the TCC is as follows:
Find a flat section of road where it is safe to perform the test.
Reach a steady speed and keep the gas pedal depressed with one foot. While observing the tachometer (or listening for an increase in engine RPM), with the other foot depress the brake pedal enough to activate the break light switch but not enough to engage the brakes. When the brake light switch activates, the TCC receives a signal to disengage. With the gas pedal being held steady, release the brake pedal and the engine RPM should decrease when the TCC engages.
Old cars with Automatic Transmissions did not use a TCC. I believe the TCC was put in use in an attempt to increase fuel economy.

MY EXPERIENCE WITH SIMILAR SYMPTOMS
The condition that causes that issue on my car is this:
- A slight uphill grade increases the load on the engine.
The car tends to gradually slow and it is necessary to depress the gas pedal to maintain speed.
- Depressing the throttle pedal (manually, or automatically with cruise control engaged) signals the torque converter clutch to unlock when the load increases slightly. (A more drastic load increase would signal the Transmission to downshift to a lower gear.) The corresponding increase in engine RPM and output is enough to compensate for the reduction in speed. When the vehicle speed, engine RPM, and throttle position stabilize to the point that the TCC will engage and the engine RPM will reduce in correspondence with TCC engagement. Now, if the road conditions have not changed, power output is not enough to maintain vehicle speed. With the increased load caused by full engagement between engine and transmission, and the cycle (surging) repeats itself until the road conditions change.

Does that help?
If not:

QUESTIONS
Please define the symptoms.
What are the road conditions when the surge occurs? (A slight uphill grade?)
What is the frequency of the surge?
Does the engine power output have a noticeable surge?
Is there a speed change related to the surge?
Does the tachometer move up and down with little or no change in vehicle speed?
Are all instrument indication in the normal range?
What else has changed?

Good luck!

Hello James,

If the engine revs up with no increase in speed, your clutch is slipping. Probably the reason the car was sold!

Alan.

the gas pedal has a sensor on it that control the throtle body. there is no more cables

Try increasing the idle speed. If you don't know, this is how to do it.
Open the bonnet and ask someone to push and release the gas pedal a five second intervals.
Observe which part is moving.
Yo will see a small adjustment screw on that lever. You will understand that by adjusting the screw you can increase the gas supply to the engine.
Slowly rotate the screw while the engine is running. (If the engine doesn't run in idle state as you have explained, ask someone to keep the pedal slightly pressed. After increasing the gas supply, ask the person to release the pedal).
Adjust it further to get a smooth running condition at idling.
Also check whether air cleaner is blocked.

a transmission flush may help, but try this first,without touching the gas,put the car in drive and let it get to top speed(about 30/35mph)STOP, then use 1/3rd gas pedal(do not move your foot)and let it accelerate to about 60/70mph,STOP....then,if safe,floor the gas pedal to about 60/70 mphSTOP.then drive as normal.
the gearbox is computer controlled and if you use kickdown a lot or you change gears yourself then it will be harsh,try only using 1/3rd of the gas pedal(boring)BUT you will see a massive increase in gas mileage/comfort....also check fluid level every 2k.

Have the idle bypass valve checked, also called idle air control valve.
If that valve is stuck in the closed position, it's not allowing the fuel to bypass the throttle plate to keep it idling, resulting in stalling, but will keep engine running if the pedal is pushed down.

Could be clogged cat converter or it could be timing out of adj.

First guess is that a motor mount is broken and lifts one side of the motor just enough to pull on the throttle cable.
Second guess is that the cable return spring is broken or missing. It pulls the injector throttle body lever closed when the gas peddle is released.