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Can only get full power with cruise control

1996 mazada protege. Has a hard time accelerating especially up hills. You can floor it and it just struggles getting up hills and such. It can do it just not quickly. However I discovered recently on a higway had the cruise set at 70 and hit a big hill that I know it does not like, it will generally drop to 55 getting up it. But this day with the cruise on it flew up the hill. Rpms went up, they never go up when I floor it, it just bogs down and struggles. But this time the car shifted and the rpms went up And the car flew up the hill.

Edit: this is an automatic transmission

Is this an automatic transmission or a standard?

Automatic transmission

The throttle cable may have slipped or stretched, so you aren’t getting full travel with it anymore. The cruise control has its own cable, which is why it’s able to deliver full power.

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Thanks that makes perfect sense

shadofax is correct but since its an older car,clean the throttle body and plate with aerosol brake cleaner to remove carbon deposits. You will feel a big difference after that.

Thanks will give that a whirl

If you can figure out how to see the throttle valve, with the car parked, engine off, ask a helper to press on he gas pedal to the floor while you watch to see if it opens fully. It would be pretty unusual for the accelerator cable to stretch so much this symptom would occur, so I’m guessing there’s something else going on. But worth the time it takes to make sure the throttle valve is fully opening.