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Rmp goes up and down

i have a 1996 ford taurus lx about 150000 miles…when i start the car the rpm goes up then drops to like 300 rpm then goes back up then back down…now if i hold on to gas padle the car runs fine then and i dont have any codes

A small vacuum leak can cause this. With the age of the car, aged and cracked vacuum lines with slight leaks are not uncommon. Check and replace all the vacuum lines you can reach

A vacuum leak is the first thing that came to mind too.
Vacuum hose is dirt cheap by the foot. Replace the lines one at a time to prevent crossing the connections by accident.

Check the brake booster for a vacuum leak in the booster diaphragm too. Rather than guessing, you could ask your mechanic to temporarily clamp off the vacuum hoses one by one to see if one of them causes the the rpms to stabilize.

Other things can cause this too. A faulty or sticking IAC for example, or sticking throttle plate.

It sounds like the Idle Air Control valve is hunting for the proper position.

The next time the idle does this, open the hood and with the handle of a screwdriver rap on the IAC valve. If the idle speed changes replace the IAC valve.


One other idea, if the battery has recently been replace, or the cables removed, this can confuse the car’s computer how to adjust the idle. When the battery is disconnected, it has to relearn how to idle properly. In some cases it’s been reported here that the only solution is to take the car to a dealership where they can use the manufacturer specific scan tool to teach the car to idle properly again.

I find out I need battery that has 800 amps…this car has a battery 630 amps in it…

I was wrong I need 650 amps and that whats in the car

i will and a video on here so you guys can see what’s rpms doing