Hello, I have a 98 S70 automatic that I’ve been trying to get back on the road. I am having some trouble resolving an issue with the RPMs. When i start the car it idles normally no problem. Though when the gas is pressed the peddle takes force to rev and the RPM rockets to 6 and stays there long enough to force me to have to turn it off. I just put in an aftermarket throttle position sensor assuming that could be the issue but it didn’t work. Any feedback is helpful. Thanks for your time!
Ive added a link to a video I took
https://youtube.com/shorts/0eltfF1OHMw?feature=share

 

Sounds like something is binding in the throttle cable. I would check that first. Was it doing that issue before the throttle position sensor was replaced. If not, then the "new" sensor might be installed incorrectly or is faulty.

 

A 98 has a throttle cable - should be simple to see if it is sticking. The throttle position switch won't make the idle get out of control like that - but your idle valve/iac/idle motor/whatever you want to call it - will - they can stick - have you checked yours for correct operation?

Idle motor should look something like this

 

Just a thought, disconnect the throttle cable and work the throttle where the cable connected by hand and see if it sticks or revs like crazy. If not, then the issue is in the cable itself. If so, then.....
Hoonk, I agree that the throttle position switch/sensor won't cause runaway RPM unless it is somehow binding or sending an incorrect signal to the computer (yeah, a long shot I admit). I also agree that it could be a bad idle air control unit, but I'm leaning toward the throttle cable binding up somewhere since the OP says the gas pedal is hard to press down. It only takes a little bit of throttle movement for these engines to really rev when they are not under load, and if the cable moves an inch and a half or so then sticks............. Either way, I'm interested in seeing what the end cause of this one is.