99 3.4 stuck at 3,000rpm, revving on its own when coasting

Help y’all.

One day, after 10 min on the highway, my truck starts revving up to 3k rpm on its own. My truck is manual transmission, so I put it neutral but it revs even higher.

I replaced the TPS, idling rpms climbed even higher to 3,200.

deep cleaned throttle body, replaced IAC, lubed linkage, shimmed butterfly valve, cleaned MAF sensor, checked for vacuum leaks, no change.

When coasting to a stop at a light, the rpms rev on their own erratically. Gas mileage is AWFUL. when accelerating, I remove my foot off the gas pedal and after about 2 seconds the rpms drop dramatically like I just dumped the clutch and the truck lurches. Almost like the butterfly valve is sticking, but it didn’t appear to be when I checked it during cleaning. This problem arose suddenly, which makes me think it’s not the butterfly valve position.

the truck is at the mechanic currently and they can’t find the problem either.

I did just recently replace the alternator, but I don’t know what that could have to do with anything. I replaced the tstat as well (I saw this fixed other’s problems on other threads) but that did nothing. I was experiencing a dead miss a while ago before I had the main fuse box wire replaced. This truck has a history of electrical problems, and the erratic nature of these symptoms makes me think it might be electrical.

I’m stumped.

 

My 89is doing similar, almost like it wants to idle as high as the revs go. Frustrating for sure, I’m gonna get her in the shop ASAP next week.

 

Have the mechanic check for a broken firewall where the throttle pedal is attached