I recently bought a 1989 V6 manual trans Pickup with 233K on the clock. Starts every time just fine. If I throw it into gear and start driving right away it pulls strong throughout the entire rev band. But sometimes when I pull to a stop, the idle begins searching. It even starts searching if I cold start and let it sit for a couple minutes. When I say searching, I mean bobbing. Up, down, up down... steadily. The exact RPMs? Not sure. My truck doesn't have a tach.

• Now, when this idle bobbing happens the motor revs freely when I'm in neutral. But when I put it in gear and begin to pull forward, it totally bogs down and barely creeps forward. I mean like .5 mph. I have to limp it to the side of the road, shut it off, restart and do it all over. It NEVER dies or stalls during these episodes. When I shut the engine off and restart, it usually pulls away just fine until it starts happening all over again.
• Here's what's been recently replaced:
• fuel pump and fuel filter
• No proof, but the HG recently done by the PO
• I replaced a number of crusty and cracked vacuum lines
• New intake boot
• I also replaced the gas cap

It doesn't quite get to the 12 o'clock mark on the temperature gauge, but it sits at 11 o'clock. Not sure if this is normal for a Yota. My old BMW stands at attention at 12 o'clock. In troubleshooting, I unplugged the TPS while it was running and it stalled pretty quick. I've read about plenty of possible culprits including timing, TPS and EGR. My symptoms don't seem to quite line up with any of those. Like I said, it makes good power whenever it's not acting up. I've read about the coolant temp sensor and it seems like a possibility. I've also read about an air pocket in the cooling system could also throw the air/fuel mixture off. I've researched this a lot, as it's a big, big issue. Any input or questions here would be much appreciated!

 

Dirty/carboned up Idle Air Control Valve (IAC)? Or as Toyota calls it the Idle Speed Control Valve (ISC). Surging idle speeds is one of the symptoms of this.
Part Location - See Figure 12: https://www.autozone.com/repairguide...00c152801ccbd9

Guide to cleaning: https://automobileglobe.com/how-to-p...alve-cleaning/

You might want to de-carbon the throttle body too while you have the access.