I have a 97 4runner with the 3.4 5vz-fe engine and have a strange problem.

At first the engine wouldn't start cold unless I pushed the gas pedal down then after it got warmed up it was fine. So I replaced the IAC, (probably should have just cleaned it), after that the idle went up to 1500-2000rpm and wouldn't come down. The CEL hadn't come on yet but after the iac replacement I drove about 1000 miles before the CEL came on, and hasn't gone off since.
I went through hell trying the basic fixes:
Checked the temp sensor and replaced it ..
Replaced the TPS,
Smoked tested the isolated engine first, then the whole vacuum system for leaks, there was one in the rear but the rest of the car is good.
Tried a new ECU but after all of this the idle is still high.
Bypassed the wiring harness and plugs from the ECU to the tps and temp sensor and iac.
All of this has made no difference and the idle is still high.
Checking the OBD and it says that the loop that is supposed to close after the engine is started never closes after all of this was done.
At this point I have found that the iac that is in there is working, and I have artificially fixed the problem by bypassing the plug, taking the 12v from the center pin and running the top pin to ground, thereby closing the iac which has lowered the idle to 650-800 which should be about normal. Because the plug is disconnected from the iac the CEL is still on. When I checked the iac plug with the key on but engine not running there was 12v in the center pin but neither the top or bottom pin showed ground, yet when the engine is running the iac opens all of the way like there is ground going to the open pin. I am going to check this later today and find out for sure.

Question is,,,,,,,, what condition would cause the signal to the iac to make it remain open? Why does it not work properly if I have replaced all of the components that govern the signal sent to it?
At this point , at lease I have the idle down to a normal level, but the CEL is still on and I need to get this car smogged but can't while this is going on.

Has anyone had this kind of problem??

 

The 4Runner uses negative grounds as switches, or to complete the circuit if you prefer, so you won't see a ground until the ECU completes the circuit when called for. What brand was the IAC, sounds like it is still your problem, maybe clean the old OEM one and reinstall?. I know I stay away from the IAC when I clean the T-body, they are very finicky, I remove the T-body and turn it upside-down and spray it out with T-body cleaner so none gets accumulated in the IAC bore, never use brake clean here.

What is the CEL code?.

 

Thanks for your response.

Unfortunately I don't have the old iac. The code that is coming up is P0505 which is supposed to be telling me that the iac is bad.

The thing is that I have tried 3 different new iacs including a stock Toyota one and every one didn't make the CEL go away. The one on now is not a stock Toyota but an Intermotor, and it seems to work.

What else could cause the CEL to come on with this code?

 

Well the ECU rarely lies, and you've checked wiring and the pins at the plug on the IAC and ECU. I assume you are deleting the code after swapping IAC's, and you would also have to run the truck 1/2hr through its paces before the IAC settles down and any pending data can be used to recalibrate the ECU functions for optimal performance and economy. Don't expect a brand new IAC to operate properly until it's done a full warm up and a test drive then check to see how it reacts.
MAF cleaned, throttle body cables slacked off enough?