3.4l High Idle. Tested Everything I Can Think Of

Joined
Apr 5, 2017
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central america
I am trouble shooting a recently acquired 1995 T100, 4x4, 5vz-fe with a high idle (1350rpm)

Ive performed test from the factory service manual to confirm proper operation of the MAF and the TPS. I pulled the throttle body and IAC and after cleaning them, the IAC also passed resistance and physical open-close test outlined in the factory service manual. I've checked the IAC Valve circuit on the ECU and it passed. The Engine coolant temp gauge also is working correctly based on OBD2 observations. The truck idles up to 1750rpm on cold start and drops to 1350rpm when the ECT gauge reads 180 F. The throttle body valve is closing all the way based on physical observation and 9.4% according to TPS.
Ive checked for vacuum leaks. Found none, but replaced any questionable vacuum lines anyway.

In desperation, I pulled the motor/magnet off of the IAC while the truck was idling. nothing happened. I manually turned the IAC Valve shaft and was able to close the valve off so the truck nearly stalled before I opened it back up. I turned the shaft so that the truck was idling at 750rpm and it set there at 750, but when I shut the engine off, reinstalled the IAC motor/magnet and took it for a test-drive it quickly when back to 1350rpm at idle.

So, why would the IAC Valve open back up and idle the engine at 1350rpm when based on physical manipulation in could close off the airflow and idle correctly???

Could this be caused by a bad impute to the ECU (O2 sensor, AC Idle up circuit) that just isn't producing a code for some reason?

Any Ideas?

I read on another forum that it could take days of driving for the ECU to "relearn" the engine's needs, so my next move is to just drive it around and hope it fixes it's self. haha
 
Joined
Apr 5, 2017
Messages
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central america
Does anyone know how many drive cycles it takes for the ECU to "relearn" after you've done work to the IAC and reconnect the battery? (5vz-fe, 3.4l v6)

I did five drive cycles yesterday and it was all over the place. It idled anywhere from 650 to 1450 RPM. It changed (sometimes drastically) each time I turned the truck off and restarted it. I don't know if this is normal, but I've never had a vehicle do this.

The OBD2 readings on the TPS are normal/as expected during drive cycles and it's not throwing any codes. The only thing I can think of is that the ECU is "relearning" or there is something wrong that is preventing the ECU from properly adjusting the IAC Valve.

Any thoughts?
 
Joined
Apr 5, 2017
Messages
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Location
central america
Drove it again today. a couple of 1/2 hour trips on the highway with no improvement. It continues to idle differently every time I turn it off and restart it.
so...
I pulled the ECU out of my other T100( It had the same part number as the ECU in the truck I've been working on) and put it in the this truck.
With the new ECU the idle rpms ranged from 550-1450 at start up and after 15-20 minutes of stop and go driving they would be 750-950 at idle. But when I turned the truck off and restarted it they could be any where from 550-1450rpm again.
Am I just not "resetting" the ECU correctly?
I've never done anything special to reset an ECU before. It always just seemed to work.
 
Joined
Apr 5, 2017
Messages
28
Location
central america
Ok, I figured it out.

It was the IAC Valve.

At this point I was petty confident it had to be the IAC, the ECU, both, or the wiring between the two. Since I continued getting the same results with a different ECU and all my testing of the wiring seemed to indicate good connections, I decided to replace the IAC. My thinking is that there must have been something wrong with the spring in the IAC since it had passed all the electrical tests and the ECU was clearly able to control it while the vehicle was running, but it would open or close the valve too much when the vehicle was tuned off, making the ECU try to figure it out all over again.

-Something I may have failed to mention earlier was that the bake surging stopped happening once the ECU bought the idle down below 1000rpm, so I think that was just caused by the ECU trying to lower rpm during breaking like it is supposed to, but having a hard time because the idle was so much higher than it was supposed to be.

Hope this helps someone else with their diagnostics
 
Joined
Dec 29, 2005
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Craig, CO
I was going to suggest checking if the throttle body shaft was sticking. My idle would hang and be all over. If I took my toe and lifted the accelerator pedal up sometimes it would drop. Turns out the shaft and all the washers and springs and stuff on it wears. The shaft was shifting rearward and letting the butterfly hang in the throttle bore. I found an e-clip that fit and was the right thickness, kind of by chance, and it re-centered the butterfly in the bore. Smooth idles now, drops as it should.
 
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