Another p0505 quandry...

Still trying to cure this high idle. I replaced what I thought was the ect. It was the sensor near the ignition coils. Found another on the back of the cylinder head. Thank you for the info. I replaced the one that goes to the gauge on the side of the motor. the one on back of head goes to the ecu I have read. Will replace that one but Can not find a part number for this sensor. Are they the same sensor?
thanks again for all the great information and ideas...

 

The sensor the computer uses has a 2-wire connector. The sensor for the cluster gauge has a 1-wire connector, it grounds itself to the engine.

No need to replace these without testing first. You can read the live data and see what the computer is reading for coolant temperature.

 

Still at this high idle issue. Finally got a good used TB with no light showing around throttle plate. Good used Oem IAC and TPS. Installed and idle higher initially for first few drives. But then settled down to 1200 rpms when warm and almost 2500 rpms cold! Same dammit! All new gaskets and smoke tested hoses yet again..no leaks.
Checked voltage thru IAC connector with ignition key on and got around 11.5v. .hmmm shouldn't it read 12v? this is on RSc and b+. No voltage rso and b+.
Also what about the a/c kick up valve? My compressor is disconnected and pulled fuse for a/c
Power steering pressure valve?
Cel on and reads p0505 still. Will reset for up to 5 seconds but returns.
Gonna tag out soon and bring it in......
Ideas ?

 

What temperature is the ECT reading after you replaced it? Did you replace the correct sensor with the 2 wires?

And is the ECU ground on tight?

The IAC receives pulses to keep it at a certain position, so you won't get a constant 12v. It gets very rapid on/off pulses, so hard to read with a voltmeter or even know what to expect.

 

Ect now reads 175ish on live data. Replaced the sensor under the thermostat on side of block...not the one in back near firewall. Will find where the ecu ground is and check that. Thanks. This is driving me nuts...

 

The PCM monitors voltages of the IAC circuits and if it detects an incorrect voltage on one or more of the IAC motor circuits it will set P0505. NOTE: Problems with the IAC would only be noticeable at idle or when throttle is closed.

Symptoms may include: MIL (Malfunction Indicator Lamp) illumination
Engine stalls and may not idle unless throttle is depressed
Intermittently stalls when letting off the throttle
Engine may idle too high.

Potential causes of an P0505 code include: IAC motor connector is damaged (check connector and pins for damage, and moisture)
One or more IAC circuits open or shorted to ground One or more IAC circuits are shorted to voltage
IAC has failed
PCM has failed

This should be a fairly straightforward diagnosis.
Start by clearing the fault codes. Then unplug the IAC and start the engine (This may cause other IAC codes to set. Just ignore them for now).
If the P0505 code doesn't reset, replace the IAC. It is internally shorting one of the IAC circuits.
If the code DOES return after unplugging the IAC and starting the engine, then you know it isn't the IAC motor.
Visually check the wiring harness all the way back to the PCM. You may need to visually inspect the harness by removing all the insulation off the wires. It is possible that the wires could be shorting to each other or to other wires in the harness. If there are no wiring problems, and the code keeps returning with the IAC unplugged, you can check the PCM by removing the IAC wires from the PCM connector (or clipping the wires in a location where they can be repaired easily) and then start the engine (If you're not sure, don't do it. Take it to a qualified professional). Now if the code doesn't reset there is a problem with the wiring. Check for continuity on all the IAC wires. Any resistance points to a problem. Check for voltage on any(At this point there shouldn't be any voltages on the wires because they're clipped at one end and unplugged at the IAC). Repair as necessary. But if the P0505 code DOES reset after removing the IAC wires from the PCM connector (or clipping them), replace the PCM, there's a problem internally on the IAC control circuit.

Read more at: https://www.obd-codes.com/p0505

This was well written, diagnosing SHOULD be straightforward like he says.... But we all know how that goes. I feel your pain. Feel free to talk it out and bounce more thoughts or ideas here, etc.
An important thing to notice is that at the root, this code is triggered by the ECU measuring the voltage of this IAC circuit. So yeah its high idle and all that jazz but that's symptom we see. So for some reason the ECU is seeing a fluctuation of the voltage in that IAC circuit. This could be wiring in the harness, could be issue with pins in the connector on truck end which is why new IAC didn't change anything, etc
Im not the best at explaining this kinda stuff, did that make sense?

 

Yes it makes sense thank you. I'm thinking a wiring issue but will disconnect iac connector and see if code clears. Good project for this rainy New York day. Thanks for the link..will read.
Grateful for the help. This has been driving me nuts for almost 2 years. Of note is the idle has been high for over a year with no mil..it only came on recently...thanks again

 

Well..tuff to clear code as it pops back up within 4 seconds but was able to clear code and shut off motor before it returned. Disconnected iac plug , started truck and code returned.
Following the wiring now. Any easy way to access that damn plastic harness protector ?
Wanted to visually inspect wires before removing glove box for ecu

 

Good continuity , no resistance from iac wires to ecm. Only resistance from white/ red wire from iac plug to ground. Seems ecu or pcm is the issue?

 

Man that's crazy but you troubleshot/diagnosed it correctly. It's just so rarely the ECU. That post I copied from another site is great general info, but is not the FSM for TOYOTA Tacoma.
The FSM will have the specific tests for your specific vehicle..... But I still don't see how you could come up with a different diagnosis.
The wiring is showing correct resistance meaning ECU should be getting the signals from the IAC.
It's a brand new IAC so I guess that part we have to assume, of find the FSM steps fro testing the IAC itself. I don't like having to assume but yeah.
So with that info, if your testing was correct, then it's the ECU.
You have anyone around there that has a Tacoma, has to be same specs of course, that you could try throwing their Taco's ECU in yours before you drop the coin on an ECU?

 

Hold up, how much resistance we talking? Is there supposed to be resistance red wire to ground?
Ground should have no resistance right?

 

No one I know has a gen 1 unfortunately.
When checking continuity I used my multimeter and watched the needle. Put one probe on the white/ red iac plug wire and the other to ground and needle swung half way to zero. Didn't get exact number. Will check again and make note. Wiring diagram looks like that white/ red wire goes to a vsv...pressure sensor I think.

 

Put probe on the white/red IAC plug wire going to the IAC or going to the ECU?

And was the connector to the ECU disconnected when you put the other probe to ground?

 

The iac plug disconnected at iac and disconnected at ecu when checking white/ red to ground. Wiring diagram shows wht\ red wire going from iac plug to egr vsv. Only the other two iac plug wires go to the ecu directly per diagram. I have not yet disconnected/ clipped the two iac to ecu wires from the ecu plug per the link you sent. This test seemed redundant to verify wiring vs ecu problem but will do today once the rain stops. Will also dig down to this vsv..think it's hidden under the throttle body and look for anything obvious. Wondering if this vsv has anything to do with this problem? Or mabey covid 19 got into the sensor!!!! Lol
I really appreciate the help...thank you again.

 

To cruiser guy and dr z...thank you for you help figuring out this issue.
Installed a used ecu and my p0505 code is finally gone!
Idle down to 850 cold and up to 960 when warm...I will take it! That is the lowest this truck has run in 2 years! Only drove 8 miles since repair so hopefully it will get lower...
Thank you all again. It seems I have the rare truck with a bad ecu causing the p0505 code.

 

You're welcome. I wonder if electrolytic capacitors on the ECU was the culprit. They dry out over 20 years and stop working. I know bad capacitors can affect the speedometer electronics. I haven't seen much investigation into Toyota ECU problems because they rarely go bad.

 

Man that is great news to hear! Especially since you were pretty darn diligent in your diagnosing and troubleshooting. Great work hombre!