Hey, yes the mechanic replaced the IACV and that seems to have been the problem. I'm hoping it works and I'm done with my problems for awhile.

 

Update #9,125,431:

The car was running ok for a day and then stalled again. Mechanic is at least looking to find a way to keep it idling but at this point, I'm throwing in the towel.

 

That sucks, but look back what I said at the end of page 2... In my experience:

1) Temporarily solved problems mean you're almost there. What you fixed wasn't the issue, but it was closely enough related to the underlying cause that you disrupted the problem while doing it.

2) IAC valves are reliable. The connectors are not. Taking #1 into account, clipping the bad connector on a new IACV might have resulted in a temporarily good condition, but a bit of driving returned that connection to what it was before. (It's not just theory, I've seen it firsthand on my first SC.)

 

So you think it's the connector then?

 

I mean...I thought it was the connector before you replaced the valve, and doing so just reinforced my opinion, so I don't see what there is to question.

 

Ok I'll communicate that to the mechanic. Thanks.

 

^^ Seconding t2d2 here.

spdim, I have replaced the IACV wiring connector and several other old connectors on both my factory engine‘s electrical harness and later on my GTE engine’s OEM electrical harness. Toyota connectors are extremely good and long lasting but in situations like these if one of them is suspect after 20+ years of service a replacement can be warranted.

On some part of the connector’s surface there is always a five-digit number embossed as part of the casting. It may be easy to find or cleverly placed depending on the connector’s unique shape. A high powered flashlight can help as well as wiping off any caked grease with a rag.

Take that number on the IAC-V connector, let’s hypothetically call it “12345” for the sake of this post, and then add: “90980-“ to the front of it.

In my example this would now read as: “90980-12345”.

That’s how you identify each electrical connector part number on Toyota/Lexus vehicles.

Take the part number that you have identified to any Lexus or Toyota dealer and have them order it for you.

If any wires going into the old connector are frayed or damaged in any way you can have a dealer parts counter identify the correct OEM “repair terminals” based on the connector part number (they have to look it up). Only order the number that you need and generally I recommend Toyota dealers for this because their prices will be cheaper.

Installing repair wires requires soldering and sealing with heat shrink tubing so leave that to your new mechanic if it proves to be needed.

Also to note: when replacing an old connector with a new one the terminals will not just come out.

Any super fine electronics screwdriver set can be used to remove the stopper/lock/catch that keeps each wire terminal locked into the connector housing.

Once the main catch has been removed or popped into the “service” position (design varies by connector type) then each internal plastic catch/retainer for each individual wire terminal has to be gently pried up one at a time to allow the terminal to slip out of the old connector housing. These catches are very fine and require a super thin and small flathead electronics screwdriver to lift up.

I always take a reference picture of the rear wire arrangement for reference during reassembly with the new connector.

...

This is all actually much easier in practice than it may sound in writing. It’s just a bit tedious to do

Your mechanic will be familiar with all of this but I’m sharing just in case you need to know the how to as well.

I think t2d2 is right. You are almost there.

Replace that IACV connector and see if that solves your issue. Again, if any wires going into it are frayed or damaged then those specific wires need new OEM replacement terminal repair wires applied as well (with soldering and appropriate diameter heat shrink to seal the connection again).

 

That's really helpful thanks.

Okay so the new mechanic did a work around, put the old iacv back in, disconnected it from the computer, and manually calibrated it to operate with specified RPM. He did some other stuff to make this work. It's not perfect but the car idles. Does this make sense? He thinks those morons at the other shop may have still put in a bad pcm but even if a new one were to be put in again, a new iacv would be needed. So this is a fix and the issue could still be something else besides these things like rotted wiring. How long this fix lasts, maybe indefinitely.

 

That doesn't make much sense. There's nothing to calibrate with these IACV's, as far as I know. They can be cleaned, but that's about it.

If you can get your hands on another ECU of the right year, that's certainly worth throwing in as an easy test.

 

^^ The ECU is located under the footrest area of the passenger side carpet underneath a heavy duty black plastic cover held on by two 10mm nuts. You need some small screwdrivers and prying tools (very cheap) to carefully remove the plastic trim that goes over the edge of the carpet and which clips into the metal retainers for it and the carpet.

Then under that black cover you can see what the ECU part number is on its metal casing.

Or... go by the model year, model type, transmission type and emission type (or by the car's VIN number) and with those correct parameters you can identify the right ECU part number for your SC in this thread:

https://www.clublexus.com/forums/per...s-by-year.html

If you do need a good ECU, sometimes Lexus still has certain part numbers but only some of them. And they are expensive through Lexus if they do happen to still have the correct ECU part number that you need.

There are various 1992-2000 SC ECU/PCM units on eBay at any given time. Automatic ECUs are very common. Also Tanin Auto Electronix can help you source a good one if need be since they also do repairs and diagnostics to our ECUs. They also keep a couple of SC shop cars on site for in-vehicle testing and final QC which is nice.

...

I don't think there is much if any adjustment in our Idle Air Control Valves. The ECU also has no adjustments so I don't know how the mechanic did what he said he did. That is unless he cut into the wiring harness and added extremely specific type resistors or something like that which would require extensive knowledge of the electrical wiring diagram and IACV operation via the factory manual's description all to make a temporary fix.

I would focus on getting a good correct part number ECU that has been capacitor serviced and additionally look into getting a new IACV either OEM or aftermarket from Rockauto if the OEM is unavailable.

And remember that not all dealers carry the same prices. Some are MUCH more reasonable than others if you are shopping for a new OEM part.

 

Isn't there a fuel cut VSV for automatics? I wonder if that's it?

 

Okay is the ECU the PCM? If it is, that's what was supposedly replaced in June. That shop has a 2 year warranty on work but was balking at replacing it again. The new guy I took it to said that might be it but the other shop would probably claim that if they replaced it again and it acted up, that it is not the problem.

I'm obviously a novice with this stuff and whatever the new mechanic did to get my car to idle and not stall with the old iacv, I'm not sure.

 

I'm assuming they are interchanging the terms.

This is a SC300 ECU. (Image from Tanin). It's located under the passenger floorboard behind a black panel.

IACVs are not calibrated.

TPS is calibrated. Maybe they meant that?

Also to note- if they just replaced this with some junk yard part, there's a small chance the capacitors are toast since this is a common ageing issue with these ECUs. That could be causing these issues as well.

There are 2 places that repair SC300 ECUs - Sia and Tanin.

BTW - Tanin no longer has the service on their website but they will do the repair if you contact them beforehand.



generic sc ECU

 

Yeah, ECU and PCM are the same thing. PCM is the more comprehensive term, referring to powertrain (including transmission) control, not just engine.

ISFFUN's comment about TPS vs IACV is logical, as far as what the mechanic may have been calibrating.

 

Any updates? Curious.