To be honest i'm not sure what they do to test the ECU, but I know that most shops can do a service to see if the ECU is performing accurately if you suspect the ECU is faulty.

 

Thanks Murdoc. I still havnt gotten around to checking what is going on with my car but now im really suspecting that it could be either distributor caps or ignition coils. We'll see.

 

So its been a while since ive been on this forum. Up until this point my car was running fine with periodic maintenance, but now the bad idle is back. My mechanic and i agree the iacv needs to be replaced; i got 2 used iacv's and they both performed badly. The idle surges then drops, i want a brand new iacv but the Lexus dealer says that part for the 92-94 Lexus SC400 is discontinued. Now what? The so called good units online are a gamble. What can i do if a new part is no longer being made?

 

go standalone and get rid of IACV completely.

 

Is this something a mechanic can do or is this something for a DIYer? How would i go about doing this? Thanks for your reply

 

You need to explain this further for me, has it been done with success or is this a theory you have that it will work?

 

I think you need to re describe whats going on, maybe with a video. It can be lots of things.
Is the idle just sometimes high, and then sometimes low? or is it constantly swinging between the 2 every few seconds.

If it is constantly swinging, then the idle valve is probabaly ok, that is what the ecu uses to adjust the idle.
So if it wasn't working, the ecu would not be able to swing the idle 1200 rpms which is really quite alot.

I would look into the tps then as when the tps is bad or in the wrong position, it swings the idle exactly like that.
Anyone who works on toyotas knows that.. including the 2 people who posted in post 13 and post 16 who literally said it solved their exact same problem.
For some reason everyone just skipped over what they said.

Almost every mechanic who has no idea what they are doing fiddles with the tps and leaves it in the wrong position.
I would assume that is the kind of mechanic who would claim they do and say stupid stuff like the power isn't being distributed correctly.
The stock ecu is super sensitive to tps position. you can try loosening the 2 screws and turning it some to see if it stops doing it, or even just unplug the tps.
It could also be the fuel ecu or the main ecu having issues, both are known to go bad from time to time, but it 99% sounds like your tps.

If sometimes it is high, and sometimes it is low, without any swinging back and forth every few seconds, then yes a bad iacv could be the issue or an ignition component.
Your engine has 2 ignitors, 2 coils, 2 rotors and caps, and 8 plug wires, 2 coil wires and 8 plugs. just changing plugs and plug wires is not a full tune up on this engine.

 

A few days ago on first start up, the idle was high which is normal; but as the engine temp raised to the normal range the idle was still high. Normally when proper engine temps are reached, the cars idle is supposed to drop under 1000 rpms. Instead my rpms stayed at around 1500. As i drove to the shop i stopped and put it in park thats when the rpms started to jump from like 1000 to 2000+. When i got to the shop the mechanic adjusted the tps to no avail, and told me he would clean the throttle body and the iacv. The car started to run as it should but the following morning when i start it up, the car stalled. I start it again giving it gas to get the rpms up for a few minutes then it idled low but it was running so i bring it back to the shop and the tech checked it out and said the iacv had to be changed. I bought a so called rebuilt unit online and that part turned out to also be bad. With the rebuilt part installed; the idle would jump high between 1000 to 2000.rpms every 9 to 10 seconds and not change. So now im trying to find a new iacv to fix this problem .