I've done a search on the subject and couldn't find any definitive answers so I'll ask you all again. I have the benefit of watching my Apexi SAFC in monitor mode while this phenomena is happening. Anything you experts (London BIll, etc.) out there can deduce is greatly appreciated!
First, let me say that I did notice several posts that had similar symptoms but different problems. There are several which I believe have the same problem as my car.
My car's vitals:
'92 SC300, 93k miles. Had all routine services done on time. Has K&N cone intake w/ cold-air inlet along w/ dyno-tuned Apexi SAFC.
Here are my symptoms
When car has been operating in normal to hot temps (more prevalent in hot temps), idle stumbles to less the 600 rpm and eventually stalls. This happens when foot is off the accelerator and the throttle position should be zero. SO this could happen when your foot is on the brake pedal at a stoplight, or when you are coasting on the highway, whenever you take your foot off the accelerator. This occurs without the AC on. Turning the AC on bumps up the RPM, but essentially masks the problem. Sometimes you can temporarily alieve the symptoms while sitting in traffic by going into Neutral and stabbing the accelerator to move the throttlebody until it returns to 0% closed position when you take your foot off the accel pedal).

When this rough idle and stumbling occurs, I do notice on my Apexi SAFC that everytime the throttle position value starts to wander above the zero % value - w/ the foot off the accelerator! Essentially, the car thinks that the throttle plate is open (when it really is not since your foot is off the accel) and adds too much fuel to eventually stall the engine. I am fairly certain of this.

I am hoping that this little bit of info will add some help in diagnosing this common problem.

What I don't understand, and this is where I need your help on, is what causes this? I've already cleaned the throttle body numerous times. The last time it happened, I immediately checked the throttlebody and it was very clean.
Is it the Idle control valve?
What is the difference between the ICV and the IAC?

Vacuum leak? I replaced about 95% of the hoses w/ aftermarket silicone stuff. Some weren't the exact same size, but I thought it was close enough. Am I wrong? Is this a no-no?
The only hoses I could not get at were the ones underneath the throttle body, those that you cannot reach unless you took the tb off. hmmmm

I haven't seen any recent posts on this. Not sure if someone found the solution or just let the issue die. If you've found your solution, pls post.
THANX in advance!

 

Check your throttle position sensor. They are adjustable on my 93 SC400 via 2 slotted screw holes, just loosen them and rotate. You'll need various thickness feeler gages, a good ohm meter (Fluke), and the factory service manual.

DJL

 

thanx for the reply. Is the TPS prone to such issues only when it reaches a certain temperature? My stalling issues do not happen all the time. There is a correlation to ambient/operating temperature, the hotter it gets, the more likely it is to happen.
I've got the shop manual, can you point me to where I can find the procedure?

 

Yes this is an old post, but did you find a solution to the problem. I am having the same problem with my 95 sc300 and curious if there is a solution now.

 

My first reaction to your symptom - if it happens after the car is fully warmed up, it may be delivering too much fuel because the ecu thinks the engine is still cold and is still delivering a rich mixture - - I have a v8 so my manual won't help, just goin' on my gut here - -

 

My 94 with 117K started doing what you describe at about 100K. I finally had enough and had Lexus dealer diagnose. A day later he suggested wires, caps, and rotors but atleast wires since they sparking all over. Did only wires (oem) and it runs like a champ now. Dealer say's wires tend to die at or near 100K. Also said that if the wires didn't solve the problem then the next thing was to test the coils.

P>S> Had plugs done at earlier service so thats why he didn't suggest them.

 

Check TPS. Should be about $90 for the toyota part.