:duh: I have an ongoing similar (but not identical) scenario whereby the car fires up, and runs rough (or stalls) for a minute or two (or mile or two) then runs fine, and starts first time until left until absolutely cold (overnight) when the process repeats itself.
So far have replaced the coolant temperature sensor, and adjusted the Throttle Position Sensor, and cleaned the throttle body, all of which made the car run better, but it still runs rough and stalls when completely cold, and once stalled is difficult to start. Have left it overnight with a technician to see if he can replicate and diagnose the problem. I have also had the inlet vacuum checked, and it showed 17 bar, rather than the 20 bar listed for the engine, but have checked all the hoses etc to no avail. Will keep the board posted, but have had a few false dawns on this problem, so lets wait and see.
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Good news! Got my son's Lexus GS300 back yesterday and it does not seem to fail like i originally stated. I waited till today to post because I wanted to see if it would start and run fine. My son got it back yesterday afternoon, we drove it, let it sit while hot, then started it-seems to run fine. He drove it some last night and went to work this morning and no dreaded phone call yet of any failure. Time will tell, but it looks good so far. Here's what the Mechanic found: The car is a 1993 GS300 with the straight six. He started from the beginning forgetting everything he did before and checked for all diagnostic levels that he could. Interesting thing he explained was that being that this car is a 1993, Lexus does not share some of its diagnostic code info. The car again showed no error codes, but after the car warmed up it would be very difficult to start, Idle would be around 1500 rpm when cold started in idle. This mechanic started his Mechanic school working on the Supra in 1993 while at Toyota. He got this Idea to fool the diagnostic computer and enter the data saying the Lexus was a supra with the 2JZ engine. (says its the same) when he did this a status log revealed that the throttle sensor position sensor which has an Idle contact swith integrated in it was stating that the car was not in the idle position when the car was actually in idle. To give some background on the TPS and what it does or effects I checked a great website called: http://www.autoshop101.com/ where they explain that the TPS engine mode; idle, part throttle, wide open sensed, air-fuel ratio correction, power increase correction, fuel cut control are things it affects. the Idle contact switch was the part that was out of whack. He made the adjustment to show that the Idle sense was correct when in idle-the rpm went back to 700 rpm in idle and the car seems to start every time. The Mechanic says that somebody had to change the sensor position throwing it out of spec. (now only the Lexus dealership) did any work on the thottle body when they changed the spark plug wires. Now I asked if the Idle control valve that was replaced was any good, and he said that it was bad. (I still question that answer since the problem was still present after it was replaced) If it was bad in conjunction with the sensor out of adjustment then the computer must have been making adjustments to muddy the situation. (He reminded me, there was an error code in the beginning that pointed to the Idle control valve that left after he replaced it) So, It runs now and lets hope it stays that way. That website i mentioned previously explains things with toyota's. I hope that this will help someone else. If the Mechanic was telling the truth about fooling the computer that it was a Supra engine of the same year, maybe you can have your mechanic use the Toyota adapter and info and see if it shows you more on your car. Otherwise you have to go to the dealership and trust that they know what they are doing and that they are "honest". I was told that cars from 1995 or newer by law had to reveal there diagnostic info to mechanic outside of the dealership. Thanks for your help and input. this is a great site for people like myself.