Finally cured the above, and it cost me only a call out charge of £60You may have seen prevoius posts regarding my GS300 Mk1 (1997 with 127k), which started, but wouldn't run when cold, and idled erratically when warm, typically at 500 to 600 rather than 750 to 800 rpm.
I thought my mechanic had fixed this by thoroughly cleaning the throttle body and idle air valve (cost of an hours labour and two cans of carb cleaner). It was certainly better, but not right.
Chris, my mechanic, said he needed some help, especially as Lexus had had it in for diagnostic work, and charged £150 to say they couldn't find the fault. He rang Martin, a mobile auto electrician, who offers suport services to local garages, and has all of the latest diagnostic gear.
Martin didn't even hook up to his p.c., just listened to the fault, checked that the engine management light was not on, and in 3 minutes fixed the problem.
Here is what he did.
On the front of the throttle body are two sensors. One is the idle air control valve, the other Martin described as the "Throttle Pot". If you look from the front of the engine the idle air controller is on the left hand side, and has a grey electrical connector, and the "Throttle Pot" is on the right hand side, and has a black electrical connector.
The Throttle Pot is secured by two screws, one of which, at the top of the unit, has an elongated slot, allowing some positional adjustment, Martin loosened this half a turn, and moved the unit anticlockwise about a millimetre; the idle speed increased by 350 revs. He switched the engine off, and left it off for 10 secods, and re-started the engine, which still revved at around 1100 rpm. He switched off, left for a few seconds, and moved the unit clockwise by less than half a millimeter, and tightened up the securing screw. The engine was restarted, and idled perfectly. Marin then started and stopped the engine 10 times to ensure that the ECU had "learned" the new setting, and hey presto, solved. No £500 idle air valve, no £300 throttle controller, just 15 minutes, and 30 years experience. I have pictures of the unit, and will be happy to send them to anyone who has this problem.
