Any Smog Techs?

[92Lex]

A 96 LS400 came into the shop after failing at a Test Only station yesterday. I grabbed my scan tool and monitored all the parameters shown on the data stream and found the idle speed too high. The ECT reading was .7 volts, so I thought maybe the coolant temperature was lower than the sensor was indicating...I checked it with the infrared pyrometer and found that it corresponds to the temperature reading. There was nothing wrong with the ECT to cause the fast idle. I installed a new IAC valve but the idle speed is still too high...so I went ahead and adjusted the throttle stop screw..to my fkn amazement the idle still remains high.

Where should I go from here? PCM doesnt have control over idle speed?

[VBdenny]

I think they should idle at about 600 RPM. My local Lexus dealer told me that the idle is computer controlled and can not be adjusted. I do know that they car has to be well warmed up to idle at 600 RPM. WHen cold they idle as high as 1800 RPM which seemed excessive which was why I asked the dealership.

[flanker271]

My 92 idles at 450 RPM when warm without a/c on. With a/c on it jumps up to 650 RPM when warm.

[95LS400Bob]

I'd say vacuum leak. It's getting air somewhere to raise that idle speed.

[flanker271]

I'm getting very little air coming through the vents when a/c is turned on. Does this have anything to do with the vacuum leak?

[92Lex]

1800 rpm is normal on a cold start. I still cant figure out what my problem is though...Im not too exited about replacing the computer right now though..its an expensive nonreturnable part. Would suck if I have to tell the cutomer I cant do anything about it. The idle is computer controlled but the computer can only work within certain parameters. I was thinking maybe an air leak before the O2 sensor?

[SRK]

To generate the additional RPM the engine needs air, so an air leak between the throttle plate and the engine is worth looking into. The base setting of the throttle plate is also important to put the IAC valve into its operating range. A leaking brake booster can also cause this condition.

[92Lex]

Damn, guess what....I measure the IAC counts and it read 0....so I guess the PCM was trying to reduce the idle speed all along without any luck. I finally fixed the problem though, it was a massive leak from the brake booster! Thanks SRK, and everyone else.