Rough idle on startup when cold out

[Balthazar]

Hello,

Car is a 1990 ls400. Problem only exists when it is chilly out, maybe 55 F or lower. On startup, the engine sputters and idles low and emits white smoke from the exhaust. After about 20 to 150 seconds, the idle abruptly becomes normal again. The lower the temperature outside, the longer it takes to become normal.

I think it is not getting enough fuel because pressing the throttle a little does not make the problem go away. Also white smoke is indicative of a lean condition. The rpms do raise when I press it, but still sputters. The abruptness of the change leads me to think it is a signal based issue, not a plugged injector, fuel filter etc.

What could be causing it to not get enough fuel when it is cold out?

Possibilities could be

1) Intake air temperature sensor - reading the air as warmer than it is, therefore not sending enough fuel to compensate for the denser charge. I know this is part of the MAF assembly.

2) Mass airflow sensor - not sensing as much airflow as it should and lacking fuel, I know I have the Karman-vortex style MAF so cleaning is not an option.

3) Air leak - but wouldn't that problem exist at all ambient air temperatures?

What else could it be? The thing that confuses me is the way it abruptly fixes itself. It seems like the ecu is getting feedback from the oxygen sensors and compensating once they get warmed up. I just want to see if anyone has any insight on this phenomenon before I buy 5 parts and fail to solve the problem. I already replaced both main oxygen sensors recently.

[Exhaustgases]

White smoke is usually raw fuel and that would be a RICH condition not lean. It could be something to do with the cold start injector. And actually its the opposite of what you are saying its more like there is too much fuel pressure. I'm not sure of the duration of the cold start injector and it is shutting off and not leaking if all does clear up.

The time you mention means things like O2 sensors are kicking in ok since that time is about when it will go closed loop, so it could be something with the cold start fuel system or it could be the basic start and pre closed loop routines and that points to the ecu itself. Have you read about the capacitor problems that they can have?

Smell the smoke.  If its not gas then could be coolant and that would drag it down some till it clears out. Problem there is it would be all the time not just when real cold. There is a cold start switch that turns on the cold start injector and of course its related software in the ecu.

[Balthazar]

Thanks for the useful information.

I sure hope its not the ecu. I read about the IAC valve that leaks power steering fluid into the intake manifold. Could it accumulate power steering fluid before starting which then has to burnt off? Either way I am going to delete that and plug it up and see if that helps first... after the shop gets done with something else lol.

[Exhaustgases]

http://www.clublexus.com/forums/ls-1st-and-2nd-gen-1990-2000/261358-the-first-gen-power-steering-bible-16.html

For a plug go to the very last post #238, and that IAC can be fixed as well so don't toss it.

The plug needs to be done just like in that link. Anything else can and will end up leaking, it needs to seal on the seat. And yeah that valve leaking could be the cause of the smoke, and maybe the rough running to, didn't think of that. And if nothing has been done to the ecu it maybe needing the caps changed, its just part of getting old. There are even cases of the old solder on circuit boards breaking down and needing to be reflowed, just not to long ago read something about a radio needing that.