I have a low idle. The only thing I haven't done is replace the idle air control valve and/ or the ecu to relieve the low idle issue... Here's the scenerio. Its a 94 sc400 (CA emissions) when first starting the engine cold the idle is around 750 rpms. Then as it slowly warms up the idle starts to curve downward like its supposed to. But it keeps getting lower and lower all the way down to the point where the car starts to shake.

Does this sound like the idle air control valve is working properly? I guess its working somewhat b/c the idle starts to lower as the car warms up. But it doesn't idle high enough when the engine is completely cold and once it warms up to normal operating temperature the idle drops too low.

I have cleaned the valve. So basically to sum up my question. Even though it idles down after the engine starts getting warm is it possible that the idle air control valve isn't working properly (as in something electronic)? As in opening far enough to let the correct amount of air pass by. Nothing seemed to be seized when I rotated the shaft inside the valve that moves in and out. There was no resistance when turning it by hand. Maybe something electrically is wrong with it and it doesn't allow the valve to open completely? Anyone know about these idle air control valves and exactly how they operate?

I know they receive the signal from the ecu to change the motor to adjust the idle and I know they get dirty.

Also what generates the tach speed? Is there a sensor that tells the ecu what rpm the engine is running at?

 

I had a similar issue with my sc300, although it never shook or stalled. I unscrewed the 2 screws on the ICV and rotated it right as much as possible (to force open the butterfly) and problem solved. I now idle ~7-800 w/ AC off and ~1100 w/ AC on

 

Have you checked for a vaccum leak?

 

right check vaccum's and make sure o2's are healthy (clean/working)

 

I personally haven't checked, but I had a toyota dealer check for a vacuum leak. I will probably replace all the hoses anyway pretty soon. I have replaced some so far.

 

Could a faulty/failing 02 sensor cause these symptoms?

I don't have a check engine light coming on, but I am pretty sure they are the original 02 sensors and they have 200,000 miles on them. I know your supposed to replace them every 100,000 miles. Just haven't had any reason to yet..

Since you guys are telling me to check other things do you think the iacv is working properly from what I have described?

 

Where exactly is the ICV? Anyone have a picture? That way I can tighten it and see if my idle will be better.

 

It is located right in front of the Lexus emblem.
Black round housing.

 

I don't really understand when you say unscrewed the two screws and rotated it all the way to the right as much as possible to force the butterfly valve open. Are you referring to the TPS sensor, Habitat?

I don't think you can rotate the idle air control valve itself.

Any possibility you guys think it might still be the idle air control valve? The first thing the factory repair manual states about the idle being off is to check the idle air control valve. So before i lose my mind replacing everything to fix this problem what do you think about my iacv condition?

 

Sometimes you have to clean the air assist hose connecting to the IACV/Throttle Body. I do that like every 3 months on my RX300 and makes a world of difference, otherwise it shakes on the idle and can die.

 

The hose that comes from the air intake that goes to the iacv? Its about an inch in diameter that one?

 

In my RX, there is a hose off of the IACV over to an area by the injectors, I would estimate several inches and that was quite dirty last night when I was cleaning it out. The IACV is a problematic issue with our vehicles.

 

I don't remember seeing any other hoses other than the one thick hose that connects to the intake plumbing. The other two hoses I believe carry coolant. Do you think that my valve still could be having some issues even though the engine idles down (rpms) from cold to hot?

 

If the timing belt jumped a few teeth could that cause a low idle and or misfire? Also when i check the voltage with the car running it was 13.6 volts shouldn't it be closer to 14.2 or something. I replaced the alternator with a re-buiilt Lexus denso model about a year and half ago. Someone said that when they had an alternator problem they started having a low idle. Should I have the alternator tested. Is 13.6 on the low side for a new re-built alternator. I don't have any check engine or battery lights coming on...

 

umm, your timing belt jumping a few teeth would be drasticly bad. it is probably fine. if you can run all the accessories in our car with the headlights on and they dont dim to a point where the alternator makes a whining noise, its probably got some life in it. i think i have a similar problem like yours.
im gonna try and clean the valve too, and check those small hoses, they are really small and makes sense they would get dirty

i know the iacv is part magnetic and part electrical signal on the sc, even if the electronics is shot it should technically work to some degree, although probably not a very good one. if yours is moving around well according to temp and load, i would think it works and there is restriction somewhere.
let me know if u find it