IACV Technical Question

[lexus082]

I don't know the iacv inside and out so here I go. 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?

 

[tsirovy]

There are sensors on the underside of both camshaft/timing belt sprockets and below the crankshaft/timing belt sprocket. I am certain these sensors control tach readings. But i would suspect if your low idle shakes the car that not all of your spark plugs are firing. Do you have a CE light on the panel?

 

[lexus082]

Well I checked to make sure they were all firing by pulling each plug wire one by one. Each time I disconnected a plug wire from the cap you could tell the difference. So that leaves me to believe I'm running on all 8 cylinders. I wish I had a check engine light. No lights just a very low idle which causes the engine/car to shake. It hasn't stalled out yet. Is it possible my timing is off? Could that cause a low idle? Maybe the belt slipped a few teeth or something?

 

[tsirovy]

It is very probable your timing belt slipped a few teeth. Usually you will have diagnostic codes if your timing is off alignment. Pull the distributor covers then align your crankshaft mark to 0 and see where your camshaft markings align. Personally I dont assume that your engine idles low because of an iacv which causes the engine to idle higher normally. Making any clear diagnosis will be difficult without codes. However it probably shouldnt be ruled out. Once having similar symptoms with low idle I recall experiencing improved idle performance after replacing a faulty alternator caused by a powersteering fluid leak directly onto the alternator. Next you should confirm that your timing belt is aligned.

 

[lexus082]

If the timing belt slipped and was off a few teeth could that cause a low idle and or a misfire? I had the power steering leak already about a year or so ago. I replaced the pump and got a re-built oem Lexus alternator. I tested the voltage at the battery with the car running ansd it was at 13.6 volts. I thought it would be more closer to 14.2 volts or something. Should I also have my alternator tested? Thanks

 

[ozvenom]

idle speed .

I have found that the bearings in the idle speed control motor have been drying out and getting hard to turn so the motor cant adjust its self to the correct speed it is only driven by magnetic force so it does not take much to stop it working . Some of the stepper motors can be serviced if it has screws holding it together then it can be pulled a part and some oil injected into the bearings to free it up. i use an insulin needle as its very fine and i can get it past the seals. the later model ones are a sealed unit got to buy a new one.
Regards oz.

 

[stonewis]

These bearings can be replaced. The replacement bearings are cheap and the process is easy to do. After removing the bearings, just take them to any bearing store and they will match them. I replaced mine recently and it cost less than $15 US to do it.