The check engine light came on my 95 Previa (98000 miles) and it started idling high once engine was warmed up.
I took it to my mechanic who put it on the diagnostic computer and said it showed "lean mixture".
He cleaned the mass airflow sensor and the light went out. On the way home, the light came back on. After 2 hours of searching and tweeking, he said he "thinks" it's the Idle Air Control Valve becuase when the engine is revved, it the rpm's do not come back down all the way. He put a carb jet in one of the vacume hoses (to restrict air flow) and it fixed the idle high and rev problem, but the check engine light is still on and it idles uneven in nutral when warm, but idles fine when in gear.
He said it was only a temp fix until I can afford a IAV.
Ever since then, the idle has gotten progressively lower when started cold, but only until the engine warms up. Now it won't idle when started cold without giving gas peddal until it warms up a bit.
He's not 100% sure about exactly what part it needs and I don't have the $$$ to start buying parts that won't solve the problem.
Any help is greatly appreciated!

First off, you don't need an O2 Sensor for the problem you speak of above. Your computer, when started cold, runs in something known as "Open Loop", which is a pre-set amount of variables that the computer defaults to when 1. The engine is cold and 2. If one of the main sensors goes way out of specs or ceases to work all together (this will turn your check engine light on). When your computer receives a given temperature from your coolant sensor and gets an O2 sensor reading, the computer then goes into "Closed Loop" which means your timing, idle, amount of fuel delivered to your injectors depending on engine load and such are NOW controlled by the computer. Since you stated that the problem is a "Cold Driveability Problem", the O2 sensor can be ruled out for that comes into play AFTER the engine warms up and goes into "Closed Loop."

Get a Hayne's Manual on your car and follow the steps for a "Cold Driveability" problem pertaining to your idle and you can find the problem yourself. Yes the IAV may be the culprit if the mechanic you took it to had to clean your MAF for if the IAV is open too wide (by a part that's giving a bad signal or a "Lying Sensor" that's out of specs), then oil will go back up the intake stream and leave oil in the MAF. My advice is to get a Hayne's Manual and take the carburetor jet out of the vacuum line, for the "jury rig" is only helping you when the engine is warm...insofar as keeping the engine's idle down when it's warm, but isn't helping while the engine is cold and warming up. With the restriction in your vacuum hose makes you feel all is good, it could damage other sensors for they're reading a vacuum that's not supposed to be there and calibrating your timing, fuel dwell, or delivery and O2 sensor to have readings, but Wrong readings.

I guess we need to know more information as to how high the idler's going, when it started and all the problems you had when the vehicle was cold. This appears as a Cold problem not a "Warm Driveability" problem or "Closed Loop" problem.

~Stillin~

Stillin, thank you very much for the detailed responce!
I'm in Hawaii and finding manuals such as a Haynes for a discontinued Previa will probably happen only by ordering online, but I will do so today.

As far as other info you need:
There were no cold drivabily issues at all before the "jury rig".
Before the jury rig, it would start and run fine cold, but once warmed up, the idle while in nutral/park would go from approx. 800rpm to maybe (I'm guessing) 1200 or 1300. Nothing really drastic, but enought to know there was something going on.
I look forward to any more comments or suggetions you may have...

Thanks again for your time and expertise.
Mike

Mike, I had the same problem on a BMW 325i. Other than the change in the names of certain parts...and possibly a translator, I'll put below what I wrote to a BMW "Stealership" that had my car for 3 weeks and still couldn't find the idle problem. Your idle going up and down the way it is, sounds like it's either your IAV (ICV for BMW) being wide open by means of a signal from your computer or if your vehicle has a separate module for your IAV (on BMW it's called the ICM, idle control module). Should you have three wires going to your IAV, then take a voltmeter and check the Resistance between the middle prong and each of the two outer prongs...mine was 40 Ohms and then measure the Resistance between the two outside prongs, mine was 20 Ohms, thus telling me the IAV (ICV) was good but getting a "bad signal" from the computer to stay wide open. One thing you need to do is change the oil after you solve the problem for the reason it idles high is the fuel injectors are trying to compensate for the idle, but putting too much gas in the cylinders thus the computer cuts off the fuel supply until the engine almost dies, then the computer uses the fuel delivery again until it gets too high and this process repeats itself. Your IAV may be getting a bad signal but to isolate your IAV, then voltage and Resistance tests are in order. If they check out good, then start looking at either the computer or a separate module that controls your IAV. I'll attach what I wrote to the BMW "Stealeraship" so that they could have a history of what and how my vehicle was running. I checked every single sensor and when they all checked out, I bought a Computer from Programma (has a 3 year warranty), and that solved my problem. Your O2 Sensor will always be lean in an overabundance amount of air that your IAV is putting into the engine. Before throwing parts into it, ask on this forum what the voltage readings and Resistance values should be on the IAV. The idle problem you have is a result of the IAV being wide open all the time and the computer attempting to correct it through fuel management. Your gas mileage will go to **** until this problem is corrected, but below will explain how the IAV or ICV works. It's esoteric in nature so you may need a translator...or print it out and take it to your Toyota Dealer and ask if they've had problems like this before. Remember, your O2 Sensor is good in reading a lean condition...especially with too much air coming through and around the throttle body. See Below:
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The ICV in my car does not have an Idle Control Module (ICM) but is controlled by the Motronic 1.1. The idle speed is controlled by modulating the amount of air which bypasses the throttle via an electronically controlled closed loop stabilization circuit. The ICV control signal appears to be PWM (pulse-width modulation), or "dwell" meaning the DME (Motronic 1.1) varies the duty cycle instead of the voltage to change the valve opening. The ICV's shutter appears to remain almost all the way open at all times except when checking the resistance of the valve or applying voltage to it to test it. Since it remains almost completely open, it appears that there is no "current" to the valve even though the ICV passed all voltage & resistance checks on my part.

In an ICM (which I don't have), is an analog circuit mounted on two circuit boards with a flexible connection. The circuit consists of an assortment of resistors, capacitors, op-amps, etc. It's job is to decrease the duty cycle of the ICV signal when RPM's drop and increase when RPM's rise, contingent upon it's various other inputs. Even if the ICV passes electrical tests, it could still be out of calibration.

I checked for vacuum leaks after adjusting the valves, tuning it up, torquing the valve cover back in place, replacing the air filter, putting Loctite® on the three fittings that go to the throttle body, running a fuel pressure check, replacing the timing belt, timing belt tensioner, water pump, thermostat and I'm sure you have a list of all the other things I performed. When it was running, I put my ear near the ICV and it seemed there may have been a slight vacuum leak between the ICV & the throttle body on the small 1" hose.(?)

On a car with an ICM, it would be relatively easy to unplug the electrical connector at the ICV and run the car in "open loop." The engine should idle up to about 1500-2000 and then oscillate back and forth between 600-1500 RPM's. According to my research, that if the electrical connector is then re-attached to the ICV and it has no effect on the RPM's, then most likely it's the ICM's at fault, however I could not perform that test since that is for a two wire ICV, not a three wire ICV and the DME controlling it instead of the ICM.

One thing I didn't try is to hook a voltmeter across the ICV terminals while the engine was running. Then introduce a "lean" condition by pulling up on the dipstick or opening the oil filler cap. If this was performed, there should have been a slight drop in voltage and then go back up until the idle speed stabilized. Since the idle speed is my problem it never would stabilize yet the DME would detect the voltage drop and then reduce the ICV current (opening it up more) to stabilize the idle. The ICV on my car seems to remain almost fully open at all times. If you had a scope, I guess you could check to see if the duty cycle changes instead. If you have an AC Voltmeter, check the AC voltage too. Since the signal is pulsating DC, you should get an AC voltage reading. If it's 0V (zero volts), then the DME isn't modulating meaning an internal problem with the DME. Since the ICV control signal appears to be PWM waveform, hooking up a frequency counter, the frequency should be fairly stable at about 145 Hz across various RPM's.

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Now with the ICV being controlled by pulse width modulation or "dwell" and all other sensors being correct, I've read that if the solenoid is disconnected, or failed, it will remain wide open. The idle rpm should go "open loop." A warm engine will rev until 1200 rpm's, causing the fuel injection system to intervene, cutting off fuel flow. Shutting off the injectors causes the engine to decrease in speed...cutting the engine out. RPM's drop back, and the fuel starts flowing again.This will give a surge from 700 to 1200 RPM's. Important to realize the idle stabilizer is wide open, so the RPM's are actually being controlled by the fuel injection system running then cutting out. (oscillation period for this is about 4 seconds).

It appears that what I've written above is the EXACT problem my car is exhibiting. It appears the fuel system is controlling the idle rather than the ICV or the Motronic 1.1. After chasing wires, values, sensors and the such for at least 18 months, I felt the least I could do was provide you with as much information as possible. Aside from what I thought I heard in a vacuum leak around the ICV's 1" hose (the one that directs airflow behind the throttle plate), the fuel system seems to be controlling the idle. Could it be that the car is ALWAYS in Open Loop? I've never had it idle correctly for me since receiving it as a gift from my sister.

In closing Timothy & Neil, if I can be of ANY further assistance, please do not hesitate to call me for I was once a Master Technician with Chrysler Corporation as well as their Washington Zone Technical Rep. Meaning, any problem car that no one else could figure out, was sent to me. Finally, I don't need the car back anytime soon, so should it become a headache to you or Neil, walk away from it for a while. I'm in no hurry. Please call me if you need me to come up there and assist you. Best of luck to you Neil as well as the many Technician's trying to assist with this "Problem Child."

Best of Luck,

~Stillin Saigon~

Thanks again Stillin.
I removed the jury rigging and as expected, it now starts and runs perfectly when cold, but idles high after warmed up.
I also found the supercharger to be leaking fluid.
So, I've ordered a used supercharger with throttle body and IACV from a 95 Previa with approx 125000 miles. I'll pay less for all these parts w/shipping than the cost of a new IACV.
If my problem is the IACV, then this should solve that. Either way, I needed the supercharger.

Outstanding! One thing I didn't mention is about where you live. I've had two beach houses in North Carolina and all my vehicles had problems with the salt air. If you unhook all your electrical connectors, check to see if you have corrosion in them. I use Di-Electric grease to keep the salt from giving false readings, but Vaseline® will do just the same. Hell, I even put Vaseline on all my outside light bulbs to make them easier to remove when they blow. I take it your Previa is not an "Island Car." {smile}

Best of Luck,

~Stillin~ (IMG:style_emoticons/default/57.gif)