Previa hot idle irregular

Had this Previa since new. Has 220k on it. Has always had a rock steady idle of 600 rpm with a hot engine. Recently, however, it has developed some quirks. Firstly, the idle speed would periodically drop to 400 rpm. Next it would occasionally idle at a steady 800 rpm. Now, in addition, it will sometimes pulse between 600 and 800, and between 400 and 600. These changes have happened over a period of about 4 months. It still drives totally fine, but with the number of rebellious idle behaviors increasing I worry the next one will be more troublesome, especially if it decides it wants to drop lower than 400.
Any ideas where I might start looking for culprits?

I'd start with the Idle Air Valve. It's a gizmo under the throttle body that has a twisty thing that controls that amount of air that feeds the engine while idling, bypassing the throttle butterfly. There's an electrical connector on it that controls the current to a coil of wire that twists on a magnet on a shaft, which has a split segment that opens the air passage. All driven by the computer, of course.
The rotating valve/magnet/shaft can get sticky as dirt accumulates on the valve/shaft Clean it out if if you find it so.
Hope this helps.

OK, thanks. After reading up on how the ISC works - allowing air to bypass the throttle plate - I think you might be right. When it is doing its 600 - 800 rpm pulsing if I crack the throttle valve ever so slightly with the pedal I can get dead steady idle at 700. Will probably do a throttle body, ISC and EGR cleaning at the same time.
Is there anything else to do while it's all apart down there?
Anticipating a couple of days for this for a first timer, and with no garage and the prospect of wrenching in the street in a SLC winter this will probably wait till April if it will last that long.

In the mean time, if I was to adjust the throttle stop screw so that a tiny bit of air was passing the throttle plate would this cause any problems with the electronics? That stuff is still black magic to me.

jw

As it's still too cold out to be messing with the throttle body in the street I decided upon investigating a temporary fix to the undulating idle. Since cracking the throttle slightly open with the pedal solved the problem I took a look at the pedal mechanism to see if I could do anything there. With the pedal depressed to get the smooth idle at 750-800 rpm the pedal was about 1/8" off the return spring stop, so perhaps all I needed was a little spacer in there. I glued together three dimes, but it was not quite enough. Three cents, however, was perfect. So now it idles well at 800rpm, cold starting is still OK, and it was a 3c fix. I will keep you posted on whether it remains fixed over time, and also if I ever find the true cause.

And you saved 27 cents! :LOL2:

Since the 3c accelerator pedal shimming fix only lasted a few months before the symptoms returned I'm getting ready to look at the Idle Speed Control valve (as Toyota calls it) as suggested above.
There was a '93 fresh in at the junk yard so I got the valve from that and am looking at it right now on the workbench.
The resistance measurements per manual are in spec, but is there a way to check operation of the twisty thing itself while the unit is out of the vehicle?
As it sits right now, the twisty thing (TT - love it) is open about 1.5mm. The total width of the air slot is about 4mm. If I push on the TT with a toothpick there is some resistance to motion (is this because of the magnet/solenoid design?), but the resistance is smooth and it will move to the fully closed and fully open position with little force.
Is there anything better I can do to test the operation of this one before I stick it in the van?

Attachment 5617

I have a spare and it looks exactly as the one installed. The gap/ opening sits open just a bit. I had idle probs a bit back and cleaned the ISCV up really good and just made sure valve was freely moving. I took off magnet and valve shook back and forth freely/easily. Greased it up and after cleaning, all was good.
Idles perfectly. Never did try to run hot water through it. Resistance good; valve good; according to book.:thmbup:
Mine was really stuck with gunk before I cleaned it. My idle was fast.

When the old ISCV came out of the van it looked identical, felt identical and resistance-tested identical to the one from the scrap yard. When I installed the scrap yard one, however, it worked. Problem solved (until this one gorks!).

Just an aside - I got a bit excited during the diagnostic process when I read about tapping components with a plastic mallet to see what happened, and tapping on my TPS caused the ultra low idle to initiate. However, I found I could tap on the throttle body or the ISCV and it would happen too, so it didn't mean squat... the vibration just went through the whole assembly no matter where I tapped.

Yea both of mine tested good resistance , the problem was gummed up valve. It was stuck either too shut or too open. After a good cleaning the valve really moves freely with out magnet installed and when installed, returns to preset set opening. Some say it will gum back up quickly and should be replaced but kinda pricey, so I just cleaned. Hasn't acted up yet...best of luck!:yes: