1992 Factory Shop Manual Error Re: Idle Speed Control Check Procedure?

[LoveLearn]

Family member reported that they could not restart 1992 Lexus ES-300. Manual 5-speed, so towed it home.

I discovered that flooring gas pedal allowed restarts, but if I let rpms drop below 3,000 engine stalled.

Read many posts to learn what others had found in correcting similar symptoms. On later Lexus engines,

this is generally described as the Idle Air Control Valve or IACV. According to the 1992 Factory Shop

Service Manual, the culprit part is called the Idle Speed Control or ISC, sometimes referenced as ISC valve.

Ok, for those who have not pulled any of these apart, I'll provide a short functional description.

You're familiar with "poppet valves" because one is used at your bathroom sink's bottom.

The 3VZ-FE engine uses a fancy poppet valve to control air flow into the intake system. They

control that valve's opening with a cute little electrical "stepper motor." Concentric with the

poppet valve's air-sealing end is a screw drive sort of like a screw-drive garage-door opener.

Rotate the screw one way and it opens the garage door or in this case opens the Idle Air Control Valve.

Rotate that same screw the other way to close. That rotatable shaft is rotated by a little 12 VDC 4-position

electrical stepper motor. Unlike conventional 12VDC motors, stepper motors only move by finite increments

with each new instruction. Since this motor has for positions and circles have 360 degrees, each new

instruction only rotates 90 degrees. This works out very nicely in allowing your engine's computer to

make little adjustments to how open that ISCV is at any time. Each pulse sent to the next electrical

terminal pair rotates the shaft 90 degrees which moves the poppet valve 1/4 of the distance the

worm gear shaft is cut. Rather elegant.

Now these things get really gooped-up (technical term) with lots of non-volatile gummy sticky stuff

that drops out of gasoline. Anyone who believes text-book explanations describing air only flowing

inward though induction tracks would expect that no gasoline would be flying around that far

upstream from the fuel injectors to cause any precipitation and gummy deposit build-up while driving

thousands of miles. But in reality, induction systems act more like organ pipes with air pulses

driving in and out differently with different rpms and throttle settings. Back when I was racing

motorcycles, we sometimes saw gasoline mist appearing in air several inches back from carb

intake horns. While Lexus engines use fuel injectors their intake tracks still cause fuel-laden air

pulses to fly forward and back within their induction tracks, which is exactly why all the way

back to your throttle plate, they become covered with gummy trash that drops out of fuel on its

pulsing journey into the combustion chambers. This isn't all bad because all that extra movement

helps evaporate liquid gasoline. Liquid gasoline can't burn. Only when it phase changes into gaseous

form can it burn. So the more that is in gaseous form when combustion starts the less will have

to phase change during the power stroke which reduces piston pressure "area under the curve"

that rotates the crank. Faster phase change from liquid to gaseous form improves fuel economy

and improves peak torque and improves peak horsepower. You want it. A price we pay to get

excellent phase change from this pulsing system is this gummy stuff we need to occasionally

remove.

That gummy stuff tries to stick throttle plates closed, but your foot and the steel cable easily

overcome its ability to stick those parts together. But that little Idle Air Control Valve seems

to fall prey to this sticky goop. One way to address that need to clean is to pay $417 for a new

one and labor to swap out the new for the old. While the mechanic is doing that, he / she could

with little more effort just clean your original valve so it is as clean as new. Would you throw

away one of your best dining dishes if you discovered that one had sticky cherry pie remains

hardened onto it? Most of us would not. A locked-shut ISCV is about the same situation. But

resist that urge to remove the deposits with your tongue. (silly humor)

Where is that little electric valve? Stand at the driver side of engine and follow the longest rubber

tube that extends from the 3 inch diameter rubber air tube between the throttle assembly and

the Air Mass Flow Sensor. That longest rubber tube connects to your Idle Speed Control Valve.

Make sure no children are around to hear your language when removing all those little hoses.

Removing them is the most taxing part of this procedure. One of those contains sweet-tasting

engine coolant which can poison your cat if they get a chance to drink it. So be prepared to

capture about a liter of coolant before the slow stream stops. Or you can anticipate that and

drain about 2 liters from your coolant system before disconnecting those tubes.

The black plastic housing secured by 3-too-easily-damaged philips screws has a 6-post electrical

connector. To remove that connector you must compress a spring-loaded latch which prevents

unintended disconnection. Since it has been attached for a while, it won't slide off without a little

struggle. Compress the latch tab and pull while wiggling. Don't just pull as that's much harder than

breaking it free with some wiggling. Just trying to keep this as easy as possible.

Two 12mm hex-head bolts secure the valve to the intake manifold. If the gasket which seals this

valve assembly to the intake manifold is damaged in any way, replace it. Ours appeared fine and

I cleaned the mating surfaces well before reinstallation, so I reused it saving a trip to the dealership.

Naturally the book suggests replacing it whether its good or bad. Why trust mechanic's judgement

calls? I might write the same advice were I writing for a Factory Shop Manual. If there's a chance

that it won't seal well, don't reuse it.

Now your ISCV is in hand. If you want to clean it rather than just replace it, you need to remove

those too-tight-to-easily-turn philips screws. I broke them free by simultaneously applying torque

with two tools. Mainly I used a 4 inch Vice Grip brand pliers set and a very tight-fitting philips screw

driver. Hints when trying to apply maximum force to philips screws. Select only the most tight-fitting

screw driver possible. If you feel even a few thousandth of an inch slop clearance, it's not tight

enough. Tap the screw driver handle end with a hammer. Not with destructive force, but sufficiently

hard to accomplish two goals. First goal is to firmly seat the screw driver into the screw's receiving

pocket. Second goal is to "disturb" the screw threads which reduces their break-away torque

threshold. This may sound silly, but it works. Work smarter not harder. When turning philips screws,

most of your force should be pushing the screw driver into the ramp-like philips screw pocket because

torque tends to push philips screw drivers out of their mating pockets. Terrible design. The rest of that

hand's force is used to turn the screw. With both locking pliers and good screw driver you'll be able to

break those screws free without ruining their philips pocket heads. That's an easily made error.

Now you are about to lift off the black plastic cap which contains electric stepper motor windings.

Some parts will become free to fall as you remove that cap, so don't do it while standing in

your grass. Over a table would be much better. What are these parts? First are 3 identical short

steel tubing spaces. They go inside the screw holes that hold the cap on so when you tighten the

screws you don't break the plastic. They absorb the load. Also you'll see 2 thin washers about

1/2" diameter. They are not the same. One is flat while the other is bent so it can apply spring

load against the rotor shaft end which prevents slop and adds just a little friction so vibration can't

rotate that shaft. I did NOT see which washer fit in first. But on reassembly I put the flat washer

next to the plastic housing and the spring washer right below it which complies with conventional

practice. Also you'll see 2 very nice sealed ball bearing sets that should spin freely.

I cleaned this with a starting fluid spray can. Brake cleaner spray would probably do as well.

DO not remove the white lubricant grease from the worm drive. It needs to turn freely. The spring

washer applies enough end load to prevent it from rotating with engine vibration. Recycling an old

tooth brush works well for this kind of cleaning.

Prop the throttle plate cable control bell-crank to the open position so you don't need to hold that

open during cleaning. I used a plastic screw driver handle to secure the bell-crank open. Now use

spray cleaner like starting fluid or brake cleaner to dissolve the goop as far inside your induction

track as you can reach with rag wrapped on a thin stick. That tooth brush helps with this. My

companion was astonished that this clean-appearing Lexus has so much black nasty goop anywhere

as what came out of that induction track. Old socks with heal holes make excellent rags. Nice if

they are white so you can better determine if you are still removing goop.

I was about to reinstall this assembly when I decided to run the Factory Shop Manual electrical

test procedure on it. It says to test by simultaneously applying +12 VDC to the center pair of 6

electrical connections. One small alligator clip easily spans both. The other 4 connections are all

negatives, numbered COUNTER CLOCKWISE starting from Top Right as S-1, S-2, S-3, and S-4.

Factory Shop Manual says test closing by touching ground to these terminals in this repeating

sequence: 1, 2, 3, 4 and repeat. Sure enough, the poppet valve closed by many little steps.

Then it suggests testing opening by cycling through this repeating sequence: 4, 3, 3, 1, 4 and repeat.

No, I'm not making that up. Not only is it NOT the reverse order of the correct closing sequence, but

it doesn't even include position 2!! Very suspicious. I tested it using that procedure. It failed to close.

Yes it jumps back and forth by tiny increments, but it will NOT close.

Now in frustration, concerned that I might need to either replace that absurdly-priced valve

or rewind its tiny stepper motor winding if it had a short or open winding, I went on line. AutoZone

also shows this valve and they include Densco's test procedure. Closing sequence is the same as

the Factory Shop Manual, which tested good. But their opening sequence is: 3, 2, 1, 4 and repeat.

Why they don't just list it as 4, 3, 2, 1, I don't know. Maybe Japanese logic? But by repeating both

become the same sequence. I ran that Densco test procedure and the valve tested good!!

I reinstalled it and the engine now idles like new.

Be patient when you first restart and do install with the valve fully open rather than closed. The engine

computer will keep tweaking at that position one increment at a time until it restores normal idling speed.

In our case, it took about 10 minutes by which time the engine was full warmed. Top up with antifreeze.

If you're a person who would throw away a $417 dinning plate just to avoid washing it, by all means,

feel free to throw away your gooped up stuck Idle Speed Control Valve.

I know this was long and rambling and included more information than most posts. But if I had had

this to read before I would have had a less frustrating experience. Cost for R & R at dealer for this

problem is way north of $500. Some have reported as much as $800. Just our local 7% sales tax on

that would be another $56. Lots of choices. Hope this helps other 3VZ-FE engine owners which includes

1992 & 1993 Lexus ES-300 and 3.0 V-6 Camrys for the same years.

John

[soarhero]

nice post. Thanks alot.

[soarhero]

According to the original shop manual that I bought at Lexus dealer, the ISC and IACV are actually two different parts - at least for the 1992 ES300!

[sailboat165]

You've taken the time to write up a very detailed description that I appreciated as I'm sure others have as well. Thx