6G Celicas Forums

it is definitely set up in a slot-n-groove sort of contraption so that the valve will only work one way. If you take it off and flip the valve over the wrong way when you put the spring back on the valve will not be in the right place to control the airflow at all, it will be wide open and your **** will try to idle at 4500.
I had to suck on the vacuum port while I put my thumb over the little square hole the ISCV breathes through at the bottom of the butterfly to be sure I had the valve on the right side before I put the spring back on.
so it does prevent it from being All the way open. I imagine that if the magnetic coil completely failed that little valve would just spin around on there of its own free will which would really suck.

That ISCV looks very similar to the Idle Air Control (IAC) valve on my 2000 Tacoma that I was discussing above. The IAC valve is attached to the TB using four phillips screws (that are hard to get loose). In that photo, you can see at least two of the four screws and see how the ISCV is connected to the TB, just like the IAC is in the Tacoma. The central feature in this photo, where you see two phillips screws, is a cover that would cover the bi-metal spring being discussed. If I remember correctly, there was some kind of cover (less striking) on the Tacoma IAC.

I didn't take that apart since it seemed to be factory preset (notice the yellow paint in the photo?). Also notice in the photo that if you loosened the two screws you could 'adjust' the preload on the spring by slightly twisting the plate and re-tightening the screws. There may or may not be such a spring in the Tacoma IAC, since it is definitely computer controlled (i.e. there is a connector coming off the wiring harness that plugs into wires in the IAC).

I looked up the TB in my shop manual for my 7A-FE engine of my 94 Celica ST, and there was an Idle Up valve that seems to be controlled by engine vacuum, but no coolant hoses running to it. She isn't home today, so I can't visually verify that.

I DID take the IAC valve off of the Tacoma TB, and cleaned and tested it according to its shop manual. Seems the function of the IAC is to allow air to bypass the butterfly valve inside the TB when it is closed (IDLE), so the IAC allows some air to be going into the intake manifold during idle, else the engine could not start.

TESTING THE IAC: When I had the IAC off, and cleaned, I could clearly see the 'valve' was a 'shutter' (rectangular metal door that sounds just like the square hole reported by VavAlephVav) that closes -- or opens to let air bypass the butterfly valve). There is, on the Tacoma, an electrical connector to permit the ECU to control the opening of the IAC valve. There are three electrical pins going into the valve, the center of which carries 12 volts (when under test) and the other two go to coil windings that control opening and closing of the 'shutter'. Two kinds of tests are done on this valve. The first test is to determine the resistance of each coil winding. Though different, these resistances are both in the neighborhood of 20 ohms. The second test is comprised of putting 12 volts on the center pin and then ground on the other two pins, one at a time, and checking to see the 'shutter' opens with 12 volts across one winding, and that it closes with 12 volts across the other winding. The solenoid may be working against. the force of a spring to insure that a failure would leaves the 'shutter' open (to guarantee an ability to idle), but I cannot say for certain. I do not know if the spring in the IAC of the Tacoma, if there is one, is bi-metallic or not.

Perhaps the Celica ISCV is identical in function to the Tacoma IAC valve, but instead of a computer voltage controlling its opening, it uses temperature to control its actions mechanically, by causing a bi-metal spring to wind and unwind, whereas the IAC on the 2000 Tacoma the ECU uses Engine Coolant Temperature (ECT) to decide how much to open or close the IAC valve? But VavAlephVav mentioned that his valve had a vacuum port that he had to suck to determine which direction to install the bi-metal spring. There is no vacuum port on the Tacoma IAC.

This post has been edited by Langing: Sep 12, 2014 - 12:42 PM

yeah, the center tube on the bottom of the throttle body is a vacuum line running to the AC idle-up valve.

guys the reason there is coolant going through the TB is to prevent it from freezing open. in certain climates where its really cold and humid its possible for it to freeze open. thats bad. so the warm water prevents that.

It has nothing to do with the IAC. that is all ECU controlled.

Smaay is correct, but what is the purpose of the spring? The meaning of life? Why does everyone like Will Smith so much? Life's mysteries man...

The spring is actually a bimetal spring and the temperature makes it relax so that the valve will close further than it would with no water running through it.

In other words, removing the coolant lines to the TB may result in higher RPM idle, until the TB heatsoaks enough for the heat on it to transfer onto the spring and relax it.

Yes the IACV is ECU-controlled but there is also a mechanical aspect to it.

Which is why next time I'm working on the engine I'm going to remove the OEM unit and replace it with a simpler stepper-type valve from a VW or a GM engine. (standalone ECU perks)

Ford IAC's are easy to work with also, but they're kind of bulky, but they're nice and sell contained as well with just a flange needed.

With that strong throttle spring that immediately closes the butterfly valve whenever you take your foot off the pedal, there are really places cold enough with high humidity conditions that can freeze the throttle wide open? For real? Are you absolutely certain? Like people drive their cars on the Bering Sea, maybe? Any place where the temperature drops well below freezing, I would think the humidity drops correspondingly as moisture in the air freezes. That should be true unless you are tossing sea water through your air filter, TB, and into your intake. Yet here is a fellow from Chicago (pretty cold there) who claims that a fellow from Southern California (pretty not cold there) is correct in stating this as fact. Can anyone cite a reference to validate their claim?

Hey, I sorry if I seem stupidly stubborn, but I learn something new every single day, and I am happy being wrong when I am because I learn from mistakes. I've actually never heard of a throttle freezing (like from ice) wide open, and can't imagine a way for it to happen in a real car driving on earth. Maybe when it is "really cold" (like way below zero F) the intake air, being so damn cold, doesn't require much moisture in the TB for it to freeze, or maybe it is just at the point of freezing where it becomes a problem? Maybe it has something to do with the physics of the formation of ice crystals or something? If true however, someone please show me a reference so I can believe it. If not a reference, has anybody experienced this phenomenon personally?

I remember people on this website who swore to me that cast iron absolutely could not bend without shattering, yet, after spinning my wheels over that true belief for almost a month, I was finally able to demonstrate that proposition wrong when it comes to the steel that is used to make Toyota steering knuckles, plus I found six incidents posted on Toyota car websites where other people reported bent ears on steering knuckles. Some fellow on this website even reported Pep Boys heating the ears on his steering knuckle and bending them back in place (how they got them perfectly straight I'd still like to know). Without a shred of doubt, the ears on my steering knuckle were bent. Another one put in its place solved my rotor dragging problem.

In conclusion, when I examined the illustrations in my 1994 Celica ST shop manual this afternoon, I could not find coolant hoses going to and coming from the TB. So, what you guys are telling me is that, given the right weather conditions, my Celica's throttle can potentially freeze wide open?

Pressure drop causes the air to condense and cool to a greater degree than ambient conditions. That's why airplanes have carb heat elements to prevent icing. Given the right weather conditions the throttle could form ice which could cause it to stick or the IAC to stick. It's not going to create ice at WOT because there is less pressure differential. If it's going to ice, it would be at steady state low throttle cruise when manifold vacuum is the highest for the longest amounts of time, or when engine braking down a steep grade for a long period of time. It's more of a problem with carbs since the fuel charge is evaporating and removing more heat from the air. Pressure temperature relationships are cool. Get it?

Agree that pressure/ temperature relationships are cool. Sometimes they lead to counterintuitive realities. Thanks for stepping up to the question. What you said is making sense.

You did a slight gloss-over when you said "Given the right weather conditions the throttle could form ice." Can the throttle form ice without the presence of moisture, such as when the ambient air is extremely cold? In the deep of winter, even here in NC, the ambient air is quite dry, as any moisture has precipitated out. Homeowners frequently run humidifiers in their homes to keep some moisture in their inside air, to make themselves more comfortable, or to eliminate static electricity.

If I understand you, ice formation in the manner you describe (pressure drop), the weather conditions would be in the neighborhood of 'near freezing' rather than deep freeze conditions. For ice to form in the TB, there must be moisture (water) in the intake air. In a deep freeze situation there is no moisture, so no ice can form. Or is my understanding incorrect on this point?

Low temperature but high humidity. Say when it's foggy and just around freezing, like when you have snow on the ground but a warm front with moist air pushes through, all that humidity turns to a thick fog bank but it's still in the high 30's maybe low 40's. That's when throttle icing is likely to happen. Also at higher altitudes as well where you can have cooler air with higher humidity. You can use the same charts pilots use.

toyota makes cars that are sold all over the world. its cost effective to make them all the same way. chances are i wont see those conditions in So Cal. but if i drove up to North Dakota, i just might see those conditions.

My opinion is that the possible gain is so tiny and unlikely that the effort isn't worth it given the potential for things going wrong.

My idle is fine. And my car hasnt exploded.

Yes, and you live in a generally very warm and dry region. If say you lived north of the mason-dixon line and drive the car in the fall, winter, or spring, or take it on trips to different elevations and areas of the country I'd suggest against this.

Actually last winter we got a good deal of ice and I had no problems. But yeah, if I lived in an area where I feel this would be a concern I would of left it stock.

The odds of it happening are fairly slim since heat is conducted through the head, into the intake, and to the throttle body anyway, the coolant just warms it up faster when it's cold outside to make absolutely sure it won't ice. Once the whole engine and engine bay is warmed up then that coolant flow doesn't matter really, which is another reason why the bypass makes little to no difference since it would only possibly lower intake temps when the whole engine is cold, unless you have a non heat conductive intake manifold or throttle body spacer.

Bitter and Smaay. . . thank you for helping me to understand. I was gone all day today but was very happy when I returned to find that Bitter had cleanly defined the conditions under which throttle icing can occur, conditions that make sense to me as the only way it COULD POSSIBLY happen, given the laws of physics, and Smaay followed up with a rationale from Toyota's point of view. Since becoming a member it's been easy to see that the both of you are quite expert and skilled (relative to many others) in auto mechanics/engineering, whereas I am just learning, and normally I accept what you say without question. This time I needed more pinpointed attention to my questions than you offered until I challenged the fuzzyness of your answers. Now I fully believe that my Celica's throttle may very well freeze up under those precise conditions, so I should always be self-aware and ready, prepared with memorized actions to take to avoid a potential disaster should it ever happen to me.

Let me remind you of the patent I referred to earlier as a way to help those people on the fence about shunting their TB coolant flow. By letting engine coolant flow through the TB at start up, then when the engine is warmed up, shutting it down, the patent holders claimed that they were able to shorten engine warm up time from 250 seconds to 178 seconds -- the main object of their patent was to speed up the process of engine warm up -- and they cut out approximately 30% of the engine warm up time. They considered that a very positive advance in automobile technology. With Celicas that do have coolant flowing in the TBs, you realize the same speedup in engine warm up time. Defaulting that coolant flow not only subjects you to the possibility of finding yourself in a bad situation due to throttle freezing (most likely prior to reaching normal engine temperatures), BUT you also loose the advantage of a faster warm up.This all suggests that we should NOT do what this thread was begun to show how to do. For me, it is a null issue because there is no coolant flowing in my TB (94 Celica ST).

This post has been edited by Langing: Sep 13, 2014 - 9:53 PM

I'm about to reconnect mine onto my TB.

I'm only somewhat worried that it'll increase my IATs, since I'm boosted.

I'll bet it makes no discernible difference.