The past couple months I've been focusing on trying to improve the driveability of my Evo, it has been through quite a battle the last couple years after a stock block explosion, and there were many small things that weren't quite right which added up to make it not the best mode of transportation.

Some of the problems I had were a result of debris which somehow made its way into the IAC motor opening. After a sudden increase in idle "hunting" or "ranging", I made the rookie mistake of using spray cleaners to clean up the throttle body before doing it the right way and disassembling. Spray cleaners and scrubbing might "look" like they're doing a good job at cleaning up the throttle plate and opening, but after doing some research I discovered a few things that I was not aware of when it comes to how the throttle body is setup from the factory. I'm putting the thread together to help out anyone else who might find themselves in the same situation I was.

Boost leak test showed that the high pressure shaft seals I had installed years ago were leaking. So I started by following this how-to:

https://www.evolutionm.net/forums/ev...aft-seals.html

I highly recommend the Mil Spec high pressure seals... some pics of the teardown..










The Mil Spec seals..





A little threadlocker blue on the plate screws..



Ok, now using feeler gauges to get the plate as centered as possible...



Here's where it gets interesting. By holding the throttle body up to the light, you can see a slight gap all the way around it where light (and therefore air) can get through.



I was having an issue where letting off the gas to coast up to a stoplight, the engine would stall or come close to it. Some of it was tuning, but some of it was from this excess air getting past the plate when the throttle is closed, think of it as "less control" the IAC has over the incoming air at idle.. if 100% of the air was coming through the IAC motor pathway (and the fast idle air valve pathway next to it) the idle air motor will have "control" over a larger percentage of incoming air at idle than if there is another path the air can use to get into the engine, such as this leak around the edges of the plate. From the factory, there is a black film used to seal this gap up, while letting a larger "gap" be used between the metal surfaces themselves (which is a good thing, since when we go and tighten down big t-bolt clamps onto the throttle body, you could run the risk of making the plate "stick" open

The black film used by the factory also looks like.. dirt, soot, buildup, etc... something that you will be compelled to clean Unfortunately using spray cleaners and scrubbing will result in the gap where light is shining through on my throttle body in the picture.

After doing some reading on the boards here I came accross a product by Dow Corning called "Molykote 321 dry film lubricant" which I was not able to purchase due to some type of restriction... however If you cannot find that, this product is a suitable replacement:



It's a dry film lubricant that is made for harsh environments and temperatures. I did 3 coats on the back side of the throttle and plate, letting it dry between them, and one coat to the front of the throttle. (intercooler pipe side)



you can knock down the "edges" a little with a light sanding, or just use 2 coats and they won't be so defined.. since you can't tell now there is an extremely bright light behind the throttle body in the last pic... let it dry completely, there is no restriction of movement and now the "coating" is restored.

This did wonders for drivability, even with tuning issues still present, the car no longer stalled out, the IAC has enough "control" over the airflow to keep it running even with lots of other gremlins that had not been sorted out yet at the time. I would recommend taking the time to center the throttle plate and check for this gap if you're going to be installing shaft seals in your throttle body, it's definitely worth the extra effort.

 

Great Info...

Just wonder can we just replace the TB with S90 70mm TB but maintain the 65mm OEM Intake Manifold?

 

Thanks! I'm not sure on your question, mine is actually stock, however the intake manifold is Buschur ported... at some point I'm going to step up to a ported throttle body but I've got a couple other items to get straightened out first..

 

great write-up!

 

Very nice, good to know

 

Thanks! and thanks for the guidance tracking down that garbage that ended up in the IAC motor opening!

 

Nice find on the 842! The Dow brand was expensive.

 

So I just made the same noob mistake . I found this.

LOCTITE Anti Seize, Moly Dry Film.

http://www.grainger.com/Grainger/LOC...Compound-5E202

I will pick it up tomorrow, and see how it works. ****ing california sucks, most of the stuff is not legal for purchase in CA. I cannot even buy Gray RTV here anymore.

 

it gets worse every time I hear something about it... no gray RTV???? wtf

 

Nice writeup Liqquid.

Yea CA sucks - cant buy gray RTV or a firearm worth defending yourself with.

 

This may be off topic but, does anyone know where you could aquire replacement screws for the TB plate to shaft?

 

Thanks!

 

Just ordered two cans of molykote (All my friends are having idle ranging issues not being fixed by the IAC sensor) for 28$ a pop shipped (ironically from a warehouse in cali) I'll give it a shot and get back at you guys.

OP - thanks a ton I have been trying to figure out why my idle has been dropping ever since i bought the car. I have already replaced 2 iac sensors hoping to fix it with no luck...

 

Any body have any ideas? Thanks in advance.

 

No luck at the Mitsu dealer?