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coolant bypass mod

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Old Apr 27, 2007, 08:31 AM   #1
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coolant bypass mod

i did some research here on evom and on google about this mod... it seems like it's really a free mod that you can benefit from. i ran into dyno sheets where this mod actually made a proven difference. we probably will not get the same kind of gains as say, a LS1 would... but i'm confident that we will get something on our cars.

here's some background:

Originally Posted by Fourdoor View Post
The DSM and EVO throttle bodies have two mechanical and one electric control of idle speed.

The electric control is the stepper motor controled by the ECU that opens up a bit to raise idle, and closes to lower idle.

The first mechanical idle control is the Base Idle Set Screw (BISS) The BISS is a screw that blocks off a passage that allows airflow to bypass the throttle plate. You turn the screw out and more air bypasses the throttle plate raising your idle, turn the screw in and less air is bypassing the throttle plate and your idle goes down.

The second mechanical idle control is the Fast Idle Air Valve (FIAV) The FIAV is a plastic/wax plug that blocks off a passage that allows airflow to bypass the throttle plate. when this plug is cold is shrinks and blocks less of the air passage and more air bypasses the throttle plate raising your idle, as the coolant heats up, the plug gets warm and grows blocking more of the air passage so less air is bypassing the throttle plate and your idle goes down.

If you get ride of the coolant lines to the throttle body, the car will remain in "high idle mode" for a lot longer because the entire engine bay has to warm up before the FIAV will expand. The electric control can somewhat compensate for this, but if you live in a cold climate the compensation required will be out of it's range of adjustment and you will need to adjust the BISS on a seasonal basis to compensate.

Originally Posted by Fourdoor View Post
This modification disables the FIAV (Fast Idle Air Valve) present in the throttle body. When the throttle body is cold a "plug" shrinks down opening up extra air passages in the throttle body alowing more air into the intake manifold for a fast idle on cold start up. As the coolant warms up the "plug" expands due to the heat blocking off the extra air passages so that your idle drops back down to normal.

The regular idle air valve can compensate some if you bypass the coolant lines, but you may need to make "seasonal" changes in your BISS (Bass Idle Set Screw) rather than having cold weather automaticly compensated for by the FIAV.

Originally Posted by EVOeight View Post
In order to actually cause the throttle plate to stick, you would have to maintain a constant throttle position for a long time. If you are constantly increasing and decreasing the throttle position, you are causing the plate to move. This does not give the plate a chance to stick. On an aircraft, the throttle is put in one position and may stay there for hours. This can cause the plate to stick. I think in an automotive application we are mostly OK removing the throttlebody coolant supply...
Originally Posted by chatterton View Post
So I just bought a new 06' lancer ES and I've been looking for ways to increase HP without ripping a whole in my pocketbook. I decided to run a little experiment in regards to temperature difference. This is a before and after of temp difference on the throttlebody when doing the coolant bypass mod.


Test conditions:
Temps taken using a Kessler ST652 INFRARED THERMOMETER

Outside temp is 70 deg

Temp at TB is 84.5 deg
Car has not been running but has been sitting in the sun for a few hours. So the temp degree difference is expected.

Control conditions:

Car is started and A/C is on high.
Hood is open.
Waited until temp guage on dash got to normal operating range.
Took temp reading with car running at normal operating temp.

Again car is restarted and A/C on high and hood is open.
Waited again until temp guage on dash got to normal operating range.
Temp after DIY coolant bypass test:

As you can see there is a difference of 23 deg. before and after the mod. So does it work? I'd say so. The temperature stayed fairly consistant with the temp of the TB that I took while the car was not running.
Some may debunk these crude test results as a true air temp reading was not taken and the fact that the car was not driven. However, as a simple FREE mod.

This mod does reduce the temp of the incoming air. Since heat is the enemy when it comes to making power I think I'll stick with it.



sorry for making that so long... i just wanted to compile all the information i found about this mod here in one place. the way i performed the bypass was by removing both the stock hoses going from the head to the TB and from the other coolant line to the TB.

with both hoses removed i connected the two points to each other by using a length of 5/16" hose with clamps on either end. to plug up the connections on the TB itself i used 1/4" vacuum caps.

every little bit helps!

sorry for the low quality of the pictures... i used my cell phone--but you get the idea.

Attached Thumbnails
coolant bypass mod-0426071154.jpg   coolant bypass mod-0426071154a.jpg   coolant bypass mod-0426071154b.jpg  
Old May 3, 2007, 10:18 AM   #2
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adjustment, biss, body, bypass, coolant, dsm, evo, high, idle, infrared, kessler, lancer, mitsubishi, st652, thermometer, throttle

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