I have been building and racing racecars for a long time, and one of my favorite things is removing unnecessary crap. In a 1.6l equipped Miata that spends its life at wide open throttle, the ISC or Idle Speed Control System just so happens to fall into that category. In this guide, I am going to quickly explain how this system works and then fabricate two block-off plates to remove the Idle Speed Control Valve and the Air Valve from the NA Miata’s intake path.
Understanding the Idle Speed Control System
The ISC in the 1.6l Miata contains two valves. One is under the throttle body and regulates bypass air around the throttle plate based on an electronic signal from the ECU. This is varied based on engine conditions such as temp and rpm. The other valve is an analog system that sends air directly to the center of the intake manifold when cold. This analog Air Valve closes as the coolant warms, leaving the Idle Speed Control Valve under the TB still electronically controlled. So, it’s a two-stage system, if you will.
The system is fed from a coolant loop from the cylinder head and passes through a port in the intake manifold. Upon exiting the manifold, the coolant loop attaches to the left-side inlet of the Idle Speed Control Valve. The coolant then exits the right side of the ISCV, continuing onto the analog Air Valve. It then exits the AV through a return line that attaches to the small nipple at the bottom of the thermostat housing neck on the front of the cylinder head.
Here is a diagram with a few notes to make this clear.
Benefits and Drawbacks of Removal
(Note: Before digging deeper, if you want to troubleshoot the ISC System instead of deleting it, there is a good write-up at miata.net.)
The benefit of ISC removal is simplicity. Removal includes deleting 2 to 3 coolant lines (more on that later). It also simplifies the intake path by deleting the line from the intake tube to the ISCV, opening the door to some very simple custom intakes. Also, replacing the analog Air valve with a block-off plate makes removing the fuel rail easier. On a racecar, fewer lines equals fewer failure points.
The only drawback I can think of, which applies to street cars, is that there will be no means of varying idle on cold starts. YMMV, but in colder climates, this could be slightly problematic, although nothing a slight turn of the idle adjustment screw wouldn’t fix at the cost of a somewhat higher static idle.
Idle Speed Control Valve and Air Valve Removal
I performed this work during the rebuild of the ‘ghettocet’ engine. As such, the coolant was already drained, and the intake manifold was removed. Everything here can be done with the intake manifold attached. Realize that you need to remove a little coolant or anticipate the spillage.
Starting at the top, use an 8mm wrench to remove the analog Air Valve from the cylinder head. Then, unplug the electrical connector from the Idle Speed Control Valve under the throttle body. Next, remove the pipe connecting the intake tube to the ISCV. Cap the intake tube, or you can run a simple intake like I do on the Super Touring 6 Miata for further simplicity.
Regardless, now separate the ISCV from the throttle body. I would advise removing the throttle body first for easier access to the three screws that hold the ISCV on. Also, note that these screw heads like to strip. To help, place the Philips head onto the screws and then seat them to the screw head by tapping the handle lightly with a hammer and ensuring good contact with the screw before turning.
Block-Off Plates
Removing the two valves from the 1.6 engine exposes the tracts these systems use. We must fabricate block-off plates to seal the passages and avoid massive air leaks—the ones on the intake manifold and under the throttle body.
I would generally use a 1/8″ thick aluminum plate to make these, but all I could find was steel, which works just as well. Making the plates is simple. Clamp the removed valves’ mating surface to the steel plate. Then, run a tiny drill bit through the valve mounts to score the steel plate underneath. Next, remove the valve and drill the holes on the plate to the proper size. After drilling the holes, cut off any excess material around the drill holes. Finally, clean the plates and rust-proof them with paint.
Installing the Plates
Clean the mating surfaces before installing the plates in place of the valves. Then use RTV silicone to make gaskets and bolt the plates in. (Note: Red RTV is overkill, but it is what I had on hand.) For the ISCV block-off plate, you can reuse the screws, although I recommend getting new hardware here. (M5 x .8 x12) The analog Air Valve bolts are too long and need shorter (M5 x .8 x 16) bolts.
Coolant Bypass
To finish up the ISCV delete, the coolant route should be blocked or rerouted. Since the coolant originates from the cylinder head, I prefer the reroute to ensure the factory flow remains in place. To do this, loop the coolant exit on the side of the intake manifold back to the return below the thermostat housing. This can be accomplished with one of the small hoses no longer in use after the delete. Check out the diagram below.
Finishing Notes
This is a straightforward mod to perform on any 1.6 Miata racecar. Removing several failure points in the process. For my purposes, I use the idle adjustment screw at the top of the throttle body to add another 150 rpm to the base idle. Even on the coldest days, this adjustment is adequate to sit on the grid and warm the car up. Once the vehicle is on track, the ISC system becomes irrelevant anyway.
I run this mod on all of the naturally aspirated engines. However, I can see a scenario where a forced induction car could blow air through the gaskets at higher boost levels. This could easily be remedied with a tactful weld or internal sealing. I will find out soon enough, as the’ ghettocet’ will soon get a turbo.
