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'91 Turbo with a JDM '93 swap
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Discussion Starter · #1 ·
Since buying my car I've been hunting a low idle.

It's a 91 turbo, with a 93 jdm swap and a 91 celica jdm ecu.No CEL

Idle when cold is 5-800. Warm is about 950. Have replaced all throttle gaskets, cleaned and verified the IAC works.

Runs rich when cold, no CEL. No real issues. I have another tps but messing with it while plugged in and off the car (twisting the thing on the back of it) makes no difference.
New plugs, distributor, wires.
No vacuum leaks. Very small exhaust leak that's hardly noticable.


Only thing missing is the radiator fan switch. Aside from that, all else appears okay. I'm stumped. What else can I check?
 

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Since buying my car I've been hunting a low idle.

It's a 91 turbo, with a 93 jdm swap and a 91 celica jdm ecu.No CEL

Idle when cold is 5-800. Warm is about 950. Have replaced all throttle gaskets, cleaned and verified the IAC works.

Runs rich when cold, no CEL. No real issues. I have another tps but messing with it while plugged in and off the car (twisting the thing on the back of it) makes no difference.
New plugs, distributor, wires.
No vacuum leaks. Very small exhaust leak that's hardly noticable.


Only thing missing is the radiator fan switch. Aside from that, all else appears okay. I'm stumped. What else can I check?
I, too, am seeing my gen 2 3sgte low idle when it’s cold even after a hot runner.
 

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On FI-148 in the 91 manual there is an explanation of how to check the idle speed control valve. The other thing to check might be whether or not there is a signal getting from your ECU to the valve. It's probably a duty-cycle modulated valve, so you would look for a duty cycle that changes when the idle is above or below the set point.
 

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'91 Turbo with a JDM '93 swap
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Discussion Starter · #8 ·
On FI-148 in the 91 manual there is an explanation of how to check the idle speed control valve. The other thing to check might be whether or not there is a signal getting from your ECU to the valve. It's probably a duty-cycle modulated valve, so you would look for a duty cycle that changes when the idle is above or below the set point.
As mentioned the Valve works and I can hear it click on/off when it’s being cycled
 

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I'm not an expert on MR2 engines, especially the 93, since I don't have a book or an example to look at, but I do work as an industrial control systems engineer. When a closed loop control system isn't working first you figure out if the sensor that measures the process (RPM in this case) is reading correctly. I'm guessing this would be the tach pulse here but you may want to check in the service manual and wiring diagrams to make sure. If the sensor is reading correctly, then you start looking at the actuator (the IAC). If you can block the air coming into the IAC try that and see if the idle decreases. If there is a vacuum tube between the IAC and intake, you could disconnect it and see if the idle increases. If those both act as expected, your IAC is either not responding to the signal it's being given from the ECU, or the signal from the ECU is not correct. Most people don't have an oscilloscope, but if you happen to have one (you can buy a cheap one for $50 that will work fine for automotive stuff), you can look at the signal the ECU is sending the IAC. It should be a nice, square
Pulse Width Modulation signal. If the signal does not look nice and square like the image in the link, I would do as @DefSport says and replace the capacitors in the ECU (sounds complicated but probably is actually fairly easy and very cheap). The duty cycle should go up as the engine slows and down as it speeds up (or vice versa depending of whether the valve is open or closed when disconnected). Some multimeters will also measure duty cycle, but they won't show you the shape of the wave, which might be an issue here. If you're just checking with a meter, I would expect the duty cycle to go to 100% as the engine nears stall and also go to 0% as the engine gets above the normal idle speed. If it never gets to 100% that might be an indicator that the caps are bad. You may have to figure out creative ways of tricking the ECU to do this, such as creating a vacuum leak or obstructing the air flow. After all that's done and everything looks good, you need to start looking for other inputs to the control loop, such as A/C status, temperature, etc.
 
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