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Idle Adjustment


Old 08-12-15, 01:47 PM
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VA Idle Adjustment

I have a question concerning the IDLE adjustment on the RTEK 2.1

First off I have an S4 88 RX7 Turbo II.
There are 4 adjustments for the idle for me.
Idle Adjustment Screw
BAC Adjustment Screw
variable adjustment screw (done with RTEK)

My question is what is the best way to adjust this. The reason I am saying the is would you have the BAC screw screwed out first? Or does it really matter as long as you get it adjusted. because as soon as you turn other items if you go back and check the TPS it is different again.

Last question is the variable adjustment screw. By the time you get to this everything is adjust right already. Do you just move it all the way up to 5 volts and then readjust idle at the end?

Thanks for any input on this. Shane
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Old 08-15-15, 10:16 AM
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Shane, I've only worked on a JDM S5 but here are my thoughts based on days trying to get a good idle...

The TPS value at idle should not change if the car is up to temp and you don't touch the TPS adjustment screw. The TPS is simply a potentiometer. If it is changing then you probably have a bad TPS, the linkage needs lubing or it wasn't set correctly. Setting the TPS correctly is absolutely critical and the one thing most people screw up.

The BAC is the "bypass air control" or in the parlance of other manufacturers the "idle air control". The ECU uses it to control the idle by bypassing the throttle plates to feed extra air into the rotors. A bouncing idle for example happens because the ECU is working the BAC to chase a smooth idle but there are vacuum leaks or a poorly calibrated TPS "moving the target". In a well tuned engine the biggest obvious effect of the BAC is that when you turn on the a/c the ECU will keep the BAC open longer to raise the idle to handle the load of the compressor.

The S5s don't have the resistor based idle adjustment. The JDM S5 (at least) don't have the BAC adjustment screw which suggest to me they are obsolete or unnecessary.

My recommendation would be to read the S4 FSM and set the resistor and BAC screw to whatever starting point it recommends. And I wouldnt touch them again.

Then I'd read the S4 FSM and set the throttle linkage adjustment screw to its default. IIRC it should just touch the reaction tab at idle. Better to have a tiny gap than to have the throttle a tiny bit open at "idle". But read the FSM.

Then I'd check the TPS setting using your Palm. IIRC when the engine is fully hot (don't even think about the TPS if the engine isnt FULLY hot) the Palm will show the TPS running from 20 to just short of 100 which map to 1v to 5v. If you arent seeing that, search for Hailers and Hailers2 (there is another ECU guru whose screen name begins with S but I can't for the life of me remember what it is) and read their many posts on proper TPS adjustment. Many people half-*** setting their TPS and then whine. There is no "close enough" on the TPS.

If after all that you aren't happy with your idle (you never say what problem you're having) then you may be the victim of a kid - who didnt read the FSM - "tuning" the throttle body. If you search the forum you should find my thread on that process. There are some good ones on cleaning and painting the TB as well.

If it were me, I'd fine tune the idle rpm with the throttle linkage screw and leave the other things along.

btw, if you are tuning with the stock TMIC I strongly recommend going to HD or Lowes and putting together a PVC bypass pipe. You wouldn't want to drive with it but it makes the work you're doing much easier.

Good luck,


Last edited by vrracing; 08-15-15 at 10:20 AM.
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Old 08-17-15, 12:24 PM
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vrracing Thanks for your reply.

I seem to have figured out the idle pretty good. I am not sure if the TPS is bad or not. I don't think it is as it idles pretty good now. I would have it idling pretty good before, but when I turned on the AC it would go nuts. The idle sits at 750 now (well 740-760 moving around very little while looking at my palm pilot. I can even turn the A/C on and it is rock solid now at idle. I may try and find the FSM to check out what is in it as well, but for now I am pretty happy. I ended settings it by doing the following. Of course if I go back and check the TPS it will be at %20 and not moving due to the fact that I adjusted this multiple times and kept going back and setting it right and then adjusting everything else kind of fine tuning it I guess.

I have adjusted the TPS in the past by warming up the car and they using a ohm meter to set it at 1k ohms. I followed the guides I have read on here as well. This time however I adjusted using the RTEK and it saying to set it to %20.


Last edited by smikels; 08-17-15 at 12:27 PM.
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Old 08-18-15, 11:09 AM
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Idle adjustment is tough because there are a few adjustments, and if any of them start off way off, then it throws all the other ones way off. They all have to be in balance.

1K = 1V = 20%.
The TPS is a 5k potentiometer with 5V going into it, so every 1K in resistance, produces 1V of output. And since 100%/5v = 20% per volt, then the Rtek reads 20% at 1V (or 1k Ohm).
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Old 05-12-16, 01:34 AM
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My 1989 GTU is idling is very low. It idles around 200-400rpm. It's so frustrating.
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