Idle speed problem when car slow down - Subaru WRX Forum
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post #1 of 16 Old 06-23-2014, 04:21 PM Thread Starter
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Idle speed problem when car slow down

hello
since over week i have problem with idle speed. when i drive for exaple 30-50MPH (gear 3-4) and when i slow down before traffic light i still use gear 3 or 4 and i push brake, when car slow down up to 20-15MPH i push clutch pedal RPM drop dow up to 100 but often on 0, and car engine immediately power OFF, this is big problem because after this i cant control my car because i cant move drive whell and brake not work.
What i do:
- clean throttle by throtlle spray (i spray gently whole throttle and them clean by paper towel)
- i take off idle speed control valve and clean well
- i take off and clean MAF sensor by spray based on alcochol (that was so dirty)
- i use contact spray to reconect all conectors
after this i feel engine have got more "power" , but RPM drop down like before

Any ideas ?

PS
Sorry for my language

thanks
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post #2 of 16 Old 06-23-2014, 05:51 PM
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Any check engine lights? and what mods do you have done to the car?
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post #3 of 16 Old 06-23-2014, 05:55 PM Thread Starter
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no its not any ECU errors or check engine light
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post #4 of 16 Old 06-24-2014, 03:16 AM
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Most idle problems can be traced back to a vacuum leak. I would start by pressure or smoke testing the system.

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post #5 of 16 Old 06-24-2014, 08:25 AM
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Vaccum leak. And the 1st line on the gauge is 500 RPM.
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post #6 of 16 Old 06-24-2014, 05:36 PM
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He's saying the car is stalling, not just the common dip when coming to a stop that a lot of subaru's do, I've had my intercooler off and my car still ran at idle.
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post #7 of 16 Old 06-24-2014, 05:47 PM
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Do you have any monitoring solution, maybe an access port, OS cable or even one of those little blue-tooth deals? If you can put down a log with the stall we could figure out if its something the ECU is doing causing the failure. Could be a faulty AFR sensor of a MAF sensor. Without monitoring the ECU its rough to diagnose but If it idles smooth until its coming down off a 3rd or 4th gear pull it could be an AFR issue.

I didnt see any mods list on your profile so I dont want to make any assumptions, I would start with a log on whatever engine management your using. otherwise take it by your tuner and have them put it up on the dyno and replicate the issue, with it plugged in you should be able to ID the issue pretty quick.

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post #8 of 16 Old 06-24-2014, 06:25 PM
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Could also be your fuel pump going out, how does it drive normally? any surging or power loss under throttle?
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post #9 of 16 Old 06-25-2014, 03:41 AM Thread Starter
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Hi
Thanks for help. So when i drive normalny everything fine, no power loss, turbo worka fine. @arcticscythe i have bluetooth OBD II Scanner, can i use this + some soft for make a log when i drive?
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post #10 of 16 Old 06-26-2014, 04:52 PM
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Its been a while but i think the bluetooth app we used to use was called "torque". Im pretty sure it had a logging function. otherwise i would search your respective app store/play store for obd2 software and use it to make a log. just make sure it has parameters like AFR, Knock , AFR correction and the like

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post #11 of 16 Old 06-27-2014, 05:15 PM Thread Starter
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thank u i do this in weekend and try paste some log files
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post #12 of 16 Old 08-02-2014, 04:28 AM
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What are your LTFT and SFLT numbers? If LTFT over 5% at idle usually indicates a vacuum leak.

I suggest looking at these values at idle, 1500 and 2500 rpm. If these number settle out other than idle, it again is usually a vacuum most noticeable at idle.

As for me, my numbers were negative at idle. It was caused by a leaking fuel pressure regulator.
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post #13 of 16 Old 08-15-2014, 05:48 PM Thread Starter
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Quote:
Originally Posted by p51302 View Post
What are your LTFT and SFLT numbers? If LTFT over 5% at idle usually indicates a vacuum leak.

I suggest looking at these values at idle, 1500 and 2500 rpm. If these number settle out other than idle, it again is usually a vacuum most noticeable at idle.

As for me, my numbers were negative at idle. It was caused by a leaking fuel pressure regulator.
hi
can u explaim for me how check LTFT and SFLT please

thanks
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post #14 of 16 Old 08-16-2014, 02:56 AM
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Sorry for the delay.

PID = OBDII Parameter ID (STFT and LTFT are examples of OBDII PIDs)
LTFT = Long Term Fuel Trim PID
STFT = Short Term Fuel Trim PID

To get these ever changing values, you would utilize a scan tool. I prefer scan tools that graph since many of the PIDs change so quickly, and it is often handy to look at some PIDs together.

So, here's what I suggest. Hook up your scan tool, warm up your car to operating temperature, and have all accessories off, ensuring PCM is in closed loop. Using gas pedal, rev engine to 2500rpm; write down LTFT and STFT after they steady out, 30 seconds maybe. Using gas pedal, rev engine to 1500rpm; write down LTFT and STFT after they steady out. Bring engine to idle; write down LTFT and STFT after they steady out.

Reply with your results and/or watch the following videos:

To me, this two part series explains STFT and LTFT the best and how to use them.

Secret of Engine Problem Diagnosis- Fuel Trims Pt.1
Secret of Engine Problem Diagnosis- Fuel Trims Pt.1 - YouTube

Schrodingers Box, ScannerDanner, and EricTheCarGuy on YouTube are very good resources.

Lastly, I just had an IAC issue. The easiest test is to first reproduce the idle issue in your driveway, meaning idle is wacky (too high or too low). Once in that state, with medium energy smack the IAC metal housing (NOT the connector) with the handle of a screw driver. Maybe hit it three to four times with medium energy. If your idle slowly returns to normal, it is a stuck IAC. Again, I had the same issue, I cleaned my IAC to be spotless, but it still would stick in numerous positions, which caused too high, too low, and just plain weird idles. I also verified a stuck IAC by looking at the duty cycle of the IAC with my scan tool to verify the computer was trying to counter the current weird idle state.

Sorry for the long answer, but I hope you find something I said to be helpful.
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post #15 of 16 Old 08-16-2014, 03:07 AM
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From experience, I totally agree with Boo-Key Bear in that most idle issues are caused by vacuum leaks. The IAC is hardly the problem.

From experience, a smoke or propane tests are the best ways to find a vacuum leak. BTW, look on youtube. There are many really cheap smoke testers you can make at home for cheap.

But if you don't have those available, you could carefully visually inspect all intake tubing from the back of the MAF all the way to the throttle body. Then carefully examine all hoses on intake behind the throttle body.

Start your car, open hood, keep car at idle, and listen for a vacuum hissing sound.

The way you say your car is behaving, it is going to be a big vacuum leak, like the brake booster hose off, the throttle body disconnected from intercooler, or right after the MAF the intake came apart....
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