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Old 06-17-2019, 08:59 PM   #1
Kreiser42
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Default Idle surge, no engine light

Hey everyone,
Need some advice troubleshooting an idle surge. My car is a 2000 Impreza non turbo. Once warm the idle jumps up and down from about 1000rpm to 100rpm (almost stalls). Its quite sluggish until I get up to around 3000rpm then it takes off. It has no CEL and no codes stored. It had an intake manifold leak that I fixed but it did not fix the issue. Ive check for vaccum leaks, checked the coils and plugs, checked compression (130psi all around), changed the air filter, cleaned the MAP sensor, replaced the IACV and TPS. Where else can I look?? Im kinda at the end of my rope any help would be great

Thanks
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Old 06-19-2019, 01:19 PM   #2
yarrgh
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2001 2.5RS
Black Diamond Pearl

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Did you calibrate the TPS?
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Old 06-20-2019, 08:58 PM   #3
sub_sifu
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how and why would he do that? Just researched it, doesn't sound like an easy thing to do, or is recommended unless certain that's the issue?

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Did you calibrate the TPS?
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Old 06-21-2019, 02:27 AM   #4
Kreiser42
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No I did not calibrate the tps, honestly didn’t realize that was part of the process. However switching the tps sensor made absolutely no difference in running condition. I’ve done some more poking around I found the downstream 02 sensor has been messed with, the wires are cut and reconnect multiple times and I believe it’s an aftermarket sensor. Could this be causing the surge??
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Old 06-21-2019, 05:35 AM   #5
Aurielo24
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Quote:
Originally Posted by Kreiser42 View Post
No I did not calibrate the tps, honestly didnít realize that was part of the process. However switching the tps sensor made absolutely no difference in running condition. Iíve done some more poking around I found the downstream 02 sensor has been messed with, the wires are cut and reconnect multiple times and I believe itís an aftermarket sensor. Could this be causing the surge??
+1
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Old 06-21-2019, 12:53 PM   #6
yarrgh
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The downstream O2 sensor only measures if the cat works on your vehicle. On 2005+, it does help with fuel trims.

It doesn't matter if that's the original issue, TPS was replaced, needs to be calibrated.

There are probably threads/videos on how to calibrate the throttle position sensor. When you installed it, did you notice it's slightly variable? That's on purpose. At 0% throttle, it should read 0.45V to 0.55V. At 100% throttle, it should read 4.5V to 5V.
I've found that sticking a needle through the insulation right at the TPS is the easiest way to get a reading.

Did you remove the intake manifold? Did you tighten the throttle cable a tad tight?
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Old 06-21-2019, 02:39 PM   #7
Kreiser42
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Quote:
Originally Posted by yarrgh View Post
The downstream O2 sensor only measures if the cat works on your vehicle. On 2005+, it does help with fuel trims.

It doesn't matter if that's the original issue, TPS was replaced, needs to be calibrated.

There are probably threads/videos on how to calibrate the throttle position sensor. When you installed it, did you notice it's slightly variable? That's on purpose. At 0% throttle, it should read 0.45V to 0.55V. At 100% throttle, it should read 4.5V to 5V.
I've found that sticking a needle through the insulation right at the TPS is the easiest way to get a reading.

Did you remove the intake manifold? Did you tighten the throttle cable a tad tight?
Nah throttle cable was tightened right up from the previous owner I’m guess in hopes to hide the issue but I’ve loosened that off. Yes I did notice the tps was variable, didn’t use a multimeter to set it just kinda set the idle by ear as best I could despite the surge. I’ll have a tinker with that. I have also noticed the catalytic has been cut out and welded back in, doesn’t seem to be leaking but any issue there related to the 02 sensors?

Another quick note, it will stall with regular fuel in the tank not so much with high octane.

Last edited by Kreiser42; 06-21-2019 at 04:43 PM.
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Old 06-24-2019, 06:49 PM   #8
yarrgh
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You say that once the engine is warmed up, these symptoms occur. If you have access to the coolant temp sensor, check it against intake temp sensor (and ambient) when the engine is cold, then again after warm-up.
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Old 06-29-2019, 02:46 AM   #9
Kreiser42
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Quote:
Originally Posted by yarrgh View Post
You say that once the engine is warmed up, these symptoms occur. If you have access to the coolant temp sensor, check it against intake temp sensor (and ambient) when the engine is cold, then again after warm-up.
Yea seems like the symptoms start once the car goes to closed loop. So check the output voltage of the sensors? What is the thought behind comparing the 2 of them?

Thanks for your advice itís greatly appreciated
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