Rough, irregular RPM @ idle and low acceleration - Subaru WRX Forum
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post #1 of 8 Old 01-20-2010, 11:28 AM Thread Starter
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Unhappy Rough, irregular RPM @ idle and low acceleration

Car: 2002 WRX
Vibrant UP
Vibrant Headers
TXS TBE with Magnaflow HM cat in place of TXS cat for better emissions
Blouch 19T turbo
Perrin Intake (regular size MAF; got rid of "big" one)
Hallman Pro RX MBC (@~15-16 PSI)
Tuned for these mods to ~270 WHP/TQ

My car does not seem happy. My tuner says that at low throttle, the A/F ratio compensation is all over the board, totally erratic and unpredictable. If it wasn't so erratic, he'd be able to fix it. But due to the randomness, it's not fixable. He theorizes that it's a partially faulty ECU, and that I should get a new one, transfer the tune to it and see if that fixes it.

I am not sure if that issue is related or not, but my car is running weird at cruising speeds. When on a flat road and just coasting down at constant throttle, it is not smooth - it is jerky and annoying as hell. It's not scary like a car running out of gas, but it's not smooth like it should be. It is enough to make my head move an inch while driving. What could be causing this, and how do I fix it?

Also, the turbo doesn't spool up smoothly... at WOT it acts weird when I just floor it. When I am gently in flooring it it behaves a little better, but still, it's not what I'd call smooth, and it does occasionally hiccup quite badly at times (enough to break the car loose on anything but dry pavement).

This whole project is getting quite frustrating since I've put over $3000 into my car to get it working (swapped turbo, new muffler, custom-installed cat, replaced bad fuel tank, new injectors, MBC, tuning and labor). Oh, and LOTS of my very-hard-to-come-by spare time. I think the only thing left to replace is the car...

To add insult to injury, I failed emissions today for the third time. Both the last time and this time I was "assured" that it would pass just fine. I am roughly doubling emission requirements now, but that's another matter...
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post #2 of 8 Old 01-20-2010, 02:53 PM
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moved to problem diagnosis

2015 STi: Still Stock
04 STi, 04 WRX: SOLD
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post #3 of 8 Old 01-20-2010, 03:04 PM Thread Starter
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Just to bump this to the top...
forgot to mention that I have RC 750cc injectors
Also, the current theory is that my front O2 sensor is bad, causing the closed loop issues at low/no throttle. This still doesn't fix the boost hiccups at WOT or my emissions.
However, I am still open to other opinions!!!
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post #4 of 8 Old 01-20-2010, 05:59 PM
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You have a video of the rough idle and running ?

I can't see an ECU doing all that since the tuner is the one messing with parameters.

How long ago did you replace the big MAF ?

I'm sure you have reset the ECU... right ? Do you get any CEL's?

2002 WRX Sold
2007 WRX Sold
2007 STi totaled
2007 Foz XT sold
2007 MS3
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post #5 of 8 Old 01-20-2010, 05:59 PM
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I hope your tuner can tune a MBC pretty well. Otherwise I wouldn't be using that at all. Taking control from the ECU with a MBC denies the ECU's safe guards. The tune has to be matched to the MBC. If its a bad tune, its going to run like crap...reguardless of a MBC or not. But, something doesn't make sense. I wouldn't be blaming the ECU first. Hopefully its the 02 sensor or something else.

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post #6 of 8 Old 01-20-2010, 06:42 PM Thread Starter
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Well I just popped in the AFR/front O2 sensor, and drove it around a bit. Same problem. This leads me to think/wonder that maybe the tune wasn't compensating for these readings, and now with a functioning O2/AFR in there, he'll be able to now compensate since he'll be getting stable readings and not voltage/current all over the board (he was showing me the percent compensation readings on the AFR, and at low throttle, they would fluctuate from +20 to -20 randomly). So hopefully with a new AFR this is fixable.
If that's not it, then what else could it be?
Yes, my tuner knows how to tune for an MBC - they sell a lot of them. The only reason I got one was because there's something wrong with my electronic boost circuit, and it was basically cheaper with no performance loss with the MBC versus the labor charge to chase down the problem and fix it.
I do not have a video - my iPhone is not new enough for that. But to describe it, when I try to hold the throttle at a given RPM (by choosing a point to depress the pedal to and keeping it there), the needly mostly stays at a single RPM, but every second or three fluctuates to a different RPM, up to 300+ RPM up or down, randomly.
I recently replaced the big MAF with a normal one, but I had my most recent tune done after that replacement.
As for resetting the ECU, no, I don't touch it. As I understand it, my tuner flashes it, but I'm pretty clueless on that part of the tuning process. I do know that I don't mess with the ECU. I had a CEL that showed itself about 24 hours after getting the tune done, and it lasted one day before it disappeared on its own. None since.
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post #7 of 8 Old 01-21-2010, 09:32 PM Thread Starter
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I took the car to my tuner, he fiddled and I still have the same problem. So apparently my front O2/AFR sensor was working fine. Grrr.
My tuner thinks that it might be related to a partially faulty ECU, but he's going to do some research and get back to me.
I've been doing some reading - could this kind of problem come from a vacuum leak? I'm not sure which hoses are vacuum hoses or how to check for a leak. How would I go about diagnosing this?
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post #8 of 8 Old 02-24-2010, 09:46 PM Thread Starter
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Okay, so update. My O2 sensor had to do a little learning (I think) because now the car drives smoothly in closed loop, cruising in a given gear (usually 3rd or 4th) at ~2500 RPM on flat roads, constant speed.

But, it still does weird stuff. I think it might be experiencing some compressor stall. Not sure how to diagnose that. I don't have a boost gauge yet because I want to get one that I can interface with my Innovate OT-2 for logging purposes.

I did do some logging for fun since the OT-2 just came in today, and that's attached. I thought that the STFTs were supposed to be more constant...?

I just feel like my car should be running more smoothly considering all the effort I've put into it to get it running well. If anyone has any input, that'd be great!

Edit: apparently I can't upload the file, but if you'd like to either a) tell me how to do this (it says that it can't upload it, it's a .csv), or b) have me email it to you, I'd be happy to comply with either.
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