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Old 07-28-2016, 10:37 AM   #1
GuitsBoy
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Default Engine stumble in first and second gear after catback

So I recently replaced my 135K mile rotted out stock exhaust with a 3" free flowing exhaust, and now Ive been noticing a little bit of engine stumble while accelerating from a stop, primarily in first and second gear. It seems to happen between 2-3K rpm, and at about 15% throttle. The shifter shakes back and forth in my hand a bit as it sweeps through the rpm range, then smooths out.

Car is a 2006 WRX with 135K on the clock, and is stock aside from catback and a mild canned tune. Fueling looks good, and my fuel trims are only a few percent off. I have swapped o2 sensors, and replaced the MAF sensor as well. Ive been all over the motor checking for intake leaks and exhaust leaks (none found), and Ive also recently cleaned the throttle body. Plugs, injectors, fuel filter are all about 5k old. I have removed the air system with blockoff plates at the tubes. Knock sensor was replaced maybe 15K ago, and I am chasing some false knock from rattling heat shields, but I dont think thats coming into play here.

I'm currently running gabedude's R30 rom, with some minor tweaks.

I'm beginning to think it might be a bad plug, or bad injector. I had replaced these while trying to troubleshoot a misfire at idle, which wound up being valve clearance. The lifter buckets have been changed out, so that issue has been addressed. Compression and leakdown tests were all normal at the time. I'm thinking of maybe putting the stock injectors back in, since they weren't the original problem anyway. I can check the gap on teh plugs as well.

Any suggestions on what I should check? Is this a fairly common occurrence with aftermarket exhaust and reduction of backpressure? I would think the turbo has enough backpressure on its own, but maybe not at such low loads.

Thanks for any suggestions.
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Old 07-28-2016, 11:10 AM   #2
BATmoWAGON
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Did you get a new tune with the new injectors? Are they the same size as your original?
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Old 07-28-2016, 11:15 AM   #3
GuitsBoy
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Originally Posted by BATmoWAGON View Post
Did you get a new tune with the new injectors? Are they the same size as your original?
Stock reman injectors. No significant fuel trim change before and after.
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Old 07-28-2016, 11:22 AM   #4
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So, which is it?

You replaced the whole exhaust w/a 3" free flowing exhaust or just the catback?

If it's just the catback, then should be no issues.
If you replaced the whole thing, then you need a re-tune. 2004+ WRXs became very sensitive to even an exhaust change ( anything before the catback ).
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Old 07-28-2016, 11:27 AM   #5
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Originally Posted by Samurai Jack View Post
So, which is it?

You replaced the whole exhaust w/a 3" free flowing exhaust or just the catback?
I'm still running the stock downpipe and cat. But the engine shake was only noticeable after the new catback. Its a very subtle shake, and I doubt a passenger would be able to detect it, but with my hand loosely holding the shifter, its noticeable and has become a bit bothersome to me.
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Old 07-29-2016, 09:32 AM   #6
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If the motor ran fine before you changed the catback ...
- AND the car has the shakes after you changed the catback
- AND you changed nothing else

the I would say there is something wrong with your canned tune

OR

the shakes were there all the time, and you are only now noticing it because you changed something
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Old 07-29-2016, 09:53 AM   #7
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Yeah, I'm leaning towards them having been there the whole time. Its not noticeable unless I have my hand on the shifter. And even when looking for it, it doesnt seem to happen every time. The thing that throws me is that it is most noticeable in the first gear sweep, and less noticeable in second gear. By third gear or higher, there is no shake in the same rpm range. But if I'm really nit picking, overall partial acceleration seems really choppy, as if one cylinder is not as strong as the others.

I dont see anything wrong with fueling, and timing seems smooth enough in that area. I've been running this same tune for years without any problems.

I'm wondering what might cause these issues? A cracked plug? Loose plug that's running too hot? Clogged injector? Loose air blockoff plate? Exhaust leak? Timing slipped a tooth? KR due to rattling aftermarket turbo shield or exhaust heat shields? I mean, it could be anything.
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Old 07-29-2016, 10:47 AM   #8
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Quote:
Originally Posted by Samurai Jack View Post
If the motor ran fine before you changed the catback ...
- AND the car has the shakes after you changed the catback
- AND you changed nothing else
You didn't answer these

I think it's all in your head ... you are over-compensating
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Old 07-29-2016, 10:58 AM   #9
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Quote:
Originally Posted by Samurai Jack View Post
You didn't answer these

I think it's all in your head ... you are over-compensating
Sorry,

Motor has not run perfect since the valve clearance misfire, but its pretty darn close. I had been chasing down an issue with surging acceleration at times, which I believe is due to false knock, ever since replacing the KS. The new sensor seems overly sensitive. But I am not seeing KR when I experience the issue Im describing in this thread.

Yes, the shake either developed, or became more noticeable after the catback.

I also also went back to stock ride height and reinstalled stock springs at the time I did the exhaust, but nothing else as far as the engine is concerned.

Youre probably right that its all in my head. Ever since the the KS and then the valve clearance problems, Ive had this car under a microscope so to speak.
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Old 07-29-2016, 12:31 PM   #10
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is the catback rubbing something?
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Old 07-29-2016, 12:42 PM   #11
GuitsBoy
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Originally Posted by hondaeater69 View Post
is the catback rubbing something?
Not that I can tell. I did add a 14" bullet muffler along side the driveshaft, and its awfully close to the driveshaft support bracket. But again, I dont see it pulling timing due to knock.

It almost feels like an imbalance or rotational vibration, like the driveshaft or a tire was out of balance. But its not RPM dependant. I cant hold an RPM and maintain the shake.
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Old 07-29-2016, 12:45 PM   #12
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You should have @least 1/2" clearance all around the exhaust to eliminate rubbing possibilities.

Remember, the exhaust system moves around some. It is not rigidly bolted to the frame
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Old 07-29-2016, 12:49 PM   #13
GuitsBoy
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Quote:
Originally Posted by Samurai Jack View Post
You should have @least 1/2" clearance all around the exhaust to eliminate rubbing possibilities.

Remember, the exhaust system moves around some. It is not rigidly bolted to the frame
Its probably closer to 5/15" clearance at the moment, but since its only 2 feet away from the donut gasket and fairly rigid downpipe, it doesnt swing as much as the back end of the exhaust. I also have longer poly hangers/isolaters which are stiff as hell, and dont allow for nearly as much movement as stock. And again, no obvious knock in logs. And I log pretty much every drive to and from work lately.

Maybe its as simple as the motor mounts starting to show their age?
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Old 08-01-2016, 08:12 AM   #14
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If you are on the original motor mounts, certainly a possibility.

Now would be a good time to upgrade to GrpN motor mounts and transmission mount
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