Stuttering under medium acceleration

2008 FIAT BRAVO
72,000 MILES • 1.9L • 4 CYL • TURBO • 2WD • MANUAL
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ARAN YARDLEY
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Hi. Since I bought it a year ago, my vehicle has been stuttering in second and third gear at about 2,800-3,000 rpm's under moderate acceleration. I have (rarely) encountered it in first or as low as 2,000 rpm's, but it's nearly always in that range. When it starts happening, putting my foot down or backing off will make it stop, so it is easy to avoid, but I want to fix it.

Each stutter can be felt as a small loss of power and a hushing sound. I've had multiECUscan on it, but none of the sensors were obviously fluctuating when it was stuttering. It's no the clutch - no rpm fluctuations, and the flywheel failed that failed recently so the whole clutch kit was replaced, but it was happening before and after that problem.

See video to hear the sound it makes and get an idea of the conditions. Cheers!

https://youtu.be/y_wN_RaAh4w
Nov 10, 2019 at 6:46 PM
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JACOBANDNICKOLAS
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Hi:

I listened to the video a few times. I hear a clicking sound. Is that what you are referring to? Has anyone done any checks on the drive line as far as axles/CV joints? Does the clicking speed up if you are going faster?

Let me know.

Joe
Nov 11, 2019 at 7:08 PM
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ARAN YARDLEY
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Hi. It's not really a clicking, grinding or mechanical sound - more like a hushing/rubbing or maybe (air) rushing, kind of like a steam train. No one's specifically checked the CV joints or axles, though when the clutch was being replaced there was a leak at the gearbox that had to have the oil seals replaced, so any obvious damage at least to the gearbox end of it would probably have been spotted then. The problem was present before and after that.

I am pretty sure the sound does speed up when you go faster, but only if you maintain steady acceleration in that rpm and gear range. Increase or decrease acceleration and the sound stops.
Nov 11, 2019 at 8:09 PM
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ARAN YARDLEY
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I'll also add that it just passed its MOT last month with 0 advisories, so any obvious damage to e.g. CV boots should have been noticed then.
Nov 11, 2019 at 8:15 PM
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JACOBANDNICKOLAS
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Hi:

The only thing I was able to pick up on what a sort of clicking. I listened again, and the same thing. Is there something I should pay closer attention to?

Joe
Nov 11, 2019 at 8:18 PM
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ARAN YARDLEY
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Hey Joe, I think you're hearing the right sound (starts at 8 seconds), but it's not really a clicking, it doesn't sound like hard surfaces knocking together or tapping or grinding but more like something rubbing, slipping or air escaping from something. Might not be so easy to tell from the video. It does it in a straight line as well as cornering, and I couldn't see any fluctuations in boost pressure when I hooked up my laptop running MES.
Nov 11, 2019 at 8:41 PM
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JACOBANDNICKOLAS
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Hi again:

Are there any exhaust leaks? Have you checked the catalytic converter? I've seen the converter partially come apart inside and make noise. As the inside moves, it can block exhaust flow which would affect the turbo.

Take a rubber mallet and tap on the exhaust to see if there is any indication.
Nov 12, 2019 at 5:49 PM
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ARAN YARDLEY
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There could be... The silencer's corroded in parts and I haven't looked at the cat so it might be damaged. I'll have a look as soon as I get a chance. I should also add that it only does this when it's moving. There's no unusual sound when just revving up in neutral: it seems to be something about being in 2nd/3rd and gradually accelerating to 2900 rpm that does it. Would the exhaust system behave differently in neutral? Wouldn't it throw an emissions code too? I guess if it's something caused by vibration on a loose cat part that might make sense. Either way, I will get under it with a hammer first chance I get. Cheers!
Nov 12, 2019 at 6:30 PM
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JACOBANDNICKOLAS
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Hi:

Let me know what you find. Understand, although I hear a noise, it is so hard to identify without being there. Based on how you described the sound, I'm wondering if air is escaping from a location other than the exhaust because it is plugging or it is being forced through the exhaust.

Take a look through these links. The first explains general symptoms and the second how to test one. Even if you remove the oxygen sensor before the cat, you should be able to notice if there is a change. It's a thought at this point.

https://www.2carpros.com/articles/bad-catalytic-converter-symptoms

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Let me know what you find.

Joe
Nov 13, 2019 at 5:16 PM