15 captured repliesDiscussion pagination complete · Source accounts, not a confirmed diagnosis
THIS DISCUSSION / SOURCE-DERIVED OVERVIEW
Discussion summary
YOUR RESEARCHcatless x-pipe low rpm stumble
Leading reported evidence
Replace not too long ago
Performed · outcome unconfirmed
No successful repair reported for this finding.
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Read evidence and context
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Suggestion
The ECM uses the downstream O2 sensor readings to adjust all sorts of things so when you pulled the converters I'm not surprised the ECM didn't like it.
Leading evidence: 'Replace not too long ago' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, the most frequently mentioned possible cause is 'caused by a vacuum leak' (1 discussions; 0 report identifying it and 1 suggest it). The most frequently discussed action is 'Adjust sorts of things so' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
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Suggestion
The ECM uses the downstream O2 sensor readings to adjust all sorts of things so when you pulled the converters I'm not surprised the ECM didn't like it.
Explicit resolution reports before identified causes, performed actions and suggestions. Within each level, matching component labels precede direct query mentions and inherited thread context; successful versus unsuccessful discussion counts, owner confirmations and independent discussion support determine order. This is not a probability of success.
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Suspected cause
If it doesn�t change things, I�d perform a smoke test on the intake system, because I think this issue is caused by a vacuum leak.
Explicit resolution reports before identified causes, performed actions and suggestions. Within each level, matching component labels precede direct query mentions and inherited thread context; successful versus unsuccessful discussion counts, owner confirmations and independent discussion support determine order. This is not a probability of success.
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Suggestion
If it doesn�t change things, I�d perform a smoke test on the intake system, because I think this issue is caused by a vacuum leak.
Explicit resolution reports before identified causes, performed actions and suggestions. Within each level, matching component labels precede direct query mentions and inherited thread context; successful versus unsuccessful discussion counts, owner confirmations and independent discussion support determine order. This is not a probability of success.
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Suggestion · Original poster
My tuner confirmed that the tune had the car running extremely rich adding to the stumbling at low rpms.
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Adjust sorts of things so
No successful repair reported for this finding.
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Reported fixes
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Read evidence and context
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Suggestion
The ECM uses the downstream O2 sensor readings to adjust all sorts of things so when you pulled the converters I'm not surprised the ECM didn't like it.
0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.
0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.
Suggestion · Original poster
Car feels like it's running super rich because there is of course an elevated gas smell with no cats, but I have a feeling that is also causing the hiccups.
0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.
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Suggestion
The ECM uses the downstream O2 sensor readings to adjust all sorts of things so when you pulled the converters I'm not surprised the ECM didn't like it.
Counts describe captured discussions, not unique people or a diagnosis. Author identities are not reliably stored. Exact duplicate evidence is counted once across matching discussions; one discussion counts at most once per action/state. Multiple repairs reported together remain a combination. Suggestions and failed attempts do not establish a fix. Matches cover whole discussions and can include related faults, model variants and incomplete threads.
15 eligible posts · 0 unverified posts · 1 discussions match the subject in their title or opening post.
Rennlist: 1 discussions
0 evidence records excluded for query context · 0 excluded by outcome qualification · 0 adjusted by outcome qualification.
0 acknowledgements have no verified action link; they do not establish a repair result.
Generation: 20261026T122219Z · outcome-qualification-6. Deterministic extraction; no AI model inference. How we use sources
I recently added a catless x-pipe to my gundo mod mufflers. The sound is fantastic and the power above 4k rpm is very noticeable. With the loss in back pressure there's some loss in torque (which doesn't bother me) but also I now have a reoccurring stumble/hiccup around 1000-1500rpm under medium to heavy load which is more annoying. I have read that some others have experienced this going the same route and it could be attributed to variocam as well. What were your ways to solve it short of adding cats back in? We don't have emissions here anymore so I don't see a reason to add them back. I have contacted a local shop on a tune to potentially smooth it out. I should mention that the PO on this car added a FVD ots tune before my ownership years ago. Thanks
The ECM uses the downstream O2 sensor readings to adjust all sorts of things so when you pulled the converters I'm not surprised the ECM didn't like it. Are you sure the power is actually noticeable and you are not relating extra noise to power?
Yes, that's what I figure as well and am hoping a tune can help smooth the low rpm hiccups under heavy load (basically city driving). The butt dyno says there is considerably more power above 4k rpm. There was a post several years ago that shows +30hp and +20tq gains in the upper rev band and similar losses down low with a catless setup.
Overall, I'm happy with the x-pipe but would consider cats down the road - perhaps next season. The loss in torque at low rpms is not enough to bother me, but the hiccups create a drivability issue (provided I cannot tune it out) and the car is easily twice as loud. Very much so an exotic-like sound.
If there is room, try installing spark plug anti foulers/ spark plug extenders onto the downstream O2 sensors. It's a way to fool the ECU a bit by pulling the sensors out of the exhaust stream. This way they see less oxygen and the ECU thinks that there is still a converter in place. All you need is a drill to modify these parts so that the sensor fits into them. Lots of info on this just Google it. Tuners need to be very careful doing any kind of tuning on road cars. The EPA in the US is now extensively cracking down on them in every state. The laws are pretty much the same here too so be careful.
I have the anti-foulers/o2 spacers on the old sensors to avoid CELs. They are working well as I've logged 500km on the car since the x-pipe with no cel. Car feels like it's running super rich because there is of course an elevated gas smell with no cats, but I have a feeling that is also causing the hiccups. I'm in Ontario like you so emissions tests are a thing of the past so nothing really to crack down on!
End of the day I have a tune booked with AMT in Vaughan this week and he's confident he can smooth it out. I'll report back for others that experience this in the future... and will also report back if I ultimately add cats down the road.
Get a pro tune performed, I had the exact same issue with my 2002 C2. Slightly better with an Evo Tune but I will be taking it to a local to have professionally tuned for that issue. Also, can confirm there is about a 30hp increase measured on a dyno.
You don't understand. This has nothing to do with the emission testing that was cancelled a couple years ago. It was always illegal to mess with a vehicles emission system but the government didn't go after as many people than it does now. Just do a little Googling and you'll see. They are now heavily going after small tuning shops and when they audit them, all the customer's names will be at their disposal too. Also keep in mind, even with the best of tune, the exhaust will stink like a lawnmower without the converters.
Actually the best thing for me that improved the torque loss and stumble has been the long tube headers. I haven't had it retuned since but I will get it in after my GT3 throttle body gets here.
Thanks for the feedback. I'm going with a custom/road tune, not dyno or off the shelf. I don't really feel the need to dyno tune at this point given my mods. Hopefully it corrects the stumble. I've learned to drive around it, but it's particularly annoying on stop and go uphills.
AMT Tuning which is a local shop here in the Toronto area was able to correct my issues. They customized their "stage 2" tune on my car within a few hours and the drivability is back, no stumbling/hiccups. Still need to put more kms on it over the weekend for the ecu to fully re-learn but the initial response is great.
I forgot to add that the PO had a FVD stage 1 tune on the car which is basically an off the shelf stock tune which isn't optimized for a catless exhaust. My tuner confirmed that the tune had the car running extremely rich adding to the stumbling at low rpms.
Hey guys, similar issue on my x51 03'. Intalled FVD catless x pipe (after Fister exhaust) and developed a low rpm (600-1800) stumble or hesitation under medium+ load. Low speed driveability affected (starting on hill, driveway, 2nd gear rolling restarts) and spirited driving from a stop. Added a tune and has not addressed it through several iterations. Other mods include: K&N panel air filter and an intake air elbow (breather which was connected to OEM elbow now has a breather on it). No codes, plugs/coils replaced not too long ago. Appreciate your input!
Stick the OEM elbow back on and see if things improve. I don�t know where this breather leads, or what it does (I don�t have a breather on my C2), but it sounds like an unmetered air is sneaking past the MAF via the breather.
If it doesn�t change things, I�d perform a smoke test on the intake system, because I think this issue is caused by a vacuum leak.
