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Really rough idle, lots of black (maybe blue?) smoke...

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4 captured repliesDiscussion pagination complete · Source accounts, not a confirmed diagnosis

THIS DISCUSSION / SOURCE-DERIVED OVERVIEW

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YOUR RESEARCHReally rough idle, lots of black (maybe blue?) smoke...

Reported or suspected cause

untrue)

Reported cause

No successful repair reported for this finding.

1
Identified cause: 0
Suspected cause: 0
Reported fixes

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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.

Discussions that report identifying a cause: 1

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.

Reported cause · Original poster
I'm told by the shop that sold me the car (which told me a couple of other things that turned out to be untrue) that the plugs were changed right before I bought the car, at around 130,000 km's or so.
Original post Read captured discussion
Really rough idle, lots of black (maybe blue?) smoke...

Frequently discussed work

Clean idle control valve

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

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Discussions that contain suggestions: 1

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
If it is a 99 C2, remove and clean the idle control valve.
Original post Read captured discussion
Really rough idle, lots of black (maybe blue?) smoke...

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Leading evidence: 'untrue)' (1 discussions report identifying this cause; no attributable repair resolution). Across 1 matching discussions, the most frequently mentioned possible cause is 'untrue)' (1 discussions; 1 report identifying it and 0 suggest it). The most frequently discussed action is 'Clean idle control valve' (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.

Source reports, not a diagnosis or a success probability. Counts refer to discussions and can overlap.

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1 matching discussions · 4 captured posts examined · 0 partial discussions

Compare 2 other evidence results

Replace right

Performed · outcome unconfirmed

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

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Discussions that performed work without a confirmed outcome: 1

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.

Performed · outcome unconfirmed · Original poster
I'm told by the shop that sold me the car (which told me a couple of other things that turned out to be untrue) that the plugs were changed right before I bought the car, at around 130,000 km's or so.
Original post Read captured discussion
Really rough idle, lots of black (maybe blue?) smoke...

Clean idle control valve

Suggestion or failed attempt

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Discussions that contain suggestions: 1

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
If it is a 99 C2, remove and clean the idle control valve.
Original post Read captured discussion
Really rough idle, lots of black (maybe blue?) smoke...

Frequently discussed actions (2)

These are separate views of the same evidence. Frequency alone does not establish a fix, and these counts must not be added together.

Clean idle control valve

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Discussions that contain suggestions: 1

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
If it is a 99 C2, remove and clean the idle control valve.
Original post Read captured discussion
Really rough idle, lots of black (maybe blue?) smoke...

Replace right

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Discussions that performed work without a confirmed outcome: 1

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.

Performed · outcome unconfirmed · Original poster
I'm told by the shop that sold me the car (which told me a couple of other things that turned out to be untrue) that the plugs were changed right before I bought the car, at around 130,000 km's or so.
Original post Read captured discussion
Really rough idle, lots of black (maybe blue?) smoke...

Reported and suspected causes (1)

These are separate views of the same evidence. Frequency alone does not establish a fix, and these counts must not be added together.

untrue)

No successful repair reported for this finding.

1
Identified cause: 0
Suspected cause: 0
Reported fixes

Read evidence and context

Discussions that report identifying a cause: 1

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.

Reported cause · Original poster
I'm told by the shop that sold me the car (which told me a couple of other things that turned out to be untrue) that the plugs were changed right before I bought the car, at around 130,000 km's or so.
Original post Read captured discussion
Really rough idle, lots of black (maybe blue?) smoke...

Coverage and how to read this overview

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.

4 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: 20261026T103553Z · outcome-qualification-6. Deterministic extraction; no AI model inference. How we use sources

POST 1 / ORIGINAL POSTEROriginal post

Hi All, My 996 recently hit the 170,000 km mark and the last two times I've started it up, the engine idles very roughly (like a slow-chugging boat engine) and billows a lot of black smoke. I did a search on the site, and it looks like some people have a bit of a rough idle and some white smoke, but this is definitely not that (the car shakes pretty violently). The first time, it went away after a few minutes, but the last time, it did it for about 15 minutes (at which point I shut the car off). Any ideas on what this could be (or how I might be able to diagnose the problem)? The CEL isn't on (I think it may have come on after the first time it happened, but then went off). Thanks, MT

POST 2Original post

I would clean the MAF as well as add a bottle of injector cleaner to the gas tank. If it is a 99 C2, remove and clean the idle control valve. When were the plugs last changed?

POST 3Original post

Pull the codes, but my guess is a misfire on one or more cylinders. Likely plugs or coilpacks.

POST 4 / ORIGINAL POSTEROriginal post

Thanks to you both--I'll check it out. I'm told by the shop that sold me the car (which told me a couple of other things that turned out to be untrue) that the plugs were changed right before I bought the car, at around 130,000 km's or so. I'll change them just in case, too. I just spoke to my girlfriend who was there last time it happened and she said that she thought the smoke was dark, but had a bluish tinge--same considerations/issues? Can a dirty MAF/fuel injectors/bad spark plugs cause engine to burn oil? Thanks again, MT

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