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NCT Query - Smoke

chases0102

Hi all, my 2016 Peugot 308 failed its NCT due to smoke/emission levels.

What can I do to fix this and pass? I am worried that it will fail the retest - car does lots of small journeys etc. Driving fine though.

Any other time I have failed an NCT it's a fairly straight forward fix but this one has me worried!

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User1998

Redline it at idle for 30 seconds or so. And drive it extremely hard to the red line. You don't have to drive fast, just drive it hard in 1st and 2nd gear. See if that gets rid of the smoke.

chases0102

Thanks for that, appreciate it.

greenbin2

Always drive to the nct in 3rd with plenty of revs in a older diesel

gipi

Give it a dose of dipetane before driving it in low gear, it'll help clean up the emissions.

chases0102

https://www.boards.ie/discussion/comment/124524619#Comment_124524619

How much Dipetane would I give - and would it be best if I use it in a full/half-full/empty tank?

User1998

https://www.boards.ie/discussion/comment/124542958#Comment_124542958

Instructions are on the bottle

GavPJ

https://www.boards.ie/discussion/comment/124542958#Comment_124542958

I always throw the full bottle in if it is a diesel. Never had any problems.

If it is a diesel, fill up with HVO too. If it still fails, you have a further issue.

chases0102

Hi all,

Just to go back to this - did the retest earlier, and unfortunately failed again. Worried now!

Thanks for advice above - drove it really hard in third gear for a long time, revved it up etc. and put the Eco Clean Drive in on a quarter full of tank - still no go.

I guess my next option is to bring it to a mechanic and hook it up to diagnostics?

GavPJ

https://www.boards.ie/discussion/comment/124556108#Comment_124556108

Is it a petrol or diesel?

chases0102

It's a diesel

https://www.boards.ie/discussion/comment/124557098#Comment_124557098

GavPJ

https://www.boards.ie/discussion/comment/124557106#Comment_124557106

More than likely an issue with an intercooler pipe leaking or the intercooler itself.

Are you getting black smoke when you accelerate real hard? Did you put HVO into it

before the test? Or Diptane?

chases0102

I went into the local Motor Factors to buy Diptane - the guy recommend I use Eco Clean Drive instead, so bought that and put the whole thing in on a quarter tank. Drove it in 3rd gear for around 40 minutes on way to retest - no good. In fact the smoke readings were almost the same.

I am not seeing black smoke when I accelerate.

GavPJ

https://www.boards.ie/discussion/comment/124557118#Comment_124557118

Use diptane and HVO and try again. If that doesn't bring it down you have fuel/air problem I would think.

chases0102

https://www.boards.ie/discussion/comment/124557156#Comment_124557156

Thank you for advice, much appreciated.

Henry Ford III

https://www.boards.ie/discussion/comment/124557118#Comment_124557118

How many rpm? It needs to be quite high.

barneygumble99

Dipetane is snake oil

Cordell

https://www.boards.ie/discussion/comment/124557474#Comment_124557474

Every fuel additive is. If a diesel fails the extremely relaxed NCT smoke test then clearly it's something wrong with the engine.

barneygumble99

Agree. A good hard drive does way more for a car than putting a bottle of piss into it.

chases0102

https://www.boards.ie/discussion/comment/124557418#Comment_124557418

Quite high I think - it was a good 40 minute spin to the Test Centre, didn't put it out of third gear. I also revved it up to the red line for 30 seconds or so just before I went in. The smoke reading was almost identical to when it did the first test!

Henry Ford III

https://www.boards.ie/discussion/comment/124557649#Comment_124557649

What speed was that? I reckon you'd need to be doing c.90kph in 3rd gear.

Citizen Six

You probably need to actually drive it in the red. I had problems last year, and took three attempts to get through. I use Dipetane on a semi regular basis, as well as extra before the NCT. Last year I even added petrol to the diesel fuel after a few people recommended it. Eventually passed.

This year I drove 80km on the motorway, in third, in the red, the day before the test. Then third gear for the 12km to my local test centre. Passed at 1.120 and the limit was 1.5.

Citizen Six

https://www.boards.ie/discussion/comment/124557729#Comment_124557729

I did 120 kph in third myself.

Kevindiyer

Pretty obvious you have dpf deleted because underlying issue . Was risk you took failing nct .

Only option i see to pass it put liter of petrol into it with haft tank of diesel

GavPJ

https://www.boards.ie/discussion/comment/124562822#Comment_124562822

I have had loads of cars pass the NCT with a DPF removed.

User1998

https://www.boards.ie/discussion/comment/124562822#Comment_124562822

Stupid accusation tbh. A DPF delete doesn't even cause excess smoke

Cordell

https://www.boards.ie/discussion/comment/124562921#Comment_124562921

It absolutely does so, that's the point of the DPF. But if the engine is otherwise healthy, the smoke increase will not be enough to make it fail NCT - which only goes to show how inadequate the NCT diesel test really is.

So if the OPs car had the DPF deleted, then it was done because it failed due to other engine malfunction. As they usually do, in modern diesels DPFs won't usually fail without reason.

User1998

https://www.boards.ie/discussion/comment/124562979#Comment_124562979

If you are so certain that a DPF delete would cause excess smoke, please explain how pre 2011 diesel cars with no DPF emit no smoke? If what you are saying is true, any pre 2011 diesel would be smoking heavily.

The point of a DPF is to capture harmful soot particles which are invisible to the human eye, not to reduce smoke.

Cordell

https://www.boards.ie/discussion/comment/124563040#Comment_124563040

Even pre 2011 some diesels had DPFs and catalytic converters. And they did smoke, not heavily but enough to be visible, especially under hard acceleration.

The soot captured in the DPF is quite visible with the naked eye. Both black smoke and soot are in fact make out of the same thing, impure carbon, as a result of incomplete burning.

chases0102

Just to jump on here - brought machine to mechanic who ran Diagnostics….amongst other things, he found that there was a faulty sensor that might be the issue? I have never heard this before, just wondering how could a faulty sensor be responsible for a failed smoke test?

mickdw

Of course it could. Everything is carefully metered and anything out of tolerance due to a misreading sensor will cause havoc.