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Hi all, I am experiencing dark gray/charcoal colored smoke on boost on my 08 STI. The biggest puff of smoke comes after a daily commute drive without boost. After the big puff, the smoke reduces considerably for next couple of WOT until I revert daily commute drive again.

The car drives fine and is making decent power based on my mods on a stock internal motor (FP71HTA/DW750cc/Walboro 255L FP/Perrin TIH/HKS open pod intake/stock TMIC/decatted stock downpipe/Fuji exos). Noticed fuel consumption has increased by 10-15% despite traveling same route and similar driving pattern during daily commute.

Things I have checked/done:

1. New spark plugs replaced. Previous set that came out was not fouled.
2. New DW 750cc injectors replaced. Got my tuner to retune after injector change and he confirmed AFR 14.7 during closed loop and 11.55 on WOT.
3. MAF sensors replaced for test and it still smokes. Not throwing error.
4. Not consuming motor oil. Oil level on dipstick remains same after 4000miles where I would change them out.
5. Coolant is checked. No signs of head gasket leak.
6. Downpipe pulled and checked. No oil leak between turbo and downpipe.
7. No smoke at idling, even extended idling.

Tuner suspect the new DW injectors could be leaking or at least not closing when it should, causing leakdown which builds up during daily commute drives and causes the big black puffs during spirited drives WOT. Could O2 sensor be faulty too? no CEL noted.

Would like to seek some feedback on this issue and where else I should look pls. Understand black smoke suggest a rich tune which is safe but its so annoying I can't drive spiritedly anymore without creating a real smoke screen. It covers the rear windscreen totally which it never did before.

Thank you all in advance!
 

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black puffs are just unburnt fuel, it's when you're running a rich. I fit's only doing them at WOT i wouldn't be too worried if it's doing it everywhere i'd look into a return

If your puffing black smoke with little to no boost a leaky injector could be an issue. Definitely fuelling related. Do you have an intake leak? That could make the ECU it's sucking in more air than it thinks and trying to fuel for it.

Black - unburnt fuel
Blue - Bad. Coolant
White - Condensation
 
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