If you believe GM on this then I'd like to sell you some American Tea :)


Well-known user
This bulletin applies to all 2008-2019 GM passenger cars and light trucks equipped with direct fuel injection systems. (Includes of course C7-C8 Vettes)

Some customers may comment about black smoke, rough idle and minimal misfires on a cold start, and the MIL may not be illuminated.

With the introduction of direct fuel injection, GM has revised the cold start control system to reduce cold-start emissions.
Quicker catalytic converter heating helps meet the ever-changing emissions requirements and improve fuel economy.

According to GM, a dual-pulse injection strategy is utilized during engine cold start to reduce the time required to bring the catalytic converter up to operating temperature. This dual-pulse injection strategy lasts for about 60 seconds on cold start.

This process will cause the customer to see increased black smoke, soot, rough idle or minimal misfires during cold start and “should be considered normal.”

GM must think all customers are stupid


CCCUK Member
It must be a common GM thing then as my 68 Vette does then when I start it up (driven not very often at all). Little bit of black smoke, rough idle etc. All cured by a few blips of throttle etc. I put it down to it being old and like myself a bit grumpy when having to wake up. Its all part of the charm. :ROFLMAO:


Only a 'common' thing Capt 'coz the engine is stone cold and all of the inlet tracts being cold are not atomising the fuel fully. The engine needs an overly rich fuel mixture to compensate. Once the engine is fully warmed-up then fuel is correctly atomised and the only time you'll get the back smoke is when you 'hit the gas' and the accelerator pump does its thing and squirts a bit of neat gas in to the intake......


CCCUK Member
Soot is an unfortunate by-product of direct injection, particularly on cold-start.
Cold engine emissions are much worse than a hot engine and to make things worse the catalytic converter is not at a temperature where it is effective.
To reduce the amount of hydrocarbon emissions during cold running, and to heat the catalytic converter quicker, a lot of engines use a Secondary Air Injection system, which pumps fresh air into the exhaust port of the cylinder head in order to oxidise the excess fuel that has not been completely combusted in the combustion chamber. This goes on for a minute or so. This is why a lot of cars sound louder on cold running.


CCCUK Member
Yup. That's why some manufacturers (such as Audi) use both port and direct injection in some applications, too – they can use port injection at low loads to cut emissions, including particulates. Does help out at higher loads, though, too. And helps stop the intake tracts and valves fouling up, heh....