mobile-menu-icon
GM Authority

Why Some GM Exhaust Tips Become Blackened With Use

As governments around the world continue to ratchet up the pressure on automakers to produce ever more fuel-efficient automobiles, direct injection has gone mainstream. That’s where fuel is injected directly into the cylinder, rather than being sprayed in at the intake port or further upstream, allowing for more precise control over the fuel timing and mixture.

It helps the 6.2-liter V8 small-block LT1 engine achieve up to an EPA-estimated 25 miles per gallon (highway) in the 2019 Chevrolet Corvette.

But like any automotive technology, direct injection has its shortcomings, and some owners surely will have noticed that many modern GM exhaust tips become covered in filthy, black soot as time goes on. We’re used to associating that soot with a fuel-rich air/fuel mixture, as that’s typically been the culprit on cars fueled by carburetors and port injection; blackened, unburned hydrocarbons are scavenged from the engine and go on to coat the inside of the exhaust system as they’re pushed toward the exhaust outlet.

2015 Cadillac ATS coupe

But why would new direct-injected engines, factory-calibrated to run as efficiently as practical, and using three-way catalysts to clean up the exhaust as much as possible, so quickly develop the same tell-tale blackened tailpipes?

According to Car and Driver, which spoke to U-M Mechanical Engineering Associate Research Scientist John Hoard, the answer is simply that direct-injection engines have the same tendency as diesel engines to produce particulate matter. This is especially true when the engine is running at low loads, as there’s often inadequate swirling of the intake charge to evenly distribute the fuel particles, resulting in small, rich pockets.

The low intake-charge velocity also quite often leads to gasoline pooling on the cylinder walls, Hoard told Car and Driver. Both phenomena can lead to the production of soot, despite the air/fuel ratio being just right for safe, efficient combustion.

2019 Chevrolet Silverado RST exterior - August 2018 - Wyoming 009

Countless GM exhaust systems are at risk of being coated by charred, sooty filth as a result of direct injection. The list includes those used by the LT1 and its high-performance derivatives, the 6.2-liter supercharged V8 LT4 and LT5; GM’s EcoTec3 small-block light truck engines, including the 2019 Chevrolet Silverado 1500‘s new 5.3-liter V8 EcoTec3 L84 engine; and the new GM 3.6-liter V6 LGX engine used in the Cadillac ATS, Cadillac CTS, and Chevrolet Camaro from 2016.

Stay tuned for all the latest GM powertrain news, right here on GM Authority.

Aaron Brzozowski is a writer and motoring enthusiast from Detroit with an affinity for '80s German steel. He is not active on the Twitter these days, but you may send him a courier pigeon.

Subscribe to GM Authority

For around-the-clock GM news coverage

We'll send you one email per day with the latest GM news. It's totally free.

Comments

  1. Higher fuel pressure needed direct injection gasoline?
    Be interesting to know when most particulates (soot) are generated: cold start, in and out of fuel cut-off, power enrichment, stop/start (if so equipped)?
    Also, particulate size is important; < 5 micrometers tend to stay in the lungs
    I have a 2014 2.0L Turbo CTS and it definitely has soot on tailpipe outlets

    Reply
  2. What is the best cleaner to remove the soot from exhaust tips?

    Reply
    1. WD-40 works well although the longer you drive before cleaning the more cleaning involved, have been looking at the 10gen Accord 18/19 2.0T models and have not seen the extreme blacking on the exhaust tips on cars that have been on the road awhile. Their 2.0T [252hp/273 ft] runs on reg gas and is a detuned version of the Civic R engine some of their forum members have reported better performance results running premium fuels apparently the computer adjusts tune for fuel used, just saying.

      Reply
  3. Particulate Filters and regens coming soon to a gas direct injection engine near you.

    Reply
  4. My ’03 and ’13 Silverados with non direct injection 5.3 engines both coat the area above and behind the exhaust outlet with soot, mostly while travelling long distances, probably when pulling out to pass and stepping on it. Does it whether towing the travel trailer or not, also coats the front of the trailer. Washes off easily, thankfully because the ’13 is white. The pitch black inside of the tailpipe stays that way. Has never affected economy or reliability though.

    Reply
  5. My LGX blackens the exhaust tips after just 100 miles. Easy enough to clean though.

    Reply
  6. It too this long for someone to say something? Web forums have covered this for years.

    The 2.0 turbo Eco actually was sooty enough to stain the real bumpers. Note when they are cold the will put out black smoke.

    Reply
  7. Burning e85 or a higher ethanol blend than e15 would also help. It burns cleaner.

    Reply
    1. Interesting, though in my case it is the opposite. My 2016 ATS 3.6 engine is designed to use 88 octane fuel, but most gas in my area, 88 or 89 octane, has 10% ethanol; 92 octane is ethanol free. Using this has dramatically decreased the soot problem.

      Reply
  8. Turbos are really bad about it, just like a diesel they’ll produce some soot when you punch it during the turbo lag. I see F150s regularly throwing a puff of black soot on the highway when the punch it, makes you look twice to make sure it isn’t a diesel. Meanwhile we have T4F diesels putting out cleaner air than they take in

    Reply
  9. The next gen cylinder deactivation will solve this, IF gm goes with independently controlled ITB’s…. OMG, just imagine the amazing intake sounds ?

    Reply
  10. If they do turn colors why dont people replace them instead of complaining about them!

    Reply
    1. No complaints, just facts.

      Reply
  11. My 2015 ATS 2.0T constantly blackens the exhaust tips, have 58k mostly hyway in 45 months [93 octane gas] I clean tips and all rear flat surfaces on body of car after every 200+ mile trip, while my tips look like new I have not seen another ATS in my travels that does not have totally black exhaust tips even dealers used ATS and CTS cars on lot look bad. just saying!

    Reply
    1. Is this really a problem? Why are we spending brain cells trying to fix this?

      Reply
      1. I think we are trying to avoid having 5 micrometer tar balls permanently attached to our lungs because the guy in front of us is belching 5 micrometer tar balls.

        Reply
        1. I think you are overreacting to this so called problem!

          Reply
          1. So if a more measured reaction exists, what IS it? Do nothing?

            The industry should refresh its collective memory on the EPA/Volkswagen settlement’s public health considerations. Volkswagen could not retrofit these vehicles economically to eliminate the excessive particulate (tar ball) emissions. The EPA would not budge. The science was irrefutable. Diesel exhaust particulates–and in particular small particulates–cause cancer. Volkswagen was forced to buy-back these so called “clean diesel” vehicles and they now sit neatly arranged, parked in a huge grid, out in the desert, for what might be eternity.

            https://images.hgmsites.net/med/volkswagen_100646033_m.jpg

            How soon they forget.

            Reply
            1. Your worried about a problem that is so miniskul! If exhaust comes out of a tailpipe then it’s going to be dirty! Unless it’s a fuel cell vehicle!

              Electric vehicles will have the same effect on the planet with the use of coal burning power plants. The only difference is that it can’t be seen by the average person’s naked eye. But following a car is very noticeable!

              There are way more important things to spend our time and money on then this!

              Reply
  12. My 2012 Verano accumulates soot on Most of the exterior including all the window glass. How does that happen?

    Reply
  13. I prefer to keep all the parts of my car clean. Even want to get myself a pressure washer for the purpose. Found some options on https://cleanup.expert/best-pressure-washers/ and now I am trying to come up with the best solution. Never thought that choosing a pressure washer is as hard as choosing a car…

    Reply
  14. I can’t believe I didn’t think of this before, Heavy Duty Easy-Off Oven Cleaner.
    Works great!
    I have a 2017 Impala Premier, worth a lot on the used market but it’s a keeper.

    Reply
  15. The electric car is not saving the planet just yet. There are still fossil fuel burning and coal electric plants out there supplying the power. The enviroment is also taking a hit on electric cars doe to the rubber particle that the tires produces due to being a heavier car. The future will depend on hydrogen electic plants but to much politicians worrying about their oil stocks etc. Batteries are getting better with each year passes but the cost is still too high. I wood like to see more trains and subways powered by hydrogen and other clean ways of energy and someday fusion power.

    Reply
  16. I have black exhaust from my 2020 Chevy Equinox recently after I started a job that is 40 minutes away, Had it looked at at Monroe and was told that I have CO2 build up in my car and to have it brought back to my dealership. It explains the constant dizziness, lightheadedness, and vertigo I have had right after starting my job 5 weeks ago.

    Reply

Leave a comment

Cancel