What does “you need to blow the carbon out every now and then” mean in regards to automobiles.


What does “blow the carbon out” really do?

Occasionally, I will see a car in front of me, floor it (gas pedal), and when they do puffs of black smoke, come out the back of the car as the car hits full throttle. What gives?

In: 12

The black smoke means your fuel mix is too rich, aka there’s not enough air to burn all the fuel. In newer cars and trucks this means that something is broken or dirty in the engine.

White-ish smoke at full throttle usually means either the vehicle is using more fuel than air during the combustion cycle (in a petrol engine) and what you’re seeing is excess fuel that has been heated, but not fully consumed.

The right “settings” on spark plugs and air/fuel ratios (usually set via software these days) helps alleviate this.

Some sports cars or cars that have been modified after purchase will intentionally run “rich”, meaning there is a certain level of waste (smoke you’re seeing) that is accepted. The general reason for this is a safety measure for the engine. When changes are made to increase power and torque in a car, if the aforementioned air/fuel ratio swings the other way (when there is more air than fuel), it can do thermal damage to the engine (and beyond).

However, black smoke can also be a sign of other issues. Sometimes oil leaks in various places can cause oil to enter the combustion chamber(s). Then the smoke is coming from oil burning (or more precisely, not burning up all the way). In all but very small cases, oil should not be entering the area where petrol and air are burning.

“Blowing the carbon out” would mean to run the engine up to its highest state of tune (usually high/peak RPMs for a somewhat sustained period of time (15-120 seconds depending on context and capabilities). Someone smarter than me will have to explain whether there is any real benefit to this in modern fuel management systems.

Hope that makes sense.

I recently had this done because my check engine light came on. The dealer said come back in 2 weeks and for $199 we’ll tell you what the light means.

I went to Midas, it was my catalytic converter sensors. They did a fuel injection cleaner, told me to put higher octane gas in the car and put in some STP and drive it 100 miles. For $99, they fixed the problem in 2 hours instead of making me wait 2 weeks to find out what the problem was.

It actually took 140 miles, the check engine light went off, I passed my inspection, and I am good to go for another year. FO Ford dealer.

It’s burning oil, either because it’s a diesel engine or the driver has floored it so often that the engine is worn.

Back in the days of carburetors you could get a build-up of carbon inside the combustion chamber, which if bad enough could lead to poor running. I used to warm up the engine, pull the plugs and squirt in a little RedEx. Leave that to soak in for a while, then go for a ride, trailing white smoke behind me for around 30 minutes.

It definitely ran better afterwards.

These days manufacturers are under pressure to use as little fuel as possible and the days of carbs are long gone. The advice to “blow out the carbon” will only apply to classic cars.

I believe in this context it means to blow the carbon residue buildup, most common in diesel engines.

Driving short distances and not giving it enough revs doesn’t bring the temperature up enough for that carbon to dissolve and so it builds up, diesel is a bit oily and unlike gasoline so it has much more carbon buildup.

Blowing it out now and than mean’s basically going on a highway and driving for some amount of time in like 3000-3500 or maybe even 4000 rpm so that the engine and rest of the exhaust and intake system can reach temperatures high enough to dissolve that buildup and shoot it trough exhaust cleaning the engine in the process.