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Hi everyone, my car has been getting bad gas mileage. (see my other post). The dealership is giving me the run around. But today I was looking at the tailpipes and both of them are covered in black soot. What could this mean?
 

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As for the black fume, this is normal on those cars. I had the same situation on a Grand Am 2001. Mine is doing it also on my actual GTP and it did the same on my first GTP. You need to wash the pipes more often, that's all.

No worries.

As for Gaz Milage, make shure that it's not a computor problem. Mine works on Kilometers (metric) but when I convert to gallons it does'nt match as per some conversion tables.

My manual GTP did some 12,3L/ 100 Kms. average (city/road) on a 6 months basis with 9 500 Kms..

My automatic shows 13,1L/100 kms but it's only a few hundred Kms at this time.

Metric is universal:

Why don't you put your computor to metric a check the results.

I think that you should be able to read around 12 to 13 Liters / 100 Km.

If it's 12/13, than the metric computor is OK and the conversion could be wrong.

12.3 liters / 100 kilometers converts to 28.9 US galons.

But if you get something over 14 or 15 Liter / 100 kilometers or more, than the fuel consumption is definitively incorrect. Around 12 city / hignway. and on highway only it could go down to 9,5 L / 100 Km.

Anything over 14 L / 100 Km. is definitively incorrect.
 

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Carissa249 said:
Hi everyone, my car has been getting bad gas mileage. (see my other post). The dealership is giving me the run around. But today I was looking at the tailpipes and both of them are covered in black soot. What could this mean?
I have a 04 3.5 L and the tail pipe is clean, I mean I put my finger in the tail pipe and rub it across the length of my finger and it is clean.
Computer controlled engines with a MAF should be clean. You have a problem.
 

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^ Cars are dirty machines, carbon is a by-product of the combustion reation that occurs in an engine. It ends up at the tail pipe, end of story. Take a walk in any parking lot and look at every cars exhaust, I promise the overwhelming majority will be black. There are only two reasons for a "clean" tail pipe.
1: The owner scrubs the hell out of them periodically.
2: The car is relatively new and has low miles such that the carbon hasn't built up yet.
Don't go telling people they have a problem until you look around a little bit.
 

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I have to agree. Black tailpipes are natural and do not equal a problem. I have had my G6 now for 6 months and 14000 miles and mine have been turning black as well. I know for a fact my car doesn't have a problem and it runs just fine and gets good mileage. If I were you I wouldn't believe what the mileage on the computer says as it is not very accurate. I would see how many miles you have driven and how much gas you have to put in on the next fill up. Then figure it out from that. If you rail on your car all the time then you will have worse gas mileage. I can drive light on mine and get about 325 - 375 out of a tank but if I drive it hard sometimes I only get about 275 or so out of a tank. I hope this helps.
 

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bigbengt67 said:
^ Cars are dirty machines, carbon is a by-product of the combustion reation that occurs in an engine. It ends up at the tail pipe, end of story. Take a walk in any parking lot and look at every cars exhaust, I promise the overwhelming majority will be black. There are only two reasons for a "clean" tail pipe.
1: The owner scrubs the hell out of them periodically.
2: The car is relatively new and has low miles such that the carbon hasn't built up yet.
Don't go telling people they have a problem until you look around a little bit.
Well I must have the only car in the world with a clean tail pipe, 17 K miles. Soot is unburned fuel, a pure waste, you should check some more tail pipes before you call my statement BS. I will call the Guiness Book of records to get it in if I have the only one. BTW 35 MPG at 70 MPH, not much left for soot. :p
 

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^ Soot is NOT unburned fuel, it is a by-product of the combustion process. The first law of thermodynamics says matter cannot be created or destroyed. If you are burning fuel it will be changed into another form. Carbon is what comes out of most combustion reactions. Anything you burn will produce by-products.Catalytic converters try to take care of any unburned fuel after it leaves the engine.

Here's the equation for burning fuel in a combustion reaction:
CxHy + (x + y/4)O2 → xCO2 + (y/2)H2O

X and Y are various ratios of hydrogen and carbon that are what differentiate fuels (Propane, methane, butane, gasoline...)

In the real world things do not work out perfectly, if they did we would not have carbon monoxide and NOx emissions to worry about in our cars; only water and carbon dioxide.

In short, a real world combustion reaction WILL produce carbon atoms not bonded with any other molecules. That carbon is what turns your exhaust black.
 

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I just looked at my tailpipe today and its completely clean on the inside. You could about eat off of it. 17k+ miles on it.
 

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Black on mine. I tried wiping it off with a dry paper towel once to see if it would do anything, but it didn't.

Too lazy to clean them.
 

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bigbengt67 said:
^ Soot is NOT unburned fuel, it is a by-product of the combustion process. The first law of thermodynamics says matter cannot be created or destroyed. If you are burning fuel it will be changed into another form. Carbon is what comes out of most combustion reactions. Anything you burn will produce by-products.Catalytic converters try to take care of any unburned fuel after it leaves the engine.

Here's the equation for burning fuel in a combustion reaction:
CxHy + (x + y/4)O2 ? xCO2 + (y/2)H2O

X and Y are various ratios of hydrogen and carbon that are what differentiate fuels (Propane, methane, butane, gasoline...)

In the real world things do not work out perfectly, if they did we would not have carbon monoxide and NOx emissions to worry about in our cars; only water and carbon dioxide.

In short, a real world combustion reaction WILL produce carbon atoms not bonded with any other molecules. That carbon is what turns your exhaust black.
Not to be picky, but the catalyst on a vehicle is not their to take care of unburnt fuel. In fact, unburnt fuel entering your catalyist will likely damage it if exposed long enough. Combustion in modern engines is not perfect, but a good engine comes very, very close. The actual combustion itself is very efficient. But your point is correct, the engine does produce some carbon and it will lead to a SLIGHT buildup on the exhaust, however I would say anything more than a dirty coating would be excessive. If you can wipe your finger across it and collect a gob of soot, then you might have a problem. However, the likelyhood that your having a fuel metering/combustion problem without the ECU picking up on it is probably pretty slim.
 

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Lucius said:
Black on mine. I tried wiping it off with a dry paper towel once to see if it would do anything, but it didn't.

Too lazy to clean them.
This sounds about right. Any black you have on the tailpipe shouldn't really be something that wipes off easy. If there is so much that you can collect powder on your finger, then you would probably want to look into it.
 

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In a car, soot (carbon) is produced as a result of VERY incomplete combustion (typically due to too much fuel being delivered for the amount of oxygen drawn into the engine). Soot is the first oxidation state up from that of the fuel itself. Carbon monoxide is also a product of incomplete combustion, but is one step higher oxidation state than that of soot. Complete combustion produces CO2, which has the highest resulting oxidation state resulting from combustion.

The optimum amount of fuel to be delivered into the engine depends on many things, and is nowdays controlled by computer. The computer monitors many things such as coolant temp, manifold air temp, throttle position, engine RPM, manifold vacuum, barometric pressure, oxygen sensor, more and makes a programmed deterimination of the injector pulse width required to deliver the correct amount of fuel at any given moment. This calculation and adjustment is done many times each second. The production of soot indicates the failure of the fuel control system to provide the correct amount of fuel (at some point).

A little soot is normal, as fuel control systems are not perfect. Cars that are driven for longer distances tend have exhaust streams of higher temps and burn more of the soot out of the exhaust.

Excessive soot/black smoke, coupled with poor mileage is a slam-dunk that the fuel delivery/fuel control system is working improperly. This could be due to failure of any of the input sensors or ECU, as well as fuel delivery hardware such as fuel pump/pressure regulator/fuel injectors. Fuel delivery problems with newer cars usually turn out to be related to input sensors.

Sounds to me like the car in question here has a legit problem, unless it is being driven very hard. Full throttle and during initial warm-up of the vehicle are the only times the engne doesn't have closed-loop (O2 sensor feedback) control of the fuel delivery.

LM
 

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jp375 said:
However, the likelyhood that your having a fuel metering/combustion problem without the ECU picking up on it is probably pretty slim.
If the ECU never makes it into closed loop mode with O2 sensor feedback (for what ever reason), it will never be able to recognize that it is running too rich or too lean. This is always one of the first step in diagnosis - verifying closed loop mode with no codes set.

LM
 

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the corrosion is most likely because of acidic compounds in the exhaust, you may be using a gasoline with a higher sulfur content. try another brand.

I don't know how much blacker or sootier your all's exhaust could be than mine or for that matter the exhaust on every car I've owned since the mid 70's when they started using the catalytic converters and exhaust gas recirculators. I haven't seen a "gray" tailpipe since a 1972 Mustang I owned. It's normal to have black pipes.
 
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