Drive fast to protect your car?


I was told that I was driving my Mercedes Benz 350SLK too slow and carbon deposits were building up in the cylinders. The mechaniac suggested that one of the techs take the car for a day to eliminate the carbon by some quicker driving. They seem full of it - are they?


Yes. DO NOT let the tech have a day’s fun driving the “stuff” out of your Mercedes.
As a matter of fact, look for a new shop. Not only is this not the problem, but what he’s suggesting is unethical.

In days of old…days of carburators…carbon deposits could build up in engines under the right conditions because those old engines ran rich and about 20 degrees cooler than modern engines. If an engine was never “pushed” and always ran near idle, usually because it had much more power than was needed for daily driving, the cylinders would run cool & rich and load up. Incomplete combustion, and those old carburated engines definitely suffered from incomplete combustion, could leave residues. Fuel injected engines don’t have this problem. They burn the fuel far more completely and run leaner and hotter.

Find a new shop.


Thanks for the great insight. That’s what I figured. However, confirmation is always great.


Mountainbike is right, but having said that we still have crankcase condensation if the engine never really warms up. This type of driving is very common with senior citizens, who go to the 7-11 for a paper or drive 1.5 miles to the bingo hall. It will cause sludge in the crankcase to form, as well as water. Oil companies have a standard test in cold areas to see how many of these “aunt Minnie” trips will cause the crankcase ventilation system to freeze up and give trouble. My mother-in-law does not drive great distance any more, so her son takes her car out every 2 weeks or so and gives it a normal day of highway use to drive off the water. You don’t need to race the car to accomplish this!

We assume here that anyone with a 350SLK does more than drive to the bingo hall. As Mountainbike said, years ago, Jaguar, for instance said to “de-coke the cyliner head every 20,000 miles”. With California labour rates this would quickly put you in the poorhouse. Road & Track magazine recommended fast highway trips and winding the engine up to 6000rpm in 2nd gear on Mulholland drive, to blow out the cobwebs. I would just love to drive a sports car fast through rural Vermont or Main.


I drive 200 miles round trip on the Interstate at least twice a month when traveling to and from my home and college. I was told this is very good for my car. I guess this is why ?


Yes, defintely! You car gets the right use. If you lived in Germany those trips would be at about 115mph or so. Your car is designed to go that fast for hours without burning up.


As long as you get the engine fully warmed up when you drive it the crankcase condensation will be dissipated. Doc made an excellnt point about that being a problem for cars that never get to full operating temp very often.

While I have no evidence to back me, I think a monthly highway run is good for car for other reasons. It runs lots of fuel and its detergents through the injectors and the agitation of the fluids (gas and oil) tends to suspend contaminants such that they can be carried by the fluids and captured by the filters (or burned) rather than gumping up on the bottom of the containers (oil pan and gas tank). It’ll also bring to your attention for correction any developing problems like an irregularly wearing tire or articulated joints (nall joits or whatever) that you might not pick up if you rarely do a highway drive.

Besides…its’ fun!

Carbon buildup is no longer an issue. Multiport fuel injection systems provide a nice fine mist right into the intake ports, and modern metering controls the amount to the “Nth degree” so that the fuel is burned as completely as possible. Carbon is the carbon from the gasoline molecules, which are hydrogen and carbon (hydrocarbon), and that can get left over after the hydrocarbon molecule splits if there’s not enough oxygen or time for it to pick up a few oxygen atoms and become carbon dioxide. That’s why the old rich, cool running engines could get carbon buildup. Too much gas, too cool an operating temp (old T-stats used to be 165 degrees commonly), too large a droplet with not enough time to burn (only the surface, in contact with the xoygen, burns…larger droplets take longer to burn through).


Mountainbike excels at common sense! We were overseas for 4.5 years, and one of our children was house sitting for us at that time. Rather than mothball the 2 cars, we had him drive each one every 2 weeks or so. I never used to tell my kids to drive fast, but encouraged my son to take the cars to the mountains for the weekend. It saved us a lot of hassle; no skunky gas in the tank, engine stayed well lubriccated, etc.


While I would not let the tech go play with your car, performing the occasional “italian tune-up” by winding it up through the gears won’t do any harm.


When we had to repair a leaking head gasket on our Subaru, our mechanic commented that many of them have carbon deposits. Our engine was clean and he attributed that to our driving habits. Every day, the car sees a couple of full throttle bursts of acceleration to 80% of red line.

You don’t need to “drive it like you stole it.” However, you should use situations like accelerating up to highway speed as an opportunity for a few seconds of hard acceleration. It will do the car good, not harm.


You probably just need to drive the way you always do. IF you are concerned, just jam your foot down when you get on a sparsely populated highway. Don’t light up the tires at stop lights.


Jeff— Well, it is good for the reason others have explained about the condensation in the oil, but usually the reason why highway driving is “good” for your car is because car’s lifespans are measured in miles and highway driving gives you more miles per engine revolutions, as well as less wear on the “other” parts of the car. Hence, when we go to buy a car “highway miles” are good because it means less actual wear. If we measured a car’s lifespan in running hours, it would probably be just the opposite.


I agree with this and will add that there is no need for an “Italian Tune Up” unless your owner’s manual specifies differently. Consider too that it is possible that the memory of some posters may extend back to leaded gasoline days when combustion chamber deposits were more of a concern. At some point, and the number that I recall for lead free gasoline is 40,000 miles, combustion chamber deposits maximize and flake off at the same rate as they are deposited. If you have 40,000 miles and no problem, then you are good to go.

Consider too that many MBs are sold to older people who are not inclined to hard acceleration and seemingly have no problem.


Not needed, but this is one thing that some drivers take as a religion. Driving it fast is considered the first commandment and that is the excuse used on the wife when she complains about the way he drives. Of course that is also used in reverse when the wife likes to drive fast, she also may find religion.


Not really talking about driving fast, just using a little more of the rpm range every once and a while. I agree the most fuel injected engines are well designed to run at partial throttle, however, a little italian tune-up certainly help clear out my diesels.


So, it’s settled. The mechanic should not drive your car like a maniac. Let me do it! ;^)