Casebook

Garageman saves a mechanic from trouble with a Free State tune-up and chats carbon

CARBON BLOWOUT

Drifting has become a motorsport in many countries, despite the fact the cars in the pinnacle of racing, F1, do not drift spectacularly, if at all. To see a proper drift, you have to nd a video of Fangio or Ascari handling a Maserati or a Ferrari: the four wheels are parallel as they point towards the side of the road while the car negotiates a corner in a glorious slide controlled mainly by the throttle.

“Sideways” Sydney, one of our town’s more colourful mechanics, deserves his nickname. He even has a court case behind him to prove it. Some years ago, he crashed a customer’s car by trying to drift it the way they do in the movies. The traf c constable testifying in court said: “I saw the accused negotiating a corner on the highway with the car at a weird angle. The moment he spotted the police car, he got such a fright he slowed down. This caused the car to run into the bushes next to the road.” Sideways had to pay a hefty ne and the workshop employing him had to repair the car but his boss kept him on; good mechanics are scarce.

Many mechanics, especially the backyard fraternity, like to rev an engine up to the redline in neutral in order to “clear out the cobwebs”. This can be harmful because the pistons rely on the air compressed during the upstroke to act as a damper for the erce deceleration they face in coming to a stop at top-dead-centre. There is very little air to compress while the gearbox is in neutral; the engine is not under a load.

Some mechanics, myself included, believe a good run at high revs, known as a “Free State tune-up”, helps to clean out some of the deposits. It has to be done on the road and especially uphill so the engine has to work hard. Sideways’ boss doesn’t believe in this theory and has banned him from doing it. In fact, poor old Sideways has been asked to drive customers’ cars sedately in order to keep his job.

Two weeks ago, Sideways was caught “indulging in harmless fun” and he asked me to have a heartto-heart with his boss. I prepared for my case by consulting some SAE automotive-engineering papers written by engineers who have investigated the effect of high-speed runs on engine clean-ups.

It turns out modern EGR and PCV systems recycle oil mist through the intake manifold and this eventually leads to a carbon build-up. These deposits start to form and bond with a metal surface when piston temperatures are in the range of 195 to 290 degrees Celsius, which is normal for everyday driving. Above 290, it’s too hot for the molecules to bond with the metal or each other. At about 325, it’s hot enough for the deposits to break down and separate from the metal.

I visited Sideways’ boss and showed him the proof that an occasional bit of hard work is good for an engine. He accepted the scienti c evidence reluctantly and admitted he was most likely too hard on Sideways. He just didn’t want another mishap involving a customer’s car.

CARBON BUILD-UP

Peter Watson was such a cute little boy his family started to call him “Pixie”. During his high school days, he developed a keen interest in photography and his name gradually changed to Pixel. These days, he is known as Megapixel for a reason which can well be imagined.

Megapixel drives a 2015 Peugeot 308 1,2T. Soon after the 60 000 km service plan expired, the car began to lose its sparkle. Idling was no longer smooth and the acceleration became more leisurely. He brought the car to us and I gave the job card to August.

It was one of the rst GDI (gasoline direct-injection) engines that we serviced because most of them are still under warranty. We were all aware some tend to carbon-up the area behind the intake valve, so the rst thing August did was remove one of the spark plugs and examine the combustion chamber with a scope. The intake valve area was a lot dirtier than the exhaust valve space. August then removed the intake manifold and con rmed there was a lot of carbon in the ports.

The general opinion in the motor trade is this build-up is so hard, the only solution is to remove the cylinder head and send it to a specialist shop to clean it by using ultrasound and a glass-bead spray. I had to get Megapixel’s permission to do that and the poor man had no choice but to agree.

Apparently, on GDI engines, the area behind the intake valve often reaches that 195 to 290 degrees Celsius temperature which promotes the formation of carbon. This does not happen on indirect-injection (into the manifold) engines because the fuel in the airstream has a cooling effect so the temperature stays below 195 degrees Celsius.

The previously mentioned SAE article does also state a Free State tune-up will not clean up the carbon in a GDI engine.