Casebook
Fossil hunters, carbon deposits and blocked up Range Rovers are on Garageman’s to-do list this month Direct-injection engines carbon-up intake valves
TOO MUCH CARBON
Nelson “Neanderthal” Roode is a fossil hunter in his spare time who often spends weekends walking the surrounding veld looking for old bones that might have belonged to some of the weird creatures that predate us. Up to now, all he has found are bones that he thinks belong to a Neanderthal humanoid, but most of us are not impressed. It is dif cult to take a man seriously who regards his pets and farm animals as cousins because of a rm belief in evolution. He recently told a neighbour that cousin Suzy is not giving as much milk as she used to and cousin Buttercup is suffering from a dropped udder.
Neanderthal owns a 2006 VW Jetta with a 2,0-litre, direct-injection petrol engine, something we understand far better. At just over 100 000 km, the engine began to show a reluctance to start and was noticeably less powerful than it used to be. Soon afterwards, the check-engine light came on and the car was brought in.
August once worked at a VW dealership, so I gave him the job. He was thrilled and, upon calling his contacts at the VW shop, he learnt that this problem generally occurs in direct-injection petrol engines. These engines tend to carbon-up the intake valves, which was what was causing the problems that Neanderthal had experienced.
Wanting to see how bad the problem really was, August started by removing the spark plugs and fetching our recently acquired endoscope. This is a long, thin, exible, bre-optic tubular instrument devised for looking inside long passages. With it, August was able to see the thick layer of carbon round any of the intake valves that were open.
In the old days, we would have taken off the cylinder head, stripped it, cleaned the valves and removed all the carbon from the rest of the engine interior. This is now done only in extreme cases. Modern oils have resulted in far less carbon build-up and overhead cam engines are far too complicated to strip just to remove some carbon.
August phoned his contact at VW and was told to take the car to a company that specialises in using walnut shells in a compressed air stream to remove crud. Vintage-car restorers have been using this service for years. This was done, but it was neither cheap nor quick because the intake manifold had to be removed and replaced. The process did, however, restore the engine to its former condition.
So, why did this happen in the rst place? Well, direct injection is able to supply a more precise fuel quantity to the combustion chamber than is possible with manifold fuel injection. The consequent improvement in combustion quality and cooler temperature has enabled designers to increase compression ratios and this has resulted in a torque and fuel-consumption improvement. Unfortunately, this layout has led to carbon build-up at the intake valves.
With manifold injection, unevaporated fuel particles hit the intake valves and keep them clean, but direct-injection engines suck in only air. When a direct-injection engine is new, there is only a small amount of carbon in the intake air, but as the mileage builds up, the PCV (positive crankcase ventilation) valve, as well as the EGR (exhaust gas recycle) valve will not function like they did when they were new. As a result, as time goes by, more and more carbon particles nd themselves inside the intake manifold.
To me, this was a revealing story. I believed that most manufacturers go to a great deal of time, expense, money and facilities to test every part, every assembly and every complete vehicle under all possible motoring conditions. This direct-injection disaster showed how naïve I had been. Even the mildest test driving must have made manufacturers aware of the carbon deposit problem. It is true that it is not a universal problem; some manufacturers seem to not have this problem, but the ones that are trying to cope with it have only themselves to blame.
BAD DIAGNOSIS
This story comes from a customer who recently returned from an upcountry trip. Somewhere north, his 2014 Range Rover developed a serious loss of power. It would idle as well as ever, but it wouldn’t pull with any sign of enthusiasm.
He took it to a well-known, large workshop where a mechanic went for a drive and diagnosed a torque-converter failure. The customer did not believe this, so he visited another well-known workshop. This time, the hydraulic valve lifters were blamed.
Finally, he took it to a small workshop where the car was put on a lift and the workshop’s mechanic actually took the time to inspect the underside of the Range Rover. His investigation revealed that the exhaust tailpipe was blocked by a chunk of exhaust-baf e material that had dislodged itself from the second silencer box. The customer didn’t fancy the hassle and expense of locating a second silencer, so he asked them to replace the box with a length of pipe as a temporary repair.