Choking hazard

Many cars are suffering an automotive form of clogged arteries with carbon soot and oil deposits clagging intake manifolds, coating sensitive exhaust gas recirculation valves and jamming turbo actuator flaps. Less than 60,000km is required in some cases to coke up modern high-pressure common-rail diesels in a maintenance nightmare reminiscent of the valve grind and decoke requirements of 1950s engines.

Result: routinely a $3000 bill to dismantle the intake system, remove the thick gunk and reassemble. Labour alone is 10+ hours.

Left untreated, the fall off in emissions performance and fuel economy is an open question.

Wheels can find little evidence in manufacturer servicing schedules for any requirement to decarbonise intake systems. Indeed any formal advice to owners about the problem is scant. We know because we asked them to provide it.

At best, local dealers offer $200 “magic spray” treatments, which diesel experts say may – or may not – have some preventative benefit but can’t unclog centimetres-thick sludge.

“Even if it removes some of the stuff, where’s it going?” asks one Bosch mechanic. “If it doesn’t get stuck in the back of the valves or the piston head it will lodge in the DPF, causing other problems.”

The engineering explanation for why this liquefied soot is afflicting the modern diesel is one word: emissions. It is a clean air motivated quick fix, where exhaust gases are ventilated via the oil soaked crankcase, then back through the Exhaust Gas Recirculation (EGR) valve, where they return to the intake.

Positive crankcase ventilation, they call it. It’s a form of automotive second hand smoking where the engine rebreathes some of its old fumes in a bid for a more complete burn-off of harmful emissions.

“The engine breather is supposed to vent crankcase gases but unfortunately it also brings oil with it and the turbo endorses that by punching it straight through to the intake. Then you’ve got the EGR valve open on cold starts, so there’s some unburnt fuel going through, plus the oil. The result is black liquid glue,” says Frank Spiteri, the senior partner in his father and son Bosch service operation in Sydney’s Milperra.

A booming aftermarket fix is the fitment of a so called ‘catch can’ to harvest oil from engine blow-by before it hits the intake system. Why manufacturers don’t fit these as standard is an open question. Cost is the most likely answer and as the problem doesn’t always make itself obvious during the warranty period … well guess the rest.

The more radical fix is also the illegal one – to blank off the EGR system.

“The EGR technology is up for debate but no one wants to do it for fear of government prosecution” says Berrima Diesel’s Andrew Leimroth.

“It cannot be blocked off legally but installing a catch can may keep the inlet manifold slightly less oily and not allow the carbons to stick as much” he says.

Downstream from the engine, the problems become even more expensive.