THE GREASE CAR
How I converted a 1979 Mercedes to run on a completely free, relatively ubiquitous and only slightly smelly fuel source: used vegetable oil.
Iwas always a little uncomfortable with the ecological implications of car ownership, but when some friends began experimenting with running diesel vehicles on vegetable oil, I saw it as a way to be part of a much more environmentally responsible fuel-supply chain. I was definitely intimidated and wondered if I could manage a backyard conversion. My mechanic skills were limited, although there had been some bright moments: once, working on a San Francisco stoep I repaired a motorcycle’s fuel-tank valve with only J-B Weld and hazy memories of advice I’d once been given. I also replaced a car’s freeze plug successfully, albeit with a little handholding from a mechanic. Plus, I had serious concerns about the logistics of fuel sourcing. What eventually convinced me was another friend who got into the conversion game and was generous with both advice and fuel-collection opportunities. After several months of research I bought a 1979 Mercedes 300TD. A few months after that, I bought a used, partially complete and extremely dirty twotank vegetable-fuel system from a friend who had decommissioned it from a pick-up.
The system included a tank, hoses, a fuel-switching solenoid valve and a filter head. The additional tank is for the vegetable oil. When you first start the car, it runs off diesel fuel until the vegetable oil is heated to a level that reduces its viscosity enough that the injection pump can handle it. Once the oil is heated, you flip two switches and start drawing only grease. Coolant keeps the grease warm and flowing on the trip from the tank to the engine.
Because my tank was originally used in a pick-up, it was huge – 150 litres. Although this limited fill-ups and extended the car’s range, it also meant I couldn’t put the tank in the boot, as most converters do. I ended up lashing it down in the back seat. Having the tank inside the car raised the unexpected challenge of how to get the new lines through the firewall and to the engine. Cutting the hole (40 millimetres by 63) was truly a brutal ordeal. Because of the contours in the metal and the challenging access, a hole saw turned out to be useless. Instead I hacked away at the steel with a variety of tools, including twist drill bits, hacksaws, hammers and levers.
Once the hole was complete, things progressed more rapidly. To create the customised coolant circuit necessary to transfer engine heat to the vegetable-fuel filter, line and tank, I teed into the main coolant line coming out of the engine block and then ran it back into an accessible segment of hose just before the water pump. It was difficult to tease the air out of the system, but with lots of revving, several test drives and periodic additions of coolant, I finally got everything to circulate properly.
Next was the fuel system. The engine and injection pump required no modifications. The original diesel engine was actually made to run on peanut oil, so as long as fuel viscosity mimics that of petrodiesel, the engine will run normally. I added a length of heater hose with PEX threaded inside, which helps protect against the degradation vegetable oil can cause on plastics and rubber. A dedicated filter head and filter would remove impurities (particles and bits of food) from the oil. And two three-port solenoid valves and their associated circuitry would control the two fuels and keep them adequately separate. After adding numerous lengths of hose to allow for the proper return of unused fuel and the necessary brass fittings and hose clamps, I turned the key for the first test.
The engine wouldn’t start. It didn’t seem to be pulling any fuel at all, which I eventually realised was a priming issue. Fighting against tight and frustrating access, I had to open up the new hose clamps and fittings and unscrew the fuel filter more times than I care to remember in an attempt to carefully pour diesel fuel into any accessible air pocket. From there I would crank the starter to test my work, giving it frequent rests to cool down. Several times I had to stop everything to recharge the battery. In desperation I even topped off the diesel tank and jacked up the rear of the car to provide the system additional hydraulic leverage. All these approaches were incrementally helpful and finally the engine reluctantly rumbled to life – coughing, revving and dying and then going through the cycle again.
When it finally ran reliably and I’d completed a successful test drive, I was ready to wire the solenoid valves. I used a multimeter to identify a positive tab in the fuse box that is only live with the