How I con­verted a 1979 Mercedes to run on a com­pletely free, rel­a­tively ubiq­ui­tous and only slightly smelly fuel source: used veg­etable oil.

Popular Mechanics (South Africa) - - Skills - BY BRAD WHEEL­WRIGHT

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

Su­per­mar­ket veg­etable oil is more ex­pen­sive than petrol, but many restau­rants will give you their old stuff for free.

To cre­ate the cus­tomised coolant cir­cuit nec­es­sary to trans­fer en­gine heat to the veg­etable-fuel fil­ter, line and tank, Wheel­wright teed into the main coolant line com­ing out of the en­gine block.

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