what should I ask a mechanic to look for before buying an uncoverted diesel car that was powered with waste oil?

September 27, 2010 9:15 PM

I'm considering purchasing a 2009 VW Jetta TDI with 12k miles. During summer months, the owner used waste oil for fuel. (no separate tank or conversion kit). But we're in the mountains, and it gets brisk even in the summer. Should I walk away altogether? Can I ask a mechanic to specifically check out the fuel injection system? What else should I ask of the mechanic? Thanks.
posted by comah to Travel & Transportation (7 answers total)

Best answer: Saying 'waste oil', you mean used vegetable oil, right? If the oil was cleanly filtered there should be no long-term damage.

Post your question on TDIclub.com in the Alternative fuel section or the A6 Jetta section, and you will get some more knowledgable replies.
posted by TDIpod at 10:03 PM on September 27, 2010 [1 favorite]

Oh, and VW only approves 5% biodiesel in its TDI engines, so the remainder of your powertrain warranty may be void if a dealer detects the veggie oil.
posted by TDIpod at 10:10 PM on September 27, 2010

Response by poster: thanks! I did know about the warranty void, and I think that's accounted for in the price.
posted by comah at 10:39 PM on September 27, 2010

Waste oil is not biodiesel.

I can't believe VW doesn't allow biodiesel. According to what I've read, it is better in all ways. Because I was going to say that running a couple of tanks of biodiesel would probably shake out any residual straight oil left in the system.

Also, even straight non-winter diesel will gel up on cold days, so you have that to consider.
posted by gjc at 3:00 AM on September 28, 2010

There's a shop in my (liberal college) town that does biodiesel conversions. Basically it's just a separate tank, a system for warming the biodiesel to liquefy it in cold weather, and a big, honking, oversized replaceable-element filter. If your car has been running used oil through the OEM fuel filter, I'd have that replaced.
posted by jon1270 at 3:19 AM on September 28, 2010

gjc is right that biodiesel and waste oil are different. Sorry for helping to muddy the waters.
posted by jon1270 at 3:21 AM on September 28, 2010

Waste vegetable oil must be warmed to a certain temperature, or there are risks it will coke the injectors. This is even with a separate tank, and a separate filter system. For this reason, the adaption of a diesel vehicle includes a separate tank, filter, and a heating system to keep the WVO warm.

The process is to start the vehicle on diesel, wait for everything to get to temperature (indicated by a separate, installed sensor) and then switch to WVO.

You must switch back to regular diesel and give it time to run all the way through the lines to clear it of WVO before shutdown. See greasecar.

Are you sure they used WVO, they weren't referring to biodiesel (which as noted, doesn't require conversion, and can be used straight) ?

I don't know of many people who would drop WVO into a 2009 vehicle... The separate tank is also because veg oil goes rancid. You don't want it in your regular diesel tank, nor regular diesel filter. If they did, this car has more problems coming than you can shake a frenchfry at.
posted by iiniisfree at 7:58 AM on September 28, 2010