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PLEASE NOTE THAT THIS FAQ IS OUT OF DATE. Work is being done to move this to a wiki format with updated information, but please check out the TDIClub forums (http://forums.tdiclub.com) in the mean time for more updated information.

TDI FAQ

3) Fuel

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3) Fuel

  1. Specifications - The recommended fuel for the TDi models of Volkswagens sold in North America is Diesel fuel number 2. This is the usual type of Diesel fuel sold at retail fueling stations. Nearly one in four gasoline stations in North America also sells Diesel fuel.
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  2. Cetane - Diesel fuel is rated in cetane number (roughly analogous to gasoline octane). The common cetane rating is 40 to 45 in North America. A higher cetane rating number indicates a "premium" grade which is not required for the Volkswagen TDi. The use of a higher cetane fuel may increase performance and mileage, but generally costs more per gallon. Decide for yourself if the cost increase is offset by improvements in performance and mileage.
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  3. Winterized -Diesel fuel number 2 loses its ability to flow at temperatures below 20F (-7C.). This is caused by wax separation, and is commonly termed "gelling". The thicker wax component of the fuel may be blocked by the fuel filter although it can flow through the larger diameter fuel lines. The fuel filter in the Volkswagen TDi is heated to reduce this tendency. The heater permits the use of Diesel fuel number 2 down to a temperature of -10F (-24C). Most fuel companies "winterize" the fuel sold during winter months in cold climates. This winterized fuel resists gelling at low temperatures. The winterized fuel does not provide the same level of performance as the summer fuel, so your mileage will likely drop while using it. Be aware that the refueling range of the TDi may permit travel from a warm climate to an extremely cold one on one tank of fuel. It is recommended to fill up with winterized fuel before stopping the engine for a long time in a cold environment.
    Under warm conditions diesel behaves much like gasoline, i.e. it appears as a liquid, stinks, but is heavier and less volatile. As its temperature drops some of its less desirable properties become apparent. Diesel fuel consists of many different hydrocarbon molecules of varying characteristics, and of special interest is the temperature some solidify and become wax. The appearance of wax crystals is called "clouding", and the temperature at which this happens in a particular blend is referred to as "the cloud point". Oil companies adjust the cloud point to suit the various climatic conditions in different locations and the time of the year. The same brand name may be different in Maine from the product sold in New York and in Florida. Lowering of the cloud point is generally done by addition of heavier (higher boiling range) components (Napthalenes and aromatics) and other additives, but this also reduces fuel energy and consequently mileage suffers. Winter fuel is less economical and lighter than heavier summer fuel. As the temperature drops further, some hydrocarbons continue to remain liquid, but others form wax. The net result of very low temperatures (-50F?) is that what was liquid fuel at +50F can resemble a thick gel. Further information on diesel fuel can be located at the following web site: www.chevron.com/prodserv/bulletin/diesel/L1_toc_rf.htm

  4. Low sulfur - Lower sulfur content fuel is becoming more common as the limits of sulfur dioxide and other acid rain producing emissions tighten, and as it becomes necessary for manufacturers to use emission control components that do not tolerate sulfur in the exhaust from the engine.
    There is a somewhat mistaken impression that sulfur in the fuel acts as a lubricant for injector pumps, and this impression stems from an older chemical process to remove sulfur which also removed other chemical compounds in the fuel that were completely unrelated to the sulfur but which turned out to be important to the lubricating properties of the fuel. These refinery processes have been changed to remove the sulfur while still allowing the fuels to meet standards for lubricity, and in addition, the fuel pumps in current production have been designed to operate with the current low sulfur fuel. Volkswagen does not specifically make mention of guidelines regarding diesel fuel lubricant additives for the TDi.
    Ultra-low sulfur diesel fuel (ULSD) can be used in all current production diesel engines without any problems. As a bonus, the exhaust emissions of current production engines should improve slightly without changing anything in the engine, simply by consistently using ULSD.
    North American standards allow 500 parts per million of sulfur, which is considered "low sulfur" only by previous standards which were much higher. Modern "low sulfur" fuel sold elsewhere in the world contains 50 parts per million or less. Lower sulfur content in the fuel allows engine designers to use more advanced emission control components. .
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  5. Non-taxed (off road) - Diesel fuel sold for use on roadways in the U.S. is subject to the Federal Highways Fuel Tax. Diesel fuel for stationary and commercial marine engines is not subject to this tax. The non-taxed fuel is dyed to indicate that tax has not been paid. The dye is concentrated so that even a small amount of dye will mark a large quantity of fuel. Home heating oil is the same as Diesel and is also dyed. Evidence of dye in road use vehicles can result in hefty fines. The color of the fuel can be seen through the translucent fuel lines to and from the fuel filter. (The situation is similar in Canada).
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  6. Bio-diesel - Bio-diesel is produced from currently grown vegetation without the intervening eons for partial fossilization. The fuel produced from this source reportedly produces even lower emissions. The current supply is scarce and there are very few locations which currently offer it for retail sale.
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  7. Additives - Additives are available to correct a host of implied deficiencies in fuel content. Volkswagen specifically mentions additives for use in its gasoline fueled vehicles, however Volkswagen makes no mention on the use of additives for the Diesel fueled vehicles. This lack of mention may be interpreted as you wish.
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  8. Refueling - Refueling at automotive service stations is the same as with a gasoline fueled vehicle. Refueling at stations which primarily service the trucking industry is slightly different. The pump nozzles are a larger diameter to permit quicker filling of the large truck tanks. The filler neck of the TDi will accept this large nozzle. The faster fill rate allowed by the large nozzles may overcome the TDi venting abilities and could cause splash back, or excessive foaming of the fuel. Filling at less than the full speed will avoid these problems. Filling at truck stations has advantages during seasonal changes. The storage tanks at truck stations is likely to be depleted and refilled at a higher frequency than the automotive station. The changes of fuel blends for weather conditions are more likely to be up to date when the turn over is higher.
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    All information Copyright 1996-2002 Fred Voglmaier.
    All rights reserved. This FAQ (Frequently Asked Questions) may not be reproduced without written permission.

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