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PLEASE NOTE THAT THIS FAQ IS OUT OF DATE.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



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2)    General FAQ

  1. Types of TDI Engines - Various cars use different engines which are generally described below. It should be noted that this document primarily focuses on the 90hp 4-cylinder engines, as these are the most common, and are the only ones seen in North America.

    1.2 litre 3-cylinder 61hp - This engine is used in the "3-litre" (referring to fuel consumption of 3 litres per 100 km) versions of the VW Lupo, Seat Arosa, Audi A2, sold in Europe. The injection system uses the "pump-nozzle" or PD system, and the turbocharger is a variable-vane type.

    1.4 litre 3-cylinder 75hp - Used in various Polo-based models sold in Europe. The injection system uses the "pump-nozzle" system, and the turbocharger is a variable-vane type.

    1.9 litre 4-cylinder 90hp (A3 type) - used in various models sold until approximately 1998 in Europe and North America. Within this document we will call this the A3 engine even though it was also used in other models, because the most popular cars sold with this engine were the A3-chassis Golf, Jetta, and Vento sold from 1994 until early 1999. Not all model years were available in all markets, and this engine was replaced by the newer type (here called the A4 type) gradually over a period of years.  Other models sold with this engine include the B4 Passat (1995 - 1997), VW Sharan minivan, and various Audi models.

    The injection system uses a distributor-type pump. On cars with transverse engines, the A3 engine can be distinguished from the later type by the spin-on oil filter on the front of the engine facing down at an angle, the air filter housing is at the right front corner of the car, and the two black plastic pipes to the intercooler lead towards the left front corner of the car. There are other differences, but these are the most easily visible.

    The turbocharger is a conventional wastegate type.

    1.9 litre 4-cylinder 110hp (A3 type) - used on the same models as the 90hp but not available in all markets - in particular, not available in North America. Visually it is almost impossible to tell apart from the A3 90hp engine, except for the red "I" in the TDI badges used on the car in various places.  The engine itself is identical to the 90hp model.

    The turbocharger is a variable-vane type, and the injection system uses a distributor-type pump which looks the same as that of the 90hp model.  Obviously there are differences in the ECU, and in the plumbing and controls for the turbo, since the turbo is not the same as for the 90hp model.

    1.9 litre 4-cylinder 90hp (A4 type) - used in various models available in Europe and North America from 1998 on, this gradually replaced the previous engine as the car models were changed.  Within this document we will call this the A4 engine because the most common application is the A4-chassis Golf, Jetta, Bora, New Beetle, and various models from Seat and Skoda using the same chassis.

    The injection system uses a distributor-type pump. The oil filter is a cartridge type, and sits in a vertical container on the front of the engine towards the left of the car.  On cars with transverse engines, the air filter housing is at the left side of the engine compartment and the two black plastic pipes to the intercooler lead towards the right front corner - exactly the opposite of the arrangement used on the previous model.

    On North American models, the turbocharger is a variable-vane type. On European models, the turbocharger is a conventional wastegate type. The North American A4 engine is essentially a Euro 110hp A4 engine but with engine controls optimized for emissions rather than performance.

    1.9 litre 4-cylinder 110hp (A4 type) - virtually identical in appearance and specification to the 90hp except for the engine controls; cars using this engine have a red "I" in the TDI badges.  Not available in North America.

    1.9 litre 4-cylinder 115hp "PD" - again similar to the A4-type engine but with the "pump-nozzle" injection system. Although the peak power is only slightly more than the 110, maximum torque is much higher and the engine has lower exhaust emissions. VW intends to gradually replace the 110hp models with this engine. Not available in North America, reportedly because the exhaust emission controls used on this engine cannot be used with fuels containing greater than 50 parts per million of sulfur, whereas at this writing (1999) fuels in North America can have as much as 500 parts per million.

    Models with this engine have badges with a silver "T" and red "DI".

    The injection system is the "pump-nozzle" type, and the turbocharger is a variable-vane type.  The lack of the injection pump mounted on the front of the engine is the obvious visual difference.

    1.9 litre 4-cylinder 150hp "PD" - Few details are available at this writing. Models with this engine apparently will have badges with "TDI" in all red.

    2.5 litre 5-cylinder - used in various larger models available in Europe. The injection system uses a distributor-type pump.

    2.5 litre V6 150 hp - used on various Audi-based models (including the VW Passat) starting in 1998.  Has 4 valves per cylinder, and uses a distributor-type injection pump.  The advantage of using 4 valves per cylinder for a diesel engine is not so much for better breathing, as it is to allow the injection nozzle to be located precisely in the center of the combustion chamber and oriented vertically.

    2.5 litre V6 180 hp - differs from the 150 hp model only in the use of common-rail injection instead of the distributor-type pump.

    3.3 litre V8 - used on the Audi A8 available in Europe.

  2. Cold Starts - This is one of the big questions that non-TDI drivers have: How well will it start in cold weather? The answer is that they behave quite well. Volkswagen was confident enough in the cold-starting ability of this engine that there is no engine block heater designed to suit the TDI engine.  For those living in truly cold places - we're talking Edmonton or Winnipeg cold, those in balmy Boston have nothing to worry about - there are ways of making the block heater designed for the previous 1.9 turbodiesel fit this engine, it's not easy but it can be done.

    Diesel engines operate on a principle of compression ignition, rather than a spark ignition as in a gasoline fueled engine. The air within the Diesel engine's cylinder is compressed much more tightly than a gasoline engine, usually 2 to 2 1/2 times more tightly. This high compression heats the squeezed air to a temperature that causes the Diesel fuel to burn as soon as it is injected. Cold temperatures suppress the tendency for self-ignition of the Diesel fuel. "Glow plugs" are used to create a hot spot within the cylinder to help force ignition. The glow plugs are small electric heaters which are turned on before the starter is operated. The amount of time required for these heaters to obtain a sufficient temperature to ensure ignition depends on the engine's temperature. When the coolant temperature is above 9 C, the glow plugs may not come on prior to starting. On cold winter nights, they may take several seconds to heat up (7 to 10 seconds is typical).

    Many measures have been taken to ensure reliable starts in cold weather, but there are some factors beyond the control of the car.  More than one person accustomed to gasoline engines has merrily hopped into the car during cold weather, stuck the key in the ignition and turned it all the way to the "Start" position (which prevents the glow plugs from operating!!!) and then wondered why the car acts up. The proper procedure is to switch the key to the "On" position and wait for the yellow glow plug lamp to go out before cranking the engine. The amount of time you have to wait ranges from none whatsoever (if the coolant temperature is above 9 degrees C), to about 10 seconds (if the car has been sitting outside in -10 C for some time). If this is done, the engine normally starts with perhaps a second of cranking, even at -10 C.  Owners have reported starting their engines at temperatures below -30 C, which is about -20 F. Better make sure the battery is healthy, at those temperatures - but that's no different from any other car.

    The other factor beyond the control of the car, is the quality of the fuel. In Canada, diesel fuel must be provided to stations "winterized" to expected outdoor temperatures as low as -45 C in some areas.  The most common source of problems is when one purchases fuel at an out-of-the-way station, which may go months between refills of their underground tank. Prudent and experienced diesel drivers go to stations that have lots of traffic during cold weather, to avoid getting a tank full of summer diesel in the dead of winter.

    The problem with summer diesel is that it "gels" or "crystallizes" below a certain temperature. The TDI engine is capable of operating at temperatures below what could normally be expected for a given fuel, because the fuel filter is heated by fuel being returned from the injection system. If fuel gels up in the filter, the engine will generally start, but won't have power to do much other than idle for a few minutes until the fuel filter warms up ... but at least you'll get going. It will only handle so much, though, and the car will not run in -20 C with summer diesel fuel. Under very cold start-up conditons (and this means in the -30 C range), you may need to wait for several minutes with the engine idling before driving off to allow the fuel to be warmed. Otherwise, power will be impaired or the engine may stall as the injector pump will be starved for fuel. Because of the long range of a TDi, when you are driving from a warm to a cold climate it may be prudent to fill up with winterized fuel in the destination area before the system cools down. If you are in the unfortunate situation of a completely gelled fuel system and the car will not start, the only cure is place the car in a warm garage for a few hours.

    For extra insurance, diesel fuel anti-gel additives are available at truck stops and many auto parts stores. There are some additives which can be added "after the fact" to a fuel tank which is already gelled, and during extremely cold spells it is highly recommended to carry a container of anti-gel additive in the car. Using a portion of gasoline or kerosene in cold weather, as a substitute for an anti-gel additive, is not recommended, because these fuels do not have the proper lubricating characteristics and cetane number.

    It is normal to have somewhat reduced power and slightly higher fuel consumption when using winterized diesel fuel.

  3. Warm-up - Because the TDI engine is so efficient, it puts less heat into the cooling system than comparable gasoline engines. A TDI engine will not reach operating temperature by idling. There is no point starting the car a few minutes before you plan to use it in order to have a warm interior - it isn't going to happen. Heated seats are definitely a worthwhile option for those living in a cold climate!

    The best way to warm up a TDI engine is to start up and GO, after minimum essential warm-up. It's generally recommended to drive gently for the first few minutes, until the temperature gauge approaches the normal range.  When done this way, the warm-up period is not too different from that of a gasoline engine.

    If one gets stuck in traffic with a cold engine, now there's a dilemma, because the engine won't generate enough heat to warm up. If this happens, one suggestion has been to introduce some electrical load to make the engine work a little harder, like the headlights, rear-window defroster, and heated seats if you have them.

  4. Shifting Gears - Much has been made about what the best shift point is for maximum economy. It is known that the engine "likes" running at around 2000 rpm, and the turbo starts kicking in at about 1500 and is going full tilt by 2000. The author prefers shifting at about 2500 rpm during normal acceleration, then when cruising speed is reached, selecting a gear which puts the revs between 1500 and 2000.

    For absolute maximum acceleration with a stock engine, shift at about 4000 rpm, because power drops off quickly beyond this.  There is little to be gained by shifting at 4000 compared to shifting at 3500.

    For those not accustomed to manual transmissions, the TDI engine is among the easiest to learn with. The engine quickly and automatically builds up torque if the idle speed starts dropping, and the engine doesn't race away like mad upon the slightest touch of the accelerator pedal like many gas engines do. It's possible to smoothly let out the clutch and start off from a stop, then shift to second, and then to third, without touching the accelerator, and the car will pull it!

    For those who don't want to shift for themselves, there is limited availability of an automatic transmission with the TDI engine.

  5. Performance - It has been said that "people buy horsepower, but drive torque". This statement completely describes why the TDI engine is so easy to live with during day-to-day driving, despite the seemingly low horsepower rating!

    A typical multivalve 2-litre gasoline engine may have its maximum horsepower (perhaps 120 or so) at 5500 to 6000 rpm, and its maximum torque (perhaps 120 lb-ft or so) between 3500 and 4000 rpm, and is probably geared to run about 2700 rpm at 100 km/h. In top gear at highway speed, the engine is below its peak torque, and probably makes around 51 hp at that speed.  A downshift is usually needed to make a quick pass, or to get up a steep hill.  If the car has an automatic transmission, the torque converter will probably unlock going up moderate hills, in order to get the engine closer to its peak torque.

    With the TDI engine, maximum horsepower (90) is available at 3750 to 4000 rpm, and maximum torque (155 lb-ft) is available at just 1900 rpm. The engine runs 2100 rpm at 100 km/h and makes about 62 horsepower at that speed.  Hey, that's more than the gas engine ... by quite a bit, too! Thus, no downshift is required to make a pass or to climb virtually any highway grade. And if the car has an automatic, it won't unlock the torque converter, because peak torque is right there already.

    If you enter a drag race between these two cars, the gas car will probably win, because drag racing is about horsepower-to-weight ratio and little else. But who drives like that every day? Most people don't. Even people who think they do - usually don't.

    For what it's worth, owners report 0 to 60 mph times with a stock car ranging from about 10.5 to 12 seconds (mostly depending on the weight of the car) and top speeds well in excess of what one ought to be doing on North American highways. In other words, not too different from the performance of a 2-litre gas engine in a similar car. The cars that the VW TDI is installed in were all designed to handle autobahn cruising at 160 km/h (100 mph) with ease and all models will exceed that easily, so if you're buying a diesel in the hope of reducing the number of speeding tickets you get, you'd better find a different excuse!

    And for those so inclined, there are ways to significantly boost the performance of a TDI engine, discussed elsewhere in this document. These modifications don't change the basic character of the engine - it makes both more power and more torque - and they generally have little effect on fuel consumption unless you use the extra power all the time. One owner has reduced the 0 to 60 mph time of his New Beetle TDI to a hair over 7 seconds through suitable modifications - nothing exotic.

  6. Noise, Vibration, and Smell - If you're comparing the TDI to diesels of yore, there is no comparison.  It compares more favorably with 4-cylinder gasoline engines. It's a little louder at idle with a little more vibration transmitted to the passenger compartment (and it's worse when cold) but hardly intrusive, especially after it is warmed up.  It's actually quieter at highway speed than equivalent gas engines, because it's turning slower, just 2100 rpm at 100 km/h.  It's especially quieter during rapid acceleration when a gas engine would be buzzing away at 5000 rpm while the TDI is loafing along at 3000.

    Diesel noise and clatter are well isolated from the passenger compartment and can only really be heard with the windows down. Most true diesel-heads roll the windows down just to hear the engine!

    Diesel exhaust smell is noticeable outside the car after starting a cold engine, but is considerably reduced compared to older diesel engines.  Fuel smells are only an issue when refueling, and only if the pump is messy and covered with diesel fuel.  You don't want to get diesel fuel on your hands.  In most cases, it's possible to refuel without spilling any without special precautions, but some owners suggest carrying some latex gloves in the car if you have to deal with a messy filling station.

  7. Longevity and Reliability - These are well-known strong points of diesel engines. There is the issue of the timing belt, which must be changed religiously at the specified intervals, or else. Aside from that, VW diesels have proven to be very durable over the long haul. Some members of this forum tend to be "anal" about oil changes and stuff ... but in a way, that's as it should be, because diesel owners are generally in it for the long haul.

    The diesel engine has certain factors acting strongly in its favor, compared to modern gasoline engines. The engine itself is very heavily constructed due to the compression ratio. Lower exhaust temperatures than a gasoline engine extend the life of the exhaust valves and the turbocharger. No spark plugs, no ignition coil, no distributor, no plug wires, and diesel engines aren't fussy about air/fuel ratio the way gasoline engines are. Although the TDI has its share of electronics and sensors, virtually all of those sensors are "non contact" sensors that work either by magnetic fields, or by solid state electronics - i.e. the sensors have no components to wear out. And the TDI lacks what is among the most unreliable components of every modern gasoline engine ... there are no oxygen sensors in the exhaust system. It does have a glow-plug system (and some owners have had the relay conk out) but this system is far simpler than the ignition system of a gasoline engine.

  8. Fuel Consumption - This is another strong point of diesel engines. The TDI engine uses about 60% as much fuel as an equivalent 4-cylinder gasoline engine. In Canada, diesel fuel is less expensive than gasoline, so fuel costs can be expected to be a little over half that of a similar gasoline car.  Using current fuel costs of C$0.65 per litre for gasoline and C$0.60 per litre for diesel, and 8.0 L/100 km for the gasoline car versus 4.8 L/100 km for the diesel car, you save $2.32 every 100 km. Payback of the extra cost of about C$1500 comes in about 65,000 km, which is less than two years for many long-distance travelers and about three years for the average driver. Maintenance costs are assumed to be about equal (which is about right) and this doesn't take into account the fact that, at least in Canada, it is virtually guaranteed that at the end of that payback period, the diesel car will be worth MORE than the gas car ... possibly by more than the original difference in price! (Example; at this writing (late 1999), the going rate for a 1996 Passat VR6 with say 100,000 km is about C$15,000 while the going rate for a 1996 Passat TDI is about C$18,000...)

    In the USA, the situation is not quite as favorable, because of generally lower fuel prices, and because diesel is relatively higher priced compared to gasoline, compared to Canada. You can do calculations similar to the above for your area.

    Besides strictly economics, there are those who prefer the characteristics of a diesel engine, regardless of the payback period!

  9. Fuel Availability - In most areas, about one fuel station out of three or four has diesel fuel, and diesel is always available at truck stops along major highways. With the low fuel consumption, and the same size tank that the gas cars use, the TDI cars have an impressive range, easily 1000 km (600 miles) or more. Availability of fuel is not an issue, although as mentioned elsewhere, it's wise to fill up at stations with a lot of diesel traffic, especially during cold weather.

    Your VW has a fuel nozzle capable of accepting the nozzle at large big-rig pumps, although you'd better set the pump at the lowest feed rate possible to prevent a splash-back!

  10. Maintenance Costs - The cost of maintaining a TDI is not much different from maintaining a similar gasoline-powered vehicle.  You need to be a bit careful with oil changes, because you need to use oil meant for diesel engines, so this item tends to cost a little bit more.  There's obviously no need to do anything about spark plugs, plug wires, distributor, or anything like that, so this cost item is eliminated.  The air filter needs to be changed once in a while, same as for a gas engine.  The fuel filter also needs changing, and it costs a bit more than one for a gas engine, but it's only once every 50,000 km or so - not a big deal.

    The only pricey regular maintenance task is getting the timing belt changed.  After considerable indecision, VW seems to have settled on a change interval of 90,000 km / 55,000 miles, but check the owners manual for your particular vehicle as it may vary.  This is little different from the change interval specified for most gas engines, but for some reason the job costs more on a TDI.  Do not try to skimp on this. Use the genuine parts; change the tensioner too; change the serpentine belt and the V-belt at the same time. If you buy a used TDI, then you can either get a piece of paper proving the belt was changed at a certain mileage and believe it (or not), or figure on getting the belt changed to make sure.  If it can't be proven that the timing belt was changed, then assume it needs to be done.  DO NOT EVER neglect this!

    Some owners have reported being charged exorbitant costs for a "tune-up". Don't be suckered into this. Find out exactly what the repair shop proposes to do in the "tune-up", because there is very little to tune up. Most places will simply do an oil change, and change the air filter and possibly the fuel filter. Rarely will such a "tune-up" include cleaning the air intake screen or the intercooler. Any owner with even slight mechanical ability is capable of taking care of every one of these tasks with the possible exception of changing the fuel filter, and the price for a "tune-up" should reflect this. Sometimes repair shops will attempt to cover up their lack of diesel knowledge by specifying countless replacement items which don't really need to be replaced. Injectors, glow plugs, whatever.

    As for unscheduled maintenance, who knows, time will tell. There are forum members at or beyond 200,000 km / 120,000 miles and still going.

  11. Can I use a remote car starter? - In general, the answer is "No", because remote car starters generally do not have provision for the glow plug waiting period in cold weather. Also, some manual transmission models do not have a switch interlock to prevent starting with the transmission in gear.

  12. Where do I find a good mechanic? - Boy, if we can find an answer for this ...

    Normal maintenance on the TDI engine is not beyond the capability of any competent mechanic. Even changing the timing belt can be done by any mechanic who has half a clue. Unfortunately, there are many mechanics out there who don't have half a clue. Some of them even work at VW dealerships. Problems with mechanics doing things wrong are not isolated to the TDI. All modern car engines are extremely sophisticated and some repair jobs - particularly "Check Engine light"-type repairs - should be left to either a competent dealer, or to a mechanic who specializes in the repair of that particular brand of vehicle.

    Talk to other TDI owners in your area, and find out where they go. IF you have an electrical or "Check Engine"-type problem, make sure the shop you take it to has the computerized equipment capable of monitoring OBDII engine controls.

  13. Venting - In the filler neck of the tank, in the "9 o'clock"" position is a little black button. This is a vent relief valve that can be pushed with the fuel nozzle when "topping up" the tank to squeeze in a few extra litres. The vent valve is designed to prevent overfilling the tank. Overfilling could cause heat expansion to push fuel up the neck and create a spill. The vent keeps an amount of air in the tank until the cap is returned. The installation of the fuel cap presses the vent button and allows air in the tank to move up into the neck as the fuel in the neck moves down into the tank. Any expansion will now burp only air instead of fuel. Topping up using the vent valve should be done only when the car will be driven long enough to consume a gallon or so before stopping the engine. Don't top off with the vent pressed and then park in the sun. For accurate fuel mileage calculations always use the same technique when re-fueling. Either always "top up" by venting the expansion chamber, or never vent and stop refilling when the dispensing nozzle shuts off. That extra half gallon or two liters of fuel squeezed into the expansion chamber will make the trip consumption appear higher if this venting technique is used intermittently.

  14. Things you may not know, but should

    • It is not recommended to let the TDI engine idle for extended periods of time. Aside from wasting fuel, causing unnecessary emissions, and not accomplishing anything (the engine won't warm up at idle anyway), the turbocharger depends on having a certain minimum level of boost pressure to maintain the condition of the seals. Extended periods of idling may cause a certain amount of oil consumption, and the oil consumption may clog the catalytic converter. Some owners who have let their engine idle for a long time report the engine running poorly for some time afterward. And no, you don't need to worry about what's going to happen if you get stuck in a traffic jam now and again, it's not THAT serious. Just don't start the engine 20 minutes before you want to leave in the morning every day, in the false hope of having a warm interior.
          
    • Old diesel mechanics may suggest mixing ATF with the fuel to lubricate the injection pump.  DON'T DO IT!  This may cause damage to the catalytic converter.  Proper additives are available to accomplish this, although even those are probably not really necessary.
          
    • Old hands may also suggest mixing a portion of gasoline with the fuel in cold weather to prevent wax buildup. The owners manual cautions against this procedure. Use proper additives which are meant for the job, if necessary at all.
          
    • Another questionable practice involves disposing of used motor oil by blending it with diesel fuel. This is another thing which sounds likely to damage the catalytic converter.  Besides, used motor oil contains extremely fine particles of solids which may be fine enough to get through the fuel filter and into the injection pump and injectors, where they cannot possibly do anything good, and are a lot more likely to do something bad.

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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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