This article shows the 3 basic types of direct injection diesel fuel injection used in TDI engines sold in North America: - the Bosch VE type injection pump, pumpe duse unit injector, and common rail diesel (CRD). Engine generations in Volkswagen TDI didn't always change with body generations. Direct injection systems inject fuel directly into the combustion chamber instead of into a pre combustion chamber or intake manifold. To keep it simple, this article focuses only on the basic differences in fuel injection. Cars outside of North America may have gotten some of these technologies before or after North America. See the FAQ button linked above to see a full list of DIY, "how to" specific to your car, and more general diesel FAQ.
Here is a quick summary of each generation. A detailed description is below.
TDI was first introduced in the 1989 Audi 100 5 cylinder 120hp TDI engine but North American didn't see TDI until 1996. The 1996-1997 Passat and some 1997 Jetta use the 1Z engine. The 1997-1999 Jetta use the AHU engine (same pistons and rods as the newer ALH engines). Since 1997 was a transition year for the TDI engine, most 97 should have 1Z but some late builds might have AHU engines. These cars were never 50 state emissions legal - you couldn't buy them new in states with CA emissions standards, you could only buy them used.
These engines share the basic engine layout with earlier VW 4 cylinder engines. They all use the Bosch VE VP 37 type rotary electronic fuel injection pump. VE stands for verteilier, German for distributor. VP 37 is the model. The pump distributes fuel to the injectors using a rotary pump instead of an inline pump used on many diesels. The injection pump is driven by a sprocket off the timing belt. There were many models of Bosch injection pumps; TDI pumps are all electronically controlled and most can be modified to fit on any 1996-2003 TDI. See 1000q: IP conversion for details on how to modify the ALH pump to fit on an older car.
In 1998, VW introduced the new ALH engine in the New Beetle (98-99 Jetta still used the AHU). Manual transmission cars with the ALH engine are still the most fuel efficient and most reliable TDI. 1998-99 New Beetle and all 1999.5-2003 New Beetle, Golf, and Jetta use the ALH engine. See 1000q: early mk4 buying checklist for a guide on what to look for on these cars.
These are all pumpe duse (PD), a new type of direct injection. It had been used in European TDI for a while before coming to North America. Check out the cosmetic differences between mk4 pumpe duse and non-pumpe duse Jetta to help identify what car you have (2004-2005 Passat TDI, BHW engine, and Touareg V10 TDI are all PD). The change to PD was to meet newer emissions standards.
PD engines have significant mechanical differences from the earlier non-pumpe duse cars. 2004-2006 4th generation cars use the BEW engine (2004-2006 Golf/New Beetle) and 5th generation cars use the BRM engine (2005.5-2006 5th generation Jetta TDI). The difference is because the Jetta changed over to the 5th gen in 2005.5. All PD use a low pressure electric fuel pump (about 6-8 psi) in the fuel tank, something that earlier TDI did not. For a buying checklist for the early pumpe duse engines, see 1000q: pumpe duse engine checklist. For the A5 body pumpe duse, see 1000q: early A5 Jetta TDI buying checklist.
These use common rail engines which saw major changes. Common rail runs cleaner and more powerful but since it's a new technology, has had some reliability and teething issues in both diesel and gasoline cars. The change to common rail was to meet 50 state and Euro emissions standards. Because of greater advertising/awareness, supply, and higher fuel prices, these TDI are by far, the most common.
While the 2009-2010 Jetta sedan uses the same body as the 2005.5-2006 Jetta, it has a completely different engine/transmission and emissions control system. The engines were the CJAA and CBEA for North America. Again, body style does not necessarily follow engine model for all TDI. The 2010 Audi A3 is the first TDI passenger car that Audi has sold in North America (they were sold in Europe for years). It uses the same common rail engine found in the newest Golf/Jetta TDI. To see more details on the 2010 Audi A3, see 1000q: Audi A3 TDI buying checklist.
The 2012 Passat TDI uses what I call "version 1.5 " of this generation of common rail engines. While retaining many basic characteristics, the CKRA engine on the Passat uses a different fuel pump, fuel injectors, turbo, and air-water intercooler like the V2.0 mk7 engines. Fuel pump failure is not yet noticeable except a few random failures but long term reliabilty after high miles is still unknown.
These also use common rail but in a V6 engine longitudinal TDI engine and with AWD. While many of the basic principles of fuel injection apply to these larger V6 engines, the engine itself is very different. See 1000q: CATA engine FAQ to learn some basics about the V6 TDI engine. The major difference to the end user is much more power, availability with AWD quattro, and that these all use urea solution injection into the exhaust to meet emissions standards (see the below videos).
These use version 2.0 of the common rail engines. The core engine is a new generation, the EA888. Major differences are integrated balance shaft, variable cooling and oiling, air-water intercooler, separate cooling loops for the head/block, and major parts revisions. The air-water intercooler fixes the intercooler ice problem and the variable cooling lets the car warm up faster. Long term reliability is unknown since it's a new design.
The myth of the heated fuel filter
A common misunderstanding on all generations of TDI is that they had electrically heated fuel filters. No TDI was ever equipped with an electrically heated fuel filter from the factory. All but a few early 1998 New Beetle TDI had fuel filters that were warmed a little bit by the return line fuel. When fuel returns from the engine it's warmed by radiant heat coming off the engine and from pressurization. When it's cold, the warmed return line fuel is recirculated through the fuel filter and back to the engine to help warm up and prevent fuel gelling. If the fuel gets too warm a thermostatic T bypass sends it back to the fuel tank.
There were a few TDI in Europe that came factory equipped with coolant heated fuel filter adapters. These used engine coolant to warm a sandwich heater element above the fuel filter as shown below. They can only be used with certain screw-on fuel filter styles. If you wish to add a heated fuel filter, it's cheaper/easier to loop a coolant hose around the fuel filter or use a stick-on electric blanket type heater.
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If your engine cover looks like one of these (mk3 on left, mk4 on right), this is what you have.
These engines use the Bosch VE VP 37 fuel injection pump. There is no electric fuel pump anywhere on these cars. The fuel is drawn from the fuel tank under low pressure suction by the injection pump and pressurized by the injection pump to over 3300 psi at the injector. Try to think of the injection pump more like a pressure wave generator than a simple water wheel type pump. Timing of the injection pump sprocket is very important. This is a common cause of timing belt job mistakes, engine running issues, and no start problems. See the full list of how to linked at the top to find the timing belt "how to" for your engine. The great pressure generated at the injector is created by restricting and focusing the wave down into a small tip, the injector nozzle.
Each fuel injector is a spring loaded, 2-stage, 5 port injector. All engines are water cooled, 4 cylinder, 8
valve inline engines. Here is a cutaway of a direct injection fuel
injector. Note the 2 stage injection - a pilot injection before the main injection
softens the pressure waves of combustion and signal the start of injection
to the needle lift sensor. The main injection occurs around 3190-3335 psi. The springs close the nozzle when the fuel pressure inside the injector
drops. Please refer to 1000q:
TDI nozzle and injector FAQ for detailed technical information and 1000q:
nozzle replacement procedure if you want to remove the fuel injectors or
replace the fuel injector tips (nozzles).
HP: 90 @ 4000rpm
Torque (ft lbs): 149 @ 1900rpm
HP: 90 @ 3750rpm
Torque (ft lbs): 155 @ 1900rpm
Here is a video explaining the basics of how this type of hole injector
works. It's not specific to TDI but gives you an idea of the spring-fuel
pressure relationship. You can also see how there's some fuel that is not
injected and is carried away. This fuel helps cool the injector and nozzle
by carrying away heat.
Here is a picture of the cylinder head of a pre-pumpe duse car. This looks like most SOHC (single overhead camshaft) cylinder heads. Compare this to the PD head shown below.
If your car is a 2004-2006 VW TDI and was sold in North America, it has a pumpe duse engine.
This includes all Jetta, New Beetle, Golf, Passat, and Touareg TDI. The
most noticeable change is the pumpe duse injectors and lack of a timing belt
driven Bosch VE fuel injection pump. If you have a mk4 Jetta/Golf/Beetle engine cover
that looks like the one below you have pumpe duse. 2004-2008 Jetta, Passat, and
Touareg TDI sold in North America were all pumpe duse.
Pumpe duse is VW and Audi's brand name for unit injector technology. Each injector is actually a miniature fuel pump - pumpe duse is roughly translated as pump nozzle. Unlike earlier TDI which had no electric fuel pump anywhere, pumpe duse cars use a low pressure (about 5-10psi) electric pump in the fuel tank, called a lift pump, to move fuel to the engine. A medium pressure fuel pump at the engine called the tandem pump, (also a vacuum system pump) at the end of the camshaft pressurizes fuel to 50-110+ psi into the fuel rail inside the cylinder head. Fuel enters each injector and when actuated by the camshaft, pressurizes the fuel up to 27,846 psi. The fuel spray is carefully timed by electric solenoid. The high stress on the camshaft and fuel injector is partly why VW recommends that pumpe duse cars use engine oil approved by VW for pumpe duse engines. Refer to: 1000q: pumpe duse engine oil for a list of some of the approved oils which are available in North America. Unlike earlier cars, both manual and auto transmissions cars use a fuel cooler in the front of the car due to the higher temperatures that the pumpe duse system puts into the return line fuel.
The camshaft has an additional four lobes over the older engines to actuate the pumpe duse injectors. Because these take up room, the camshaft valve lobes are narrower than in the past. A very common issue is worn camshaft lobes. If the engine sounds very poor at idle and is suffering from running issues, it probably has camshaft wear. See 1000q: pumpe duse camshaft inspection for more details. The only other reliability issue specific to this type of engine is ovaled injector bores (very rare issue). All PD engines will put a little soot into the fuel, turning the fuel filter black quickly, but excess could indicate compression leaking through the injector bores.
Aside from the fuel injection differences, the engine components are also different. Due to higher piston pressures, the pistons are heavier, have a smaller wrist pin, and do not have impressions for the valve heads like earlier pistons. They also feature slightly stronger connecting rods and bigger rod bearings. All engines are still water cooled and 4 inline cylinders but the Volkswagen Passat engines are now slightly different than other TDI. The Passat engine is a 2.0L, 8 valve longitudinal engine and uses different components than the 1.9L 8 valve engines. Note: although sales brochures list the VW Passat as 16v, the 16v engine was never imported to the North American market. The 16v engine was only available in Europe. Many Passat TDI have had faulty chain drives and have had the oil pump assembly replaced with a gear driven assembly. See 1000q: Passat TDI oil pump chain FAQ for more details.
Below left is a cutaway of a pumpe duse injector. Below right is a diagram of the injector in the cylinder head - the fuel rail is built into the cylinder head. Big changes vs. past injectors are in the valve spring like top which is actuated by the camshaft, the side feed fuel injectors, and the solenoid injection action.
Jetta, New Beetle, Golf: all use the 8v BEW or BRM engine
Passat: this used a 2.0L 8v BHW engine (sales brochures state 16v
but all Passat imported to North America were 8v)
VW had PD injectors that used a piezoelectric valve instead of an electric solenoid but these were never available on North American cars.
Here is a picture of the cylinder head on a pumpe duse car. You can see
the pumpe duse injectors in front of the camshaft and the rockers above them which actuate the plungers.
Every 2009 or newer Volkswagen or Audi TDI sold in North America uses the common rail
engine. Here's the VW and Audi A3 TDI engine covers as a quick reference.
Volkswagen switched to all common rail engines in 2009 for North American to meet emissions standards. Their marketing materials state that they were meeting demands for quieter, more powerful engines but it came down to meeting emissions. Older engines cannot meet modern emissions standards. The Jetta/Golf TDI use engine code CBEA or CJAA, a 4 cylinder transverse turbodiesel with 140hp, 236lb-ft torque. The Touareg/Audi Q7 TDI use engine code CATA, a V6 longitudinal turbodiesel. This page focuses on the 4 cylinnder engines - see 1000q: CATA FAQ for details on the V6 engine.
They use the same low pressure in tank electric fuel pump as the pumpe duse
to pressurize fuel to about 5 psi from the fuel tank. It then goes to a
medium pressure pump above the passenger side motor mount to be pressurized to
about 73 psi. This is about what gasoline fuel tank pumps run at. It
then goes to the high pressure fuel pump (HPFP). The HPFP pressurizes diesel fuel up to 26,107 psi (1800 bar) in an "accumulator rail",
shown below on a CBEA engine, a common tube/rail shared by all of
the fuel injectors. Unlike most common rail systems which are made by
suppliers, VW makes the common rail themselves. (Bosch makes the fuel
pump). Fuel is metered at the pump but a fuel pressure
regulator valve on the right end of the rail also controls fuel pressure in the rail by
bleeding off pressure. Pressure is a byproduct of restriction so by
closing the bleed valve, fuel entering the rail has nowhere to go and becomes pressurized.
There have been a few reports of HPFP failure (sometimes repeated) which sends metal flakes into the fuel system. This causes severe damage to it, requiring replacement of the fuel injectors, valves, pump, and filters. The fuel rail, tank, and lines must also be throughly cleaned.
Since only VW-Audi and Bosch know the real numbers and causes of failure, we can only speculate on possible causes which include faulty manufacturing, a weak design, defective parts, low quality fuel, or contaminated fuel including gasoline in the diesel. See 1000q:
fuel filter inspection for a basic check to see if there are flakes in the
The big advantage of modern common rail systems are their piezoelectric fuel injectors. They offer much better control over fuel injection quantity and duration then the older spring type fuel injectors or solenoid and camshaft drive pumpe duse injectors. Unlike older systems, they can inject fuel totally independent of cam timing. Piezoelectric injectors use a type of piezoelectric crystal that expands in response to electrical voltage instead of electromagnetic solenoids like pumpe duse injectors. The crystals can switch in a few thousands of a second, 4-5 times faster than a solenoid. The expansion of the crystal moves the nozzle needle. The needle can also be made with about 75% less mass. The effect of these faster, more precise injections is quieter, more powerful, and cleaner combustion. By injecting small quantities of fuel before the main injection, it smoothes out the pressure waves created by diesel combustion, thus quieting and smoothing out engine vibrations. Older injectors had only a pilot and main injection while piezoelectric fuel injectors can have up to 5 injections per stroke. This results in a smoother and quieter engine. The common rail injectors also feature 8 holes, which lets the same amount of fuel be injected in a finer mist, which also increases fuel economy and power.
Here is a video explaining the common rail system. It's for an Audi but the VW uses the exact same system.
Switching from 8 valve to 16 valve cylinder heads and better fuel atomization from common rail diesel injection (CRD) let the cars achieve a cleaner burn compared to earlier cars. New emissions treatments catch what is left over, making the car 50 state emissions legal. Smaller 4 cylinder Golf Jetta TDI use post injection combustion instead of adblue fluid for emissions treatments.
Mercedes Benz common rail engines have been using this technology for a while
but were not 50 US state legal due to emissions.
Generally speaking, the 6 cylinder common rail diesel engines sold in the US
like the BMW 335d, MB bluetecs, and VW/Audi TDI w/V6 engines use urea injection to meet
emissions. Part of the reason why the 4 cylinder VWs don't use urea injection because
the engines are smaller and don't have to burn more fuel to move larger, heavier cars. Here is a
video explaining the emissions treatments with Adblue urea injection on a
The smaller Jetta and Golf engines use the fuel injectors to squirt a little bit of fuel after the main combustion to burn up trapped particulates in the exhaust system during an active regeneration self clean cycle. This regeneration occurs about every 400-700 miles depending on driving style, use, and how clogged the filter is. Some particulates get burned off naturally during from normal driving. See 1000q: DPF and CRD emissions FAQ for more details on the system.
Another big engine change for the US is dual camshafts. The timing belt drives the exhaust cam and the intake cam is gear driven by the exhaust cam. The intake manifold is now in front of the engine instead behind and the exhaust manifold points up instead of down. The engine will also have pressure sensing glow plugs. The common rail engine uses a gear driven oil pump. The engine also uses a higher turbo boost than earlier engines. The CBEA engine uses about 22 psi of boost.
To search for more on common rail injection, pumpe duse, or direct injection systems like Bosch, use the bar below: If you see an error or would like to add to this article, please post in the TDI forum