MK3 vs. MK4 generational differences and interchangeable parts
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Introduction

This article describes some generational differences and some interchangeable parts between mk3 and mk4 cars in general.  In the US market, the mk3 was sold as the jetta, passat, and passat wagon only.  The mk4 was sold as the jetta, jetta wagon, passat, passat wagon, new beetle, golf, and touareg SUV.

mk3= a3/b4 (3rd generation) 1997-early 1999 jetta and 1996-1997 passat
Click here to see model year differences and model differences between the mk3 jetta and passat
mk4= (4th generation) 1999.5-2005 jetta, and golf/new beetle models into 2006, 2004-2005 passat

Aside from the obvious cosmetic differences in the body panels, suspension, etc...

Comfort/equipment 

The mk4 cars may have, depending on options/year, options that were not available on mk3 cars such as: heated seats, trip computers, 2 din radio, or in dash cd changer.  If equipped with the cd changer, your mk3 has it in the trunk.  Mk4 had the cd changer in the trunk but there was a Euro only in dash 6 cd changer.  They also have different seats and seat brackets (these are not interchangeable with earlier generation cars).  After 2004 and the introduction of the pumpe duse engine, mk4 cars were slightly better equipped.  Click here for the cosmetic differences of the mk4 cars.

The mk4 jetta, jetta wagon, and golf all share the same front doors but are different from the rear door back.  

All mk4 cars came with remote keyless entry, mk3 diesel cars can be retrofitted with an OEM VW keyless module from a gasoline mk3 jetta to have remote keyless entry.

The mk3 cars used a vacuum driven door lock system to smoothly move the door locks, the mk4 cars used an electric solenoid system which is more abrupt but more reliable.

All mk4 cars after about 1999-2000 have an immobilizer in the key.  It's a chip inside the key which will shut off the engine after a few seconds if it does not detect the correct chip.  To see more specifics of the immobilizer, see 1000q: immo FAQ.

Mk3 seat brackets are different from mk4 seats and are not interchangeable without extensive modification and welding to the seat bracket.  Most mk4 seats also have side airbags, no mk3 seats have these.

The mk3's tiny fixed center armrest was replaced by a full sized tilting armrest that was much more durable.

The mk3 passat also did not have standard cupholders or a glovebox due to the airbag location.  The mk4 passat had both.

The mk4 cars also have convenience lighting at the bottom of the doors to light the ground when the door is open.

Safety

Mk4 cars may have, depending on options/year, side and head curtain airbags, stability and traction control.  Mk3 cars only had front airbags, none models have traction or stability control.

Mk4 cars came with ABS standard in North America, mk3 cars had ABS as an option.  You may be wondering if it is possible to retrofit ABS to a non-ABS equipped car - anything can be done with enough time and money!  However, it is so time consuming and expensive that it would be easier to just buy another car that already has ABS.  You would have to change all the axles and brake rotors, change the computers and wiring, splice an ABS pump and solenoids into the brake hard lines, add a brake pedal sensor, and warning lights.  

Some Mk4 had a full size spare tire, mk3 cars use a space saver spare.

Mechanical

Few mechanical accessories are interchangeable between mk3 and mk4 cars.  Some model years alternator are interchangeable.  All 1Z, AHU, ALH starter and starter solenoid are the same in all VW TDI from 1996-2005.

Generally speaking, the maintenance on the mk4 cars is slightly more complicated and will take slightly longer than on the mk3 cars.

The front suspension lower control arm bushings on the 1990-97 passat, 1985-2005 golf/jetta/newbeetle, 1995-2002 cabrio, 1990-1995 corrado are all the same, you can replace them with Audi TT bushings for a firmer feel, all are interchangeable.  Note that there were 2 versions of the Audi TT bushings, one small and one post-recall larger bushing. 

Suspension and chassis are improved/stronger/safer on the mk4 cars.  The mk4 rear suspension no longer uses a strut on a torsion beam.  The mk4 rear suspension places the damper and spring on separate mounts on a torsion beam.

All mk3 diesel cars have 4x100 wheel bolt patterns and all mk4 diesel cars have 5x100 wheel bolt patterns except the passat, which uses 5x112.  Refer to the OEM VW/Audi wheel gallery or 1000 answered questions: wheel fitment guide for more info on wheels.  Gasoline cars of each generation may use different bolt patterns/suspension than the diesel cars.

Mk3 jetta uses rear drum brakes, passat uses rear disc brakes.  All Mk4 cars use rear disc brakes.  All mk4 cars also use sealed rear wheel bearings.  Mk3 cars use tapered wheel bearings that require replacement or cleaning and repacking of the grease and tapered wheel bearings.

All mk4 cars are pre wired for a trunk mounted cd changer.  Mk3 jettas may or may not be pre wired for a cd changer.  No mk3 passat is factory pre wired for a cd changer although it might have been done by the dealer.

Shifter linkage design and transmission are slightly different in mk3 vs. mk4 cars.  They are not directly interchangeable but are very similar both internally and externally.  See 1000q: shifter cable linkage adjustment for more details.

The mk3 cars do not have an anti-shudder valve for smoother shutting down the engine.  This valve can be useful for stopping the engine in case of an engine runaway.  Read here for more information on engine runaway.

Sheet metal on the inside edge of the wheel well obscures the end of the transmission case in the mk3 cars.  The same spot is covered by a removable plastic liner in the mk4 cars.  This is very relevant in determining how much access you have for swapping the 5th gear to increase fuel economy and lower highway engine rpms.

The engine oil filter in the mk3 cars is a spin-on metal cartridge type accessed from below, mk4 cars use a replaceable soft filter in a housing type accessed from above.  MK3 cars require you to remove the plastic engine under cover to access the oil filter.  The engine oil drain plug is easy to reach and does not require the removal of the plastic undercover.  This is a moot point because the long oil change intervals require an oil filter change each time anyways.  In mk4 cars, the engine oil filter is accessed from above and is easy to reach, but the engine oil drain plug still requires you to remove the plastic engine under cover.  So you still need to remove the engine cover and drain the oil from below anyways.  Note: some people use an oil extractor on the mk4 cars so you don't have to remove the under cover - this lets you suck the oil out fro the top.  However, it's recommended that you still drain the oil from below the car to inspect the engine.

 

Engine

Note:  the AHU/1Z engine uses the same mounting points as the older ME/MF/1V engines used in the mk2 cars so the engines can physically fit into the mk2 engine bays.  The mk1 cars need modification to the passenger side motor mount to fit the engine.

The rods in all diesel mk3 mk4 cars that use the 1Z, AHU, and ALH engine, all diesels up to and including 2003 for North American market, are the same, part # 028 198 401F . 

The bore (79.9mm) and the stroke (95.5mm) are identical.  In addition to the rods, AHU ALH engines share the same pistons.  All engines use 5 bearings for the crank.  All TDI pistons can physically fit into other generation cars.

The pistons in the mk3 jetta AHU and mk4 jetta, golf, new beetle ALH engine are the same.  They are slightly stronger than the 1Z pistons because of thicker ring lands but it does not make any significant difference in stock or close to stock power levels.  The piston ring on the newer pistons is closer to the top.  This reduces smoke and improves emissions.

The fuel temperature sensor is the same for all the engines as well, part # 028 906 040 C. See how to change the fuel temperature sensor at 1000q: fuel temperature sensor replacement.

The engine oil pump on the mk3 cars is a roots type gear drive. The mk4 oil pump is sprocket chain "gearotor" type driven off the crankshaft.  The advantage is less parasitic losses and faster cold oil pressure.

The engine oil filter on the mk3 is spin on, underneath the car.  The mk4 oil filter is cartridge type from above the car.

The fuel injection pump from the mk4 TDI is not a direct bolt on part but will work if you modify the mounting bracket and sprocket.  The electrical plug also has to be mated.

The mk3 oil pan is steel with a gasket, the mk4 oil pan is aluminum with a liquid silicone gasket.

The mk4 aluminum oil pan is easily road damaged.  This could lead to oil starvation and total engine seizure and failure.  The easiest solution is to put a metal engine under cover, replacing the plastic under cover.  The plastic under shield is a dust, sound, and aero cover, but it should still always remain on unless you replace it with an aftermarket shield because it still better protection than no protection.  It can also be damaged by improper towing with a wrecker, or wheel lift tow (the type of tow truck that lifts only the front wheels with a metal yoke, as opposed to a flat bed which pulls the entire car onto a flat bed).

The mk4 valve cover is magnesium.

The mk4 engine is mounted differently than the mk3 engine.  The mk4 engine uses a "dogbone" pendulum mount in the rear.  This helps noise, vibration, harshness.

The mk4 cars can retrofit a 100,000 mile timing belt but the mk3 cars use a different timing belt/roller and must stay to their normal 60,000 timing belt change interval.  Parts between mk3/mk4 are not interchangeable.

Water pump is driven off the accessory belt on the mk3 cars, water pump is driven off the timing belt on the mk4 cars.

The turbocharger in the mk3 cars is a conventional turbo.  The turbocharger in mk4 cars are variable vane.  

The intercooler is on the passenger side on the mk4, the intercooler is on the driver's side on the mk3.  Click here to read what does an intercooler does and to see a picture, part of the article on turbocharging.

The EGR (exhaust gas recirculation) valve on the mk3 cars is part of the intake manifold and is not removable.  The EGR valve on the mk4 cars is attached with 3 bolts and is removable for cleaning or replacement.

The mk4 cars have an anti-shudder valve on the EGR valve body.  It looks like a throttle but it only closes at engine shut down, to cut off air.  All other times it should be open.  The biggest advantage is that if you have an engine runaway, turning off the key should cut off air to the engine and should stop the engine runaway.

In the US, all mk3 TDI are direct injection.  1999.5-2003 mk4 are also direct injection but all 2004-2006 mk4 cars are pumpe duse and got some other minor cosmetic differences.
See 1000q: direct injection vs. pumpe duse for more details on this.
See 1000q: mk4 cosmetic changes to see the year by year mk4 cosmetic differences.

All mk3 cars use a solid single mass flywheel, all mk4 cars use a dual mass flywheel.  A dual mass flywheel is made of 2 pieces bound together with dampening material to help absorb driveline and engine shocks.  The significance of this is that the dual mass flywheels can sometimes weaken and fail, damaging the engine and transmission.  If you ever do a clutch job on a mk4, an option is to replace the dual mass flywheel with a solid flywheel.  This will increase transmission vibrations and increase overall noise and harshness.

Some Mk3 jetta and passat used a clutch assembly which is stronger than the later years of the mk4 a4 clutch.  The 1998-2000 mk4 diesels used a Luk clutch which is stronger than the 2000+ years that use a sachs clutch which is only rated to 180 ft/lbs of clamping force.

 

 

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