Trouble Shooter BMW's N43, N53 and N54 engines

This month we examine the issues surrounding Piezo Injectors on BMW's N43, N53 and N54 engines.

Words & Photography: Andrew Everett

Ahot topic at the moment is that of the infamous Siemens VDO Piezo injectors fitted to direct injection BMW engines (and others) since late 2007 – these will be the four-cylinder N43 1.6 and 2.0 engines, the six-cylinder N53 and the N54 twin turbo six that arrived in late-2006 as the first proper direct injection BMW engines. There have been mechanical injection BMW engines before such as the Kugelfischer injected units such as the 2002tii and pre-1976 E12 520i – there was the N73 BMW V12 in 2003, but it’s direct injection was using ordinary type injectors on a pressurised fuel rail and thus a halfway house.

The Piezo injector has not been without its troubles and this article should help you understand what they are about and why they fail.

What is a Piezo injector?

Piezo electricity is an electrical charge that builds up in certain materials when under very high pressure, crystals being one of these elements. This was discovered in 1880 by Jacques and Pierre Curie, the latter being the husband of Nobel prize-winning physicist Marie Curie. In the application of a fuel injector, the Piezo injector is a solid steel unit about six-inches long which fits into a machined hole in the cylinder head combustion chamber. This means it injects fuel directly into the cylinder as opposed to firing a fuel mist into the inlet manifold to be mixed with the incoming air charge – in other words it sort of works like a direct injection diesel engine.

The injectors are supplied by high pressure fuel from a mechanical pump, a camshaft operated plunger type on the N43 and a rotary pump on the N53 and N54. The pump supplies pressurised fuel to a rail with steel fuel lines and threaded unions to withstand the pressure – the injectors run at up to 40,000psi. There is also 200 volts and 15 amps operating on some Piezo applications so be careful!

Inside the injectors are Piezo crystals; apply voltage to them and they contract and expand incredibly quickly operating the injector nozzle. The idea is that rather than just firing in one dose of fuel, the Piezo effect injects multiple but smaller amounts of fuel per combustion cycle. These injectors work by passing electrical current through a stack of Piezo crystals, causing them to expand; as the crystal discharges the current it contracts to the original size. The expansion and contraction of the crystals displaces the fuel within the injector, causing the needle valve to open and close extremely quickly – in fact it injects fuel into the combustion chamber around four times faster than both the old type electrohydraulic solenoid injectors found on diesels and even faster that the electronic pintle type injectors. Piezo injectors also ‘talk’ to the engine ECU by way of feedback so that absolutely precise fuel metering is achieved. By altering the electronic input, the Piezo crystal behaviour can also be altered such as when you are coasting or cruising.

As well as more injections per cycle, Piezo injectors allow very high fuel system pressure and thus atomises the fuel much better for a better spray pattern. This results in improved combustion and an improvement in emissions, more torque and power plus improved fuel economy. The fact that a 320i engine gives close to 170hp and impressive low and mid range torque would vindicate the Piezo system.

What goes wrong?

Well, they wear out. It was only a year or so after launch that the N43 started to become a bit of a problem child with rough running from cold and misfires and the first revisions began. The N53 could be just as bad and the N53 and N43 injectors are the same unit. The Piezo crystal ‘stack’ when energised lifts the fuel injector nozzle by just 0.1016mm. Operating in very high combustion temperatures of 2500-degrees Fahrenheit upwards, the finely machined injector tip isn’t going to last forever and wear here is why the engine will start to run less than ideally. A Teflon seal is fitted to the injector tip where it fits into the cylinder head and a steel ‘tripod’ seal is fitted further up the injector body with the unit being clamped into the cylinder head by a steel clamp and an 11mm bolt. If anything is amiss with the sealing, that cylinder will lose compression. From here, combustion gasses will come up the side of the injector, circulate around the engine top cover and give the smell of exhaust gases. Ultimately, the injector will just be loose and an unburnt fuel air mix will be coming up past the injector as the spark plug coil is closed down by the engine ECU, often shutting down the next cylinder for safety. Driving the car in this state may well damage the catalytic converter and the NoX (Nitrogen Oxide) sensor as fuel goes through it. It can certainly soak everything in fuel.

Replacing injectors

At the time of writing there is a major shortage of new injectors for the N43 and N53 – the part number is 13537589048 with the all important index number after this. Currently the index number is 11 and this is the latest revision. Anything before this should be treated with suspicion, especially relics with index number 8 for example. The original injector number was 13517565137 and there cannot be many of these left. Any still working were obviously good ones!

Replacing an injector is not that difficult once the top engine cover is off. It’s a 14mm union nut at each end of the steel fuel pipe and officially you are supposed to renew these at a cost of around £28 each, although if it doesn’t leak once refitted, you’ve dodged a bullet. Once fitted, the replacement injector will need to be coded to the ECU with DIS or INPA BMW diagnostics and you need to know what you are doing.

Replacing the Teflon seal isn’t easy and ideally you want the special fitting tool – the stainless steel thimble type is as good as anything else. The old seal is removed with a Stanley blade and the injector completely cleaned. The new seal is left in very hot water for a few minutes, slipped into place (a plastic biro top can be useful) and the thimble is fitted to compress the seal back down to the correct size – it is only removed just as you are about to fit the injector. A light smear of grease to help it slot in is a good idea.

Can i mix injectors?

You can but chances are, the car will not run 100% when cold. Sometimes even with extensive adjustment via coding they are never quite right but should be fine when up to temperature. In the ideal world, a car with running issues needs a full set of new injectors but at £1500 supplied and fitted for an N43 or £2000 for an N53 this is an expensive fix. Be very wary of reconditioned/refurbished units – they are often a waste of time because they cannot be recalibrated and they might last long enough for you to sell the car. Used original injectors are a better idea and you’ll be paying about £75 each for Index 10 or 11 units and even then there is no guarantee of how long they will last. Only companies such as Bimmerprofs (bimmerprofs.com) who have the proper (expensive) ASNU GDI test equipment to measure and record flow rates stand a chance of cleaning and testing them properly and making sure that they are not paperweights. Bimmerprofs also designed and sell the Noxem NoX emulator, replacing the standard NoX sensor and allowing more accurate adjustment.

N54 injectors

The N54 is the twin turbocharged version of the N53. It used Piezo injectors and the current part number is 13538616079, replacing 13537585261, 13537565138 and 13537537317. The current index number is 12 meaning that they have been every bit a resounding success as the N43/N53 units. The N54 used elements of the previous M54 3.0 engine from the E46 with an aluminium block rather than the N53’s magnesium unit. The N54 was replaced in 2009 by the single twin scroll turbocharged N55 which reverted to solenoid-type injectors. Much of what has been said about the N53 applies to the N54 although the N54 was notoriously prone to high pressure fuel pump (HPFP) failures.

• 1. This injector is showing various things – the last two numbers mean it is an Index 11 unit. The two three number codes far left show the factory injector flow rates and begin with a 5 and a 2.
• 2. The Teflon seal has disappeared from this one. If the injector doesn’t seat properly after replacement, the seal will get burnt away. If the seat in the cylinder head has been damaged, it’s game over.
• 3. This BMW cross section shows the position of the injector in the head – the injector tip is very close to the spark plug and thus where the explosion happens. Further up is the ‘tripod’ seal.
• 4. This is the equipment Bimmerprofs use to check and clean an injector, without this you are wasting your time. Whilst old injectors are generally not ideal, a low mileage Index 8 that is properly cleaned and tested may be alright.
• 5. Injectors can be pretty well stuck in if they have been for a few years. This DP Tools slide puller is very good and gets them out nicely.
• 6. This photo of the head shows where the injectors fit as well as the sprung steel retaining clamps. The fifth red dot shows where the fuel pump fits.
• 7. Here's the N43 engine. removed just before the injector is fitted.
• 8. This is the ‘tripod’ seal, BMW part number 13537564751 – it’s best to replace them each time.
• 9. This is the stainless steel thimble used to keep the teflon seals compressed – it’s removed just before the injector is fitted.