BMW Injector Replacement
Wayne R. Dempsey

[click to enlarge]
 

     The fuel injectors that are in your BMW are more than adequate for stock engines, and supply more than enough fuel for maximum power and open throttle.  For your engine to achieve maximum power, it must have a air/fuel ratio maintained within a certain range.  Adding more fuel to the mixture makes it richer, and won't necessarily give you any more power.  In fact, it is typically the opposite - a richer mixture will foul plugs and won't ignite as easily.  The goal of any good fuel injection system (whether it be carburetors or Motronic fuel injection) is to maintain the idea air/fuel ratio (typically about 14.67:1) for ideal combustion and power.  Adding higher flow, or larger injectors disrupts the balance of the engine, makes the engine's fuel management system run richer, and generally decreases power from ideal levels.  Is the same thing as adding more high powered lights to your living room - if the room was adequately lit to begin with, then you won't see better - you'll see worse, because it will be too bright for your eyes.

     So what are the exceptions to this rule?  There are a few.  Major changes in the displacement or flow of the engine can cause the engine to run lean.  Examples would include if you increased the displacement of your engine, or if you added a turbo or supercharger.  The supercharger compresses the air/fuel mixture and allows more of it to exist within the same size combustion chamber.  Therefore, ideally there should be more fuel injected into the combustion chamber when compressed with a supercharger, then is normally injected on a normally aspirated engine.  For owners who add a supercharger or turbo to their car, they need to be especially concerned about keeping the engine's mixture correct - the tendency is for these cars to run too lean, which can lead to destructive problems like detonation or overheating.

     In general, you should not upgrade or replace your injectors with larger ones, unless you have made a significant engine modification that would cause the engine to run lean.  If you are replacing injectors, then make sure that you use ones that have stock flow rates for your engine - don't buy ones that have higher flow rates thinking that it will give you more power - it won't.

     So why would you want to replace your injectors then?  Well, as the engines get old, the injectors tend to fail and leak.  If you pull fault codes out of your computer, it may tell you that you have a faulty or leaking fuel injector.  See my article on Reading BMW Fault Codes for more details on how to do this.  You may also find that you can see or smell a particular injector leaking.  If this is the case, you may not have to replace the injector itself, but may only need to replace the injector o-rings.

     For the purpose of this article, I will assume that your intake manifold (which houses the injectors) is still installed inside of your BMW.  Most of the photos accompanying this article show the intake manifold removed from the car, as it is easier to show some elements without the manifold installed.  All of the injectors can be removed without removing the intake manifold though.

     The first step is to prep the car.  I like to tell people to pull out the fuse for the fuel pump (typically number 18), and then try to start the car.  The car will turn over and then die.  Do this about 1-15 times - it will help drain excess fuel out of your system.  Then, make sure that the car has cooled down - you don't want to be working with gasoline when the car is hot.  Have a fire extinguisher handy - there will be some spillage of fuel - it's nearly impossible to prevent.  Also, wear chemical resistant gloves if you don't want to get any gasoline on your hands, and make sure that you have plenty of paper towels or rags on hand to help you clean up.  Perform the injector removal in a clear, open, and well-ventilated space, and it may not hurt to have an assistant around in case there are any problems.

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     Pop off the long, thin plastic cover in the center that covers the wire harness for the spark plugs and the injectors (Figure 5).  Remove the two small bolts that fasten this wire harness box to the top of the intake manifold.  At this point, you will need to remove the connectors from each of the fuel injectors.  All six must be removed prior to removing the wire harness box.  Using a pair of needlenose pliers, reach in and disconnect the small retaining wires that hold the connectors onto the ends of the injectors.  Be careful not to drop these, as they can fall into the recesses of your engine, and be very difficult to fish out.  In most cases, you only need to undo one side of the metal clips, and the side will pop out when you lift up on the wire harness box.  Start with the injector that is closest to the front of the car, and word towards #6, which is located at the rear.

    With the clips disconnected, you should be able to remove the wire harness bar from the tops of the injectors, and move it out of the way (Figure 6).  At this point, you should have much better access to the injectors (Figure 7) - it's time to remove them from the fuel rail.  The fuel rail is the long, thin metal bar that runs along the top of the injectors.  Begin by removing the small, black, square retaining clip that fastens and secures the injector to the fuel rail (shown in Figure 8 and Figure 9).  Use a pair of needlenose pliers again to pull this clip off.  It pulls off from the side (it's C-shape), and should slide off with a reasonable amount of force.  Remove all six - this will allow you to remove and detach the fuel rail from the top of the injectors.  When all of the clips have been removed, then remove the two bolts that attach the fuel rail to the manifold.  One of these bolts can be see in Figure 10.

     At this point you should be able to pull off the fuel rail from the top of the injectors.  Use caution - although the fuel rail is made out of metal, it is easy to bend and break it.  Be very careful, and work from the front of the car to the rear - pulling and making progress slowly.  The injectors have big fat o-rings that are pressed into bores in the fuel rail - you are battling these o-rings as you lift up on it, and pull it out.  You can see one of these o-rings clearly in Figure 11.

     Depending upon the particular characteristics of your car, it may be easier to remove the injectors first from the manifold.  As you pull up on the fuel rail, some may stick in the manifold, or some may come out with the fuel rail - it all depends upon a variety of factors.  The bottom line is that it doesn't really matter.  The same fat o-ring which holds the injector into the fuel rail is also used to hold the injector into the manifold.

     When you have lifted up your fuel rail (expect some fuel spillage from the rail), you should be able to push it out of the way enough to be able to pull out the injectors.  If you don't have access, or can't move the fuel rail, then you may have to loosen up some connections on either side (it has rubber fuel line attached with hose clamps) to help a bit.   On my 325is, I did not have to remove or loosen any of these connections.

     With or without the injectors still attached to the fuel rail, you can now pull them out of the manifold.  They are held in place using the same big fat o-ring at the tip of the injector.  Simply pull straight up on the injector and it should come out of the manifold (Figure 12).  You may have to tug a little bit to get it out, but don't use excessive force.  Sometimes repeated wiggling helps.  Be careful of the injector tips (Figure 13) they are made of plastic, and are not available separately from the $80 injectors.  Do not damage them.

     With the injectors out of the manifold (Figure 14) you can now take them to be cleaned and calibrated.  Over the years, the injectors become dirty and may also not distribute flow evenly amongst all six.  It costs about $150 for all six to be cleaned, tested, and calibrated.  New injectors cost anywhere from $80-$120 apiece, making their replacement a somewhat pricey endeavor.

     There are three types of injector leaks - it can leak fuel into the manifold from the nozzle, it can leak fuel into the engine compartment from the fuel rail, and it can leak air (vacuum leak) from the manifold.  The first leak cannot be fixed at home - you need to have the injector repaired or replaced (I recommend replacement, as it will be pretty old anyways).  The fuel rail leak is easy to content with - simply replace the old, fat o-ring that seals the injector to the fuel rail (Figure 15).  This should be done anytime the injectors are out of the car.

     The third leakage area is a bit of a catch-22.  The tip of the injector needs to be removed from the injector.  While this seems easy, and indeed it is easy to remove, it is just as easy to damage the tip when you remove it.  I have not found a source for these tips, other than purchasing a new injector ($80-$120), so the risk here in replacement is very high.  The method that I used to replace one of the seals in the tip seemed to work, but it also did slightly ding and damage the green plastic, fragile tip of the injector.  Figure 13 shows this tip.

     The method I used was to first remove the old o-ring by cutting it off with a razor blade (Figure 16).  Be careful not to damage the green plastic tip when you cut through the o-ring.  Then, remove the o-ring with a pic, again taking care with the tip (Figure 17).  Finally, to get the new o-ring on, you will need to remove the tip.  The best method I figured out for removing the tip was to get a small 8-9mm crescent wrench and apply uniform pressure against the tip (Figure 18).  However, this still results in some of the plastic on the tip becoming marred.  Pressing up with the wrench using a surprisingly large amount of force will make the tip pop off of the injector.  At this point, you can attach the new o-ring and snap the tip back on.

     Because of the danger in damaging the tip of the injector, I do not recommend replacing the o-ring behind the tip unless you had a clear manifold leak.  I would almost recommend putting some sealant or silicone around the edge of the seal instead of installing a new o-ring.  I found it just too easy to damage the green plastic tip.  I'm not sure what effects a damaged tip would have on engine performance.

     If you are replacing all your injectors (Figure 19), or the o-rings, make sure that you place a very tiny, tiny bit of white lithium grease on the edges that will be pressed into the fuel rail and the manifold.  This will aid in the insertion of the injector and the reassembly of the fuel rail.  It will also help to prevent the o-ring from pinching, and will guard against tiny leaks as well.

     Installation is basically the reverse of removal.  You may find it easier to insert the injectors into the manifold first - if you have enough room.  Double-check to make sure that all of the fat o-rings are securely seated when you reattach the fuel rail.  Don't attach the two top plastic covers just yet - you will want to leave them off so you can check for leaks.  When you are ready to fire up the car, have an assistant on hand, in case there is a fuel leak.  Have them watch the injectors and the fuel lines to make sure that there are no leaks.  If all checks out okay, then button up the top two covers, and you're done!

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