Tuesday, August 23, 2011

300zx Injector Replacement

I love fuel injection.  Though my first car was carbureted, and I still use a few carbureted engines (motorcycle, snow blower), the workings of a carburetor lie in the realm of black magic.  A fuel injection system  uses a couple of sensors, pumps, fuel squirters and a computer.  That is easy for this 21st century digital boy to understand. 

My Z is now close to being 22 years old.  Though often described as "fiendishly complicated" the massive cult following of this car has made trouble shooting rather easy.  99% of the issues that might arise has already been dealt with and well documented within the on-line Z community.  Here is my repair story for diagnosing and repairing a failed fuel injector. 

Problem, symptoms, diagnostics, theory, confirmation, parts/tools, and repair.  These are the basic steps to any repair. 

I jump in my trusty Z after work one afternoon to run a few errands on my way home.  Fire her up and immediately feel that something was wrong.  The engine was running very rough.  After a few stops the check engine light appears (not flashing).  It seems like I got a problem.  I drive home before I dig into this any deeper.

The car ran rough (symptom) and felt like it was only running on five of the six cylinders.  I now needed to find the possible dead cylinder.  Engines need three major things (then there is a fat list of minor things) to run: Fuel, compression, and spark; removing any of the three would stop the combustion process.   In order to find the dead cylinder I would kill off one cylinder at a time, by removing the spark.  This is accomplished by unplugging the spark plug (coil pack to be specific for this car).  When doing so I would listen to the engine.  When you kill off a good running cylinder the engine will stumble and the idle will drop.  The engine would also struggle to run as now it was only running on four cylinders.  However when you pulled the spark to the dead cylinder no change would take place.  Performing this test, I soon discover that my #2 cylinder was not running.  This test is called a balance test. 

For starters I am going to rule out low/no compression as a source of my troubles.  I took a compression reading last year on the engine and all cylinders were good.  I have not have had excessive wear or overheating issues that might change my compression significantly. 

I had mentioned earlier that while driving with my symptom I had a check engine light.  This means that the computer is trying to tell you something is wrong.  Since the car is a 1990 it is before the days of ODBII computer systems.  I use to like ODB1 systems because you can read the trouble code by counting the long and short flashes of light in the diagnostic window on the ECU itself; no need for a reader.  However you need to get to the ECU, which is more difficult than simply plugging in a reader from a port under the dash and reading the display.  Counting the flashing lights on the ECU tells me that I have a code 51.  Checking a reference I see that I have a bad injector circuit.

I now know that I have a dead #2 cylinder.  I also have a bad fuel injector circuit error code reading by my ECU.  Sounds like I have a dead number 2 fuel injector.  This could be the reason why the cylinder is not firing as the fuel component was removed from the holy combustion trinity.  However I still need a run a few test to confirm. 

Z32's use side feed injectors.  Fuel enters the side of the injector through the mess screen.  A signal is send from the ECU to the electrical connector telling it to fire.  Fuel then squirts out the bottom of the injector into the cylinder.  A properly working injector will have a resistance reading between 10-14 ohms.  This a taken across the two pins of the electrical connection.  #2 injector showed an open circuit.  Testing another injector that I know is working, #3, I get 12 ohms.  At this point I am fairly certain that I have a dead #2 fuel injector.  I should have gone further and tested for spark on the #2 cylinder just to rule it out, but I skipped it. 

One of then reasons I am so confident that it is the injectors is because I have read many cases that were similar to mine.  The early year Z32's use a pintle style injector.  These types of injectors are susceptible to failure caused by ethanol in gas. Since most of the gas sold in the US  has ethanol mixed in, pintle injector failure is fairly common.  Though I am not sure the exact details and failure mode, it has something to do with corrosion caused by ethanol absorption of atmospheric moisture. 

This diagnostics went pretty smoothly and has taken about an hour to perform.  This initial time investment is well worth it, and much less time consuming than changing out the wrong part would have taken.  Since I now have a confirmed failure mode, I only have two more steps before having the Z on the road again.  A few quick keystrokes on my favorite Z part vendors website and second to last step is accomplished, as a new injector was being shipped to me.  The cost of a single new injector: about $100. 

The Z32 is built with a tuned length intake runners.  The basic V6 engine block is built: valve covers, fuel rails and injectors, lower intake manifold all installed, then the upper intake plenum sits on top of everything like a six legged aluminum spider.  Though great for performance the upper plenum makes working on the stuff underneath a bear.  Removing the plenum is also quite a chore as almost every fuel line, vacuum line, and half the coolant hoses are intertwined in the spiders web.  If all six injectors are to be replaced the removal of the plenum might be a good idea.  But if only one needs replacing it is better to use the "Dremel method".  The injectors for the most part are exposed between the legs of the plenum.  However not all of the screw are accessible.  The method involves taking a dremel with a carbide bit and cutting access notches in the plenum.  Though this might seem like a hack job, the process is widely accepted by Z mechanics.  The plenum itself is a cast aluminum piece with "extra" material.  I have even heard of people cutting injector screw access notches in plenums that are off the car and being polished.  These polished plenums will often have extra castings ground down as well prior to being installed on a built engine.  The pre-notched plenum  makes future injector changes a snap. 

For this job I used my dremel with a flex line attachment.  This allows me to get into the tight space a little easier.  I used two different carbide cutters.  A new cylindercal one with a flat bottom and later on a pointy one.  When cutting aluminum with a carbide bit, it is best to go slow.  Both in the spinning speed of the cutter and the amount of material you remove.  Spinning the cutter at high speeds will load up the cutter surface.  Though there is extra casting material that can be removed with no harm, it is not unheard of for a zelouse shade tree mechanics cutting holes in their plenum. 

Once enough material has been removed it is time to tackle the screws.  In all of their infinite wisdom, Nissan engineers opted to use #2 Phillips head screws on the injectors.  Stripping the screws is super easy.  Prior to any attempt to remove the screws, it is best to soak the area down with a penetrating oil.  WD-40, PB blaster, what ever your favorite is, hose it down and let it soak.  Over night is best.  The best tool to use on these screws is an impact driver.  An impact driver is different than an impact wrench/gun.  A impact driver is a hand tool that turns an eighth of a turn or so every time the back end is hit with a hammer.  When set to turn counter clockwise, this tool is great for removing Phillips head screws, as it applies both the downward force necessary to resist stripping, and turning force to unscrew the bolt.  Since I have a larger 1/2" drive impact wrench, I had to string together several adaptors in order to get it down to a 1/4" phillips bit.  Also note that a 4" long phillips bit is needed to clear plenum. 

Though a healthy portion of material can be removed in order to access the screw, the screw driver bit does not sit on the phillips screw at a 90 degree angle.  I think that removing the necessary material to do so will surely cut a hole in the plenum.  In the picture one of the phillips screw has been replaced with a socket head screw.  This is a nice upgrade, and will reduce headaches in the future.  However the socket head portion of the screw has a much higher profile, and has trouble fitting under the plenum.  I opted to keep the stock phillips in the left hole, rather than grind down more material. 

Once the screws are removed and all the aluminum shavings cleaned up it is time to finally remove the injector.  Lift off the injector cap, keeping track of all the rubber insulator below.  The injector sits in the fuel rail and is only held in place by a friction fit provided by two o-rings.  Do not under estimate the holding power of these two rings.  I have read of people using pliers, vice grips and even sinking a screw into the injector to pull it out.  Since it was a dead injector I was ready to do anything.  The soaking in oil helps this part too.  A combination of twisting and pulling works the best.  Of all the plier like tools I had, I found the 45 degree long needle nose pilers work the best.  With it you are able to grab the connector portion of the injector right below the locking tabs, providing a solid purchase.  The angle also allows for both twisting and pulling.  A few grunts and injector pops out. 

From here on it is a simple, "installation is the reverse of removal."  Once the new injector is in place and everything is buttoned up, I gave her a crank.  After a few cranks and the fuel system repressurizes she fires up and and runs perfect again. 

Ahh I love it when a plan comes together. 

Here is one of the tutorial that I followed.


  1. wish i read this 1st lolJanuary 9, 2012 at 1:31 AM

    wish i read this 1st , u were right , u wont get a 90 degree angle to get the screw driver straight , i tried and just before 90 degrees a whole appeared in the plenum !! if u have any advice on fixing this hole it would be appricated..

  2. The plenum is Aluminum. So it can be welded. But if you are the type that dremels their plenum you probably are not going to get the hoel welded. I would jsut throw a dab of JB weld on the hole. If your car is a turbo, then it will see what less than 20 psi. JB is plenty strong for that. Jsut remember to prep the surface well. Good luck.

  3. Hey Bill, just a great post. I'm having some issues with my fuel injector and I want to fix it by own. Glad I got your post. The way you have guided step by step, it would be easier for me to try my hand on it. Actually Porsche service Charlotte, NC always take care of all my car issues but this time I want to try it and I'm having lots of time to learn something new. I feel little confident after reading all these. Thanks a lot.