300ZX Fuel injection Diagnosis- by Steve Chong

Preliminary Checks

There are many ways of tackling this problem. One way is to divide the problem into 2 parts- one which involves locating "conventional " faults and the other deals with the computer controlled aspects.

Conventional fault tracing involve basic checks such as vacuum leaks, bad fuel, wiring faults, and worn or fouled spark plugs, loose alternator belt or faulty alternator. Sticking inlet & exhaust valves, worn valve seats can also cause intermittent power loss or rough running. Here are some tips:.

Vacuum leaks

It may seem obvious, but some have posted a cry for help because their engine was stumbling, and later reported they had forgotten to secure a pipe or hose, or found their intercooler had a hole in it from a parking incident!

Bad fuel

In 1994 here in New Zealand, the local refinery ran short of production, so the local gas companies decided to import a boat tanker load of refined fuel from Asia. They did this without informing the general public. Suddenly, some cars ran really badly, and after about a month, it was eventually traced to this bad batch of fuel. There has also been the odd incidence of 91 and 96 petrol being pumped into the wrong storage tank at the service station. The moral of the story is: fuel quality may not always be constant.

5.3 Wiring faults

Is your hesitation problem intermittent? If so the chances are that its an electrical connection- whether it be a connector, a relay contact, or a crimp joint in the wiring, or a soldered joint on a printed circuit board. Are there any signs of green deposits on wiring harness connectors or sensor connectors? Note that wires are crimp jointed to the connectors, so not only can you get contact failure between the male & female part of the connector, but also at where the wires run into the connector. I've noticed connectors at the front of the engine (the two temperature sensors) are much more corroded than those at the back of the engine (exhaust gas sensor, variable valve timing control solenoid etc). The other connector which can get really dirty, is the fuel pump connector, located on the outside of the car, but accessed through a panel underneath where the spare wheel is.

If you are experiencing intermittent problems, you might find page EF & EC 47 in the manual helpful. Here, there are some tips on how to try and force an intermittent fault to show up. For example, race the engine rapidly to see if engine movement causes electrical breaks, wiggle connectors etc. It may be necessary to connect up LED's (light emitting diodes) at various points in the circuit, so when the fault occurs, you can spot it immediately.

After many years, connectors can be a source of problems. Unplug any suspect connectors and squirt some WD 40 or CRC 556 into it. Inspect the wires that go up to the connector closely. If the copper in the wire has a coating of copper oxide (black colour), then you may have to dismantle the connector, clean up the wire and solder it to the terminal. What happens here is that black copper oxide is an insulator, so over time your crimp joint can go open circuit as the surface of the copper wire turns from copper to copper oxide.

5.4 Spark plugs

Are you having problems at high engine RPM (4000-7000) and wide open throttle (WOT) If so it could be spark plugs. As the engine RPM climbs, the voltage supplied to the spark plug by the ignition coil drops off slightly. ( Direct ignition is used on this engine to significantly reduce this effect, but it still happens). In addition, as the compression in the cylinder rises (under WOT), the voltage required to jump the spark gap also increases.

If your spark plugs are worn, then the gap may be greater than normal, and also, it requires a lower voltage to jump from a sharp surface than a round surface. A worn plug will have rounded surfaces from which the spark jumps. If you have other problems such as incorrect mixture or excessive oil consumption, these will tend to put coatings on the plug which will raise the voltage necessary for a good spark.

Also check the connectors and wiring between the 6 ignition coils and the transistor ignition unit, located at the front of the engine, above the alternator on the plastic cam belt cover.

Its actually quite easy to remove the spark plugs- first remove the ignition coil, and in the case of plugs 5 & 6, undo the balancer tube, roll it back to give enough room to withdraw the plugs. Be careful not to damage the balancer tube O rings, which you can reuse. When removing the ignition coil connector and coil, bits of dirt and broken connector plastic can fall into the hole where the spark plug is located. I put a plastic PVC pipe- small enough to go down the hole and big enough to go around the spark plug, then attached a vacuum cleaner hose to the end of it to suck out dirt that went down the hole.
 

Spark Plug Location :	Balancer Tube Location :

5.5 Alternator belt/ alternator/ battery

Is the problem is related to heavy electrical load?

When the alternator belt is correctly tensioned, it is able to deliver all the power required by the alternator. When it is loose, it will transmit a certain amount of power, then slip. This slipping can go on quietly, and therefore unnoticed. What can happen in this case is that the alternator can still supply power to charge the battery to start the car, but insufficient to charge the battery and supply a heavy electrical load- lights, air conditioning, stereo, wipers, rear window heater, turn signal etc.

Also, its possible for the alternator to be faulty and not be delivering full output without illuminating the charge warning lamp. An alternator can have blown main diodes, faulty exciter diodes, a faulty regulator, which can cause low output without bringing up the warning lamp. I'm sure you folks with your 1000W stereo systems can relate to this!

At the point that the electrical load becomes too heavy for the alternator & battery the voltage starts to fall, and so does a whole host of other things- ignition, fuel delivery, and the on board computers- and on comes the stumbling problems.

An electronic fuel injection system therefore relies on a good electrical power supply to function, so you need to make sure your battery, battery terminals, and charging system is in top condition.

A simple way to test the battery terminal is to start the car when cold then immediately feel the battery terminals. They should be cold. If not, there must be a voltage drop across the battery terminal lug. Remove the terminal & clean it up. Another way is to put a voltmeter on the battery terminal lugs and measure the battery voltage while you start the car. It should not drop below 10 volts.

Interestingly, older Zs used to have voltmeters as part of the instrument cluster, so it was easy to keep an eye on this sort of thing.

You can deliberately load up the electrical system to determine if the problem is load related. Turn on the lights, heater fan, rear demister, wipers, stops lights- by operating the bake pedal, cigar lighter. If you bring on the problem, then at least you have clue number 1.

To check the alternator belt tension, I bought a simple spring scale, the sort of thing you use to weigh fish with, from the local hardware shop for about $3US. Put the spring scale 1/2 way between the alternator and crankshaft pulley, pull on it to 22lbs (10Kg), and check the deflection doesn't exceed 0.453 in (11.5mm). If it does, readjust so that deflection is in the range 0.28-0.31 in (7-8 mm) for a used belt or 0.256- 0.295 (6.5- 7.5mm) in for a new belt.

A full procedure for checking the alternator is described in the manual on page EL 26.