Nissan Repair: Weak current to fuel injectors

Question
QUESTION: My 1986 Nissan 300ZX had an intermittent problem of losing power, like it was running out of gas.  At first it just occasionally stumbled then ran fine.  It got progressively worse until it barely runs.  The Nissan dealer has it and said it is barely getting current to the injectors.  I put another ECM in, and no change.

Pin test at the ECM harness shows cables are ok.  They say it is likely the Mass Airflow unit.  Would a bad MAF unit cause low current to the injectors?  I cleaned every ground I could find on the wiring harness under the hood and that seemed to help a little.

The shop manual says there is an interplay between the MAF unit and the crank angle sensor in the distributor.  Could that cause poor power to the injectors?

Let me know what you think.  I have the Nissan replacement kit for the injector harness which only has two wires to connect to the main harness, I am guessing one is hot, and the other is a ground, is that correct?  I am going to the junkyard to cut open the harness to see where the ground wire connects to the body and I am thinking of running new ground wires right to the injector repair harness and connecting the hot wire as far up the main harness as possible.  I had a corrosion problem at the TPS connection I fixed with splicing in a new connector.

So anyway it was an intermittent problem that got progressively worse, so I don't know if corrosion, a bad ground or MAF/CAS is the most likely culprit.

ANSWER: Hi John -

Is your check engine light on?

If not, then I would check or replace a couple items first, namely the fuel pump, and the ignition module. Reason is if it is an injector problem or related, normally the check engine light would come on. Voltage low at the injector has a special code that displays.

LMK and I can guide you on the testing if you need it.

hope this helps -
aloha
Calvin

---------- FOLLOW-UP ----------

QUESTION: Hi Calvin,

Fuel pressure is fine, ignition coil is brand new.
There were no codes on the ECM computer for any problem at all.
There is definitely low voltage at the injectors, the tech said the test light barely illuminates.
But it is intermittent and has gotten progressively worse over time.

Could a faulty MAF unit send weak signals to the injectors?
It has five or six wires in its' connector.
I went to the junkyard today and sliced open the wire harness to see how the injectors wiring runs.  Each injector has two leads that go all the way to the ECM. No wires run to ground at the engine.
So I guess each injector unit is grounded by the block when it is installed?

Let me know what you think.

Thanks,
         John

Answer
Hi John -

I was not speaking of your ignition coil, but your ignition module, located in the distributor. Driveability problems occur especially when the car is warmed up as it tends to fail when the base of the distributor gets hot. It is actually this module that sends the signals to the injectors as the distributor turns and fires the respective injectors. Could this be the intermittent you are speaking of? OK when engine is cold, and marginal as engine warms up?

Also, the fuel pressure could be OK, but may not pass the volume test. After the filter, pull the fuel line and place the end in a bottle. Turn ignition on and count 3 seconds. You should get at least a cup of fuel in the container. Both of these items are not part of the ECM system and will not trigger check engine lights when faulty. It's worth a check to me.

Everything else you speak of, such as the MAF sensor if faulty, will turn the check engine light on. Please be careful when checking MAF,as it operates on Millivolts (MV), so even a light probe connection might burn it out.

Answer to your question is yes, injectors find its ground when installed to the block.

Hope this helps John -
Aloha
Calvin