i've been having intermittent hard-starting, which has culminated in total non-starting.

the car has been my DD for a few months and i've replaced all the sensors, injector connectors, fuel lines and vacuum hoses. it either starts perfectly or starts after much cranking, then runs really well with no hint of fueling problems. at first i thought it was a hot-start issue, but now it will not start cold.

the plugs are dry, decent color and all are providing spark - i rested them on the valve cover and cranked the motor and saw them all fire in order. fuel pressure at the rail is 35-40 psi and fuel pump is running fine.

for some reason the injectors don't seem to be getting a signal. i put a voltmeter and then some LED's across the two terminals on the injector plugs and cranked the motor - no lights. is this the correct way to test them?

i'm pretty sure there is some kind of bad connection somewhere that is stopping the injectors from getting their signal/power/whatever and i haven't the foggiest how to even start diagnosis. i have the FSM and it is horribly confusing to an old carb guy... pin 37? continuity? wtf???

had the car towed home - for the second time, and can no longer rely on it as a useful source of transportation. the toughest part is that i absolutely love the stupid car. any help would be greatly appreciated.

Rossiz,

Your Z will eventually become reliable if you keep picking at it. Be brave. Don't listen to your daughter and/or any other naysayers. The best news, although it doesn't sound like it, is that your intermittent problem has become consistent. And where there's a consistent malfunction, you have some hope of diagnosing it.

OK, you say you're getting no voltage to the injectors. That could be a MAJOR clue. But before we conclude that's the case, we need to be certain that's what's happening.

You tested two ways. The first way was with a voltmeter. Was it an analog-type voltmeter, with a mechanical needle, or was it electronic? And EXACTLY what did you find?

Then you tested with an LED across the leads. Depending on how you hooked it up, it either wouldn't have worked, or your LED could have been destroyed unless you had maybe a 1 kOhm drop resistor in series. The best test is with a miniature Christmas light bulb, such as you would pull from a long string that you would hang on your tree. You should see it flashing dimly and slowly (once per revolution) as you crank the engine. If you pull the connector off of the coolant temp sensor (the sensor with the connector like the ones on the injectors), the light should flash more brightly.

The tests laid out in the FSM are pretty straight forward. You SHOULD be able to find pin 37 (or whatever pin) on the ECU connector. They're numbered on the connector and are in order. By testing the injectors one by one at the ECU connector, in the manner described in the FSM, you should be able to diagnose the injectors, the drop resistors (located beneath the brake master cylinder) AND their wiring at the same time.

Until you do these tests in a methodical manner, there's nothing any of us can do except to guess. So if you truly love this beautiful machine, you're going to have to fight for her!

FAIW, my GUESS, and it's only a guess at this point, is that you don't have power to either your ECU or your injector drop resistors. (How do your fusible links look? Crusty and awful?) Or perhaps the ECU is not adequately grounded. Less likely, perhaps your ECU is bad.

Oh, I'll mention we have four cars -- a '92 Saturn SL2, a '94 Mazda Miata, an '09 Dodge Ram, and the '78 Z. They are all very reliable vehicles, because I keep them well maintained. It might surprise you to learn that we consider all of these vehicles approximately as reliable. Whenever we hit the road to Richmond (about 1.5 hr), we take the Z, feeling it's safer than the Miata and more fuel efficient than the Dodge. It also has working A/C, unlike the Saturn. And of course it's fun to drive.

BUT... There were a couple of times, before I finished straightening out the Z, that it had to be towed home in humiliation. And there were times I had a HELL of a time getting her started. But not anymore. All fixed.

FW - thanks for the encouragement, i could use it.

the idea of the EFI system being reliable (as in ALWAYS starting) is as appealing as it is elusive to me.

i tested the injectors with an electronic multimeter set to "volts" and touched the two probes to the two pins in the injector plug while a friend cranked the engine. no reading whatsoever. i then put two LED's back to back in the plug (couldn't tell which pin was + or - so i had one in each orientation) and neither blinked. i didn't have a christmas bulb on hand (will put some in my tool kit). it was lucky i was at the office of my engineering consultants - they had the multimeter and LED's.

just spent the last 3 hrs online looking up SU conversions while fuming over missing dinner w/the family to go meet the tow truck and get the z home. i wanted to hate it for it's intransigence, but when i got there it just looked so damn cool i couldn't stay mad at it.

when i first got the car, i replaced the fusible links with maxifuse blocks, re-wired the primary system with 8 GA marine wire and relocated the fuses and all the relays to the cabin on the passenger side to get them out of the weather. just checked the fuses and they're all fine.

i had a new relay on hand that was the same configuration as the EFI relay (6 spades) so while i was on my hands and knees under the dash (getting used to "assuming the position" for this vehicle) i decided to test it. i first tried starting w/the old one (no love) then yanked the plug out of the old relay and plugged in the new one. the car started right up. BUT... then i plugged the old one back in to confirm it was bad and it ALSO started right up. so this is completely inconclusive to me - not sure if a relay could be intermittent, and what would cause it to be. FYI - i had cleaned all the spades on all the relays and slathered them w/dielectric grease when i did the re-wire a couple of months ago, and all looked clean and tidy when i did the swap tonight, so i don't think it was a case of the plug/un-plug scraping off corrosion on the old relay.

i think i've checked all the connections in the EFI loom - i even unplugged the the big multi-pin ECU connector hoping to find crustiness, but it looked clean and fresh (and frightening - so many teeth!!) so i just quietly put it back together and backed away. the one set of connectors i haven't been able to fiddle with are the two plugs at the dropping resistors - i'll rip them apart tomorrow and see how they look.

for now i'll leave the new relay plugged in and just keep doing test drives in circles around the block until it decides not to start again, then i'll do some more testing.

Edited April 16, 201410 yr by rossiz

To add a little to FastWoman's suggestions.

You want to test from the pins in the injector connectors to ground, with the key on, to measure voltage. You won't see any voltage measuring from pin to pin, they're at the same potential. They'll both show battery voltage to ground, it's a transistor thing.

If you're getting good strong spark, and find that you do have power to the injectors with the key on, but no injector action, it could be an ECU problem. The ECU uses transistors like the ignition modules do, and they get flaky when they get hot, and/or old.

If you don't have power to the injectors, then the fusible links, as FW says, would be the place to look. The fusible links for the FI harness are two green wires next to the battery, with white connectors. They look like two jumper wires. Check the contacts there, and also check the contacts for the EFI system ground wire, it's connected directly to the battery negative terminal.

The ECU uses a wire that runs from the coil's negative post to Pin #1 at the ECU connector to count engine RPM. Every third spark, the transistors in the ECU ground the circuit to the coils, causing them to open and squirt some fuel. It's a batch-fire system, firing all six injectors at the same time..

If you find the problem and fix it, all of this agony will fade away pretty quickly.

ok, so i'm confused now re: testing the injectors for signal... if there's power between either of the pins and ground, but both pins are +, then how does the ECU "pulse" the injector? and why are there two pins if they're both +?? and if the power is at both pins with the key on but the car not cranking, why aren't the injectors constantly fired???

sorry if i'm hopelessly stupid on this - i have no electrical training, just basic "batteries & bulbs" sort of stuff.

please explain exactly how i test the injector pulses with a christmas bulb so i can confirm their status next time the car won't start.

It's the way transistors work. When the transistor switches states, the current flows, but when you're measuring at the pins the transistor isn't in the state that grounds the circuit. Transistors are weird things.

Are you sure that you're getting a good strong spark? I had an ignition module that would give a spark outside the engine, but it was a weak orange spark and wasn't enough to kick things off unless I used starting fluid. Then it would run. With your 78 electronic ignition you should get a nice fat blue spark.

ZH - good strong, blue spark, all plugs, visible in direct sunlight - and all plugs are dry, so i'm pretty sure i'm good there.

so... do i put the two leads of the bulb to the two pins on the injector plug, or one to a pin and one to a ground?

OK, good! Here's what we've learned:

When you put the digital voltmeter to the injector connector, if you had power you'd have probably gotten SOME reading. I'm not sure what it would be, though. One lead would be at +12 (the one leading to the drop resistor assembly, which is then fed by the fuel injection relay, which is fed by one of your maxifuses, which is fed by the battery. The other lead would likely be at some fraction of a volt, but it wouldn't be a stable or meaningful voltage, and when you reconnected the injector, the voltage should have been dragged up to 12V, except whenever the ECU would pulse it. It would have been better to test for +12, referenced to ground, at each of the injectors and at the #10 pin of the ECU connector. (There is a diagram on p. EF24 of the FSM that shows you the pin layout, and there are little numbers stamped into the connector itself.)

Anyway, the fact that you read NOTHING (0 volts, right?) probably indicates no power from the fuel injection relay. And I say "probably." It would have been more meaningful to do your tests at the scary ECU connector with so many teeth! Trust me that you're not going to hurt anything by unplugging the thing and probing it. After you unplug the ECU, there are no sensitive electronics left. It's all sensors, switches, servos, power supply, etc. The only sensitive thing has been unplugged.

Incidentally, the two LEDs back to back is a good approach. But also insert a 1000 Ohm resistor in series with them, so that you don't blow the one LED that's wired in the correct direction!

Anyway, I think you found your smoking gun. It's something in the relays you moved under the dash. There are two relays of interest here: the ignition relay, which feeds power to the coil on the fuel injection relay to actuate it, and the fuel injection relay. You might have either a bad relay or a bad connection to the relay. Just be aware that parts, especially electrical ones, do not necessarily fail completely. They often fail intermittently. Your problem is (or was) intermittent, so that's the sort of problem you're looking for.

I'd pay special attention to the condition of the connectors plugging into the relays. The metal could have weakened or become corroded. And remember that the wires are crimped into those connectors. You could have a corroded wire or a bad crimp. Examine them carefully.

If I were you, I think I'd try recreating the problem with the old relay in place and the ECU unplugged. Turn the ignition on, and measure pin 10 to see if you have voltage. Then with the ignition still on, start GENTLY wiggling the relay wires and connectors to see whether you can make any of them click. If you can, then you're "exercising" your intermittent connection problem. Then substitute the new relay and see if you can recreate the same clicking. If not, then it's probably a bad relay. If you can, then it's a connector issue. Keep hunting and poking, and you'll find it.

And after you think you've found the problem, still don't stray too far from home, and carry spare parts. This car can't be your DD YET, and you absolutely should not take your daughter anywhere in it (as you don't need the negative energy right now). After you've driven the car for a while with no issues, you'll be able to certify it "reliable" and use it as your DD.

And FAIW, the more you drive the car, the more reliable it will become, just due to use. Nothing ails the electrical system of a car like sitting. A car needs to be driven. You're going through what I call the "clearing cobwebs" period.

One more thing: Just so you know how the injectors work...

The ignition relay actuates the fuel injection relay.

The fuel injection relay feeds +12 power to the drop resistors and the ECU.

The drop resistors feed +12 power to the injectors.

The other side of each injector runs to the ECU. I'll call this the "floating side" because it floats electrically.

Between injection pulses, the floating side is... er... allowed to float at whatever voltage it likes, almost as though it weren't connected to anything. So it will float at 12V. You'll see the same voltage on each side of the injector. (Don't go "Aha!" here. This is only true because the low resistances of the injector and the drop resistor drag the voltage up to 12V. When you unplug the injector and insert a very high resistance multimeter in its place, the voltage could be almost anywhere.)

When the ECU pulses the injectors, it does so by "grounding" the floating wire by creating current flow through a transistor to ground. The actual voltage will be a fraction of a volt off of ground.

And that's basically it.

Oh, and your second question to ZH: You plug the bulb (do you have one?) into both sides of the injector connector to make it flash. However, to test for voltage to the injectors, you would connect the black lead of your multimeter to ground and the other lead to either of the injector plug contacts. (One should be +12, and the other should not -- with the injector unplugged, that is.)

Actually, both sides of the plug will measure battery voltage. The reason that its' possible, I believe, is that a transistor is a resistor. You'll be measuring voltage through very high resistance. You could short it to ground but no significant current will flow, until the other side of the transistor has been switched. They're kind of like relays but harder to diagnose.

So, one side is battery voltage through the dropping resistors to the injectors - the power supply - the other is battery voltage from the transistor in the ECU. Waiting to switch states and let that current flow.

Note - I could be WAAAYYY off and the reason why both sides have voltage. Maybe there's a branch circuit with high resistance to some other part of the ECU. But I am sure that both sides show equal voltage. I've confirmed it, been confused by it, and read about it on other forums.

Edited April 16, 201410 yr by Zed Head

Zed, the transistor is actually more like a switch or a relay than a resistor. It would almost be as though the ECU had a wire from the injector that it would periodically short to ground. When the wire is shorted to ground, current flows from the battery, through the maxifuse, through the fuel injection relay, through the drop resistor, through the injector, to ground. This current opens the injector. When the short to ground is broken, the current stops, and the injector closes.

Without some component(s) to pull the voltage up to +12 (which could be via the injector, if plugged in), the voltage on that "floating" connection to the injector will fall towards ground because of current leakage inherent in the transistor. Of course there could be such components in the ECU for some purpose.

I almost added a comment about a transistor's usage as a a switch or relay but that's a whole other forum, with lots of discussion possible about nomenclature and terminology and what's actually happening.

Regardless, the two pins at the injector plug will always both show battery voltage against ground in the stock Nissan ECU setup with the key on. I just wanted clear that up to help the OP along in his quest. I even went out and measured, again, to be positive I was living in the same reality now as last time I did it, and just to see those comforting numbers on the meter.