75 280Z not starting

[zclocks]

Ok, My basic problem is my 75 280z will not start.

A couple of months ago I had a similar problem, but fixed the issue by replacing the fuel pump. Through out the summer is was hot and

not a problem starting the car. First crank and the engine started right up.  The first cooler day, below 95 F and the car will not start.

  

The fuel pressure is 40 psi when cranking and 34 psi when running, when car is running the vacuum is 18 mHg, I can get the car started by

actuating the cold start injector.This pumps in a small amount of  fuel and the car starts right up. 

I did go through all the measurements in the FI manual and discovered that there was a bad connection at the TPS. This was corrected and

the car started right up. So I shut down and tried to restart with no luck. I did go through  all the rest of the measurements on the connector of

the control unit  and everything seems OK.

 

If I had to guess I would say the control unit is not activating the injectors or is not receiving the correct input. Why then does everything work when 

a small amount of fuel is added at the AFM ? The only other observation is that the green fuse link is getting hot to touch which I believe supplys

 power for the AFM?

Any inputs would be appreciate.

Thanks ...Ron

 

[Zed Head]

50 minutes ago, zclocks said:

Ok, My basic problem is my 75 280z will not start.

A couple of months ago I had a similar problem, but fixed the issue by replacing the fuel pump.

The green fusible link might also be the headlight circuit.  i think that there are two. Which one?

What do you mean by "won't start"?  How long do you wait before trying a second time?  Need more detail.  How do you actuate the CSV?

Does the fuel pressure hold when the engine is off?  Kind of sounds like pressure leak-down.

[Dave WM]

don't forget erratic starting could be a bad ignition switch. If you have not already I would recommend you pull the switch out (easy) get it on the bench and do a though tone/continuity test on each position using the FSM to guide what should be happening esp in the start position. On indicator would be if it sometimes starts as soon as you release from start. You may find dead spots as you turn the key thru the start region.

Edited October 6, 2019 by Dave WM

[superlen]

Once the car starts, does it continue to run without the CS valve?

From what you've explained so far, (If I understand it) the problem could be the "CRANK" signal going to the ECU. The ECU supplies extra fuel during cranking based on this signal. Lets say your signal hasn't been getting to the ECU all summer. However, because it's been so hot the missing extra crank fuel hasn't caused any grief. Now with the slightly cooler weather, (yes, I know 95 isn't quite chilly but its' not 105), the missing fuel is causing a lean no start condition. When you inject extra fuel using the coldstart valve (which normally wouldn't give you much, if any, of a shot of fuel at 95f) you are supplying the missing crank fuel and you get a start up.

There are many other potential issues, but a missing CRANK signal on the ECU *could* explain it. I was just working this week on the cranking tables in the HellFire ECU replacement so it's fresh on my mind. Maybe I'm just projecting because it's so fresh, but it's an easy symptom to rule out.

 BTW, It's pin4 on the ECU connector. You should have 12v there when you are cranking.

If the car fails to stay running when you stop injecting fuel via the CS valve, then this won't be your problem. I'm a bit unclear if you have to continually fire the CS to keep it running, or just get it started.

Len

[zclocks]

Len you are correct on what is happening. I wasn't sure what happened during the crank sequence or where the input was to tell the ECU to supply fuel. 

The engine will turn over during the start sequence, there is spark, but no fuel. I used the cold start injector to to just get the motor started. Once started the 

motor runs without a problem. I still might have a small leak in my injectors , but generally the fuel pressure stays at 32 PSI overnight and several days after the motor is turned off. 

Actually, the pressure is at 40 psi when the motor won't start. As soon as the motor starts the pressure drops back to 32 psi and maintain that pressure. 

I will check pin 4 of ECU and see what I have .  

Dave thanks for your input and I will check my ignition switch. 

Zed the fuse link I was talking about is the Green one. The 75 has 2, one Black and One Green.  I did notice that one terminal of the Green was hotter than the other so I need to investigate that. 

Thanks all for your inputs and will let you know what I find.

Ron

[Zed Head]

A hot terminal is usually just a corroded or dirty terminal  Different than a hot link.  Some Scothbrite and WD-40 can fix a dirty terminal.

Have you checked the injector plugs for the opening and closing circuit, using a meter or test lights?  You can spend a lot of time assuming.  You really need to measure and observe.  No offense.  If the injectors aren't opening during the start cycle, that's not really a CSV problem.  Could be an ECU problem or a blue wire to Pin 1 problem. 

It helps a lot to read the first pages of the Engine Fuel chapter so that you'll know what is supposed to happen, and when.,

[superlen]

Ron,

Your pressure readings are acting normal. The rail absolute pressure (which is your guage reading) will drop as soon as the engine begins to run as the manifold pressure drops due to the pistons sucking in air, and the fuel pressure regulator (FPR) is always using manifold pressure as it's reference, NOT the air pressure outside the engine. Your gauge is referenced to the outside air, hence the difference.

If the car runs ok after start then it's most likely not an injector wiring issue. Although I agree with Zed, check 'em out while you are under the hood & confirm. Zed brings up a good point also about pin 1 (coil input wire). This is the main signal that times the pulses. If you are running fine after starting then in theory this PIN1 connection is fine. If it were not, you wouldn't run at all, ever. However,  this signal comes from the coil, and if it's extremely weak during crank (low bat, failing coil, corroded connector) the ECU might not be able to detect it during start, but once the bat voltage is propped up by the alternator, all is well. What's the status of your battery?

The coil seems unlikely as I would think a failing coil would show up in other places as well, but it is in the loop so I mentioned it.

 

Len

[zclocks]

Just an update as what I found with my start problem. Actually all of you were correct. I went back and reviewed my data I took on the 

ECU connector and found pin 4 voltage was low, 9.6vdc. My battery was low, 11.8 volts so I recharged and took readings at pin 4 again and it was still

low, 9.6 vdc. So I started at the ign sw and worked my way down to the connector on the FI relay.

 

The ign switch was open or a momentary resistance reading  on  bench testing. Luckily I had a new switch and I verified the old switch was bad.

I reinstalled the new switch and again checked for battery voltage at pin4 and it was still 9.6 volts? If I held the key in the start position for 60 seconds the

voltage would rise to about 11.4 volts. 

I connected everything up and started the motor. It started right up 6 times in succession. I let the motor get to full operating temp and it 's running great

and very smooth. In addition the vacuum increased from 18 hg to 21 hg?

BTW I did replace all my injector connectors several months ago and verified each is firing consistently. I installed new plugs, fuel pump, wires, cap/ rotor,

checked timing, and adjusted valves several times. 

 

I'm not complaining , but I still don't understand  why I  have 9.6 volts at pin 4 going to the ECU at start. Appears to me that something  something

is pulling the voltage down. Maybe this is normal , yet the FI manual says it should be "battery voltage". I did check the relay pins 86a to 86 and there is no resistance

and yet I have 9.6 volts output.

Before I declare victory I will try and start the car from a dead cold start and see if that makes any difference.

 

Thanks again for all the great comments and help....Ron

 

[superlen]

Ron,

Glad to hear you are making progress.

9.6V isn't normal. I honestly haven't measured any of my stock ECU samples on how much current PIN4 drains, but it shouldn't drop hardly any voltage at all.  My HellFire ECU is around a 10,000 ohm impedance, but that's just because of the values I chose. An stock analog ECU will be different, but it should still be a largish resistance (say greater than 1000 ohms). I would assume that  somewhere you have resistance Between 12V(Bat Pos) and pin4 that is allowing what little current that does flow into the ECU to drop the 12V down. The relay connections are the obvious choice (which you have already been looking at).

Resistance due to corrosion sometimes will not show up with an ohmeter test as their isn't enough current flowing to create the problem. You see this more often with really heavy loads like the starter, but it could be the problem here. Instead of measuring resistance on the relay pins, switch to voltmeter mode and measure the voltage across them during cranking. It should be 0. If you see 2 or 3 volts you've found the problem. You could also jumper around them and if your 9.6 goes to Bat Voltage, you've found out where the problem is.

I'll grab a stock ECU and do some measuring just to see what the load should be.

Len

[zclocks]

Superlen, 

There shouldn't be a relay(switch) in the path from the Ign start to pin 4. It's a straight in and out connection to the control unit(pin 4).I have a couple FI relays and will measure this path. 

The connections are 86 to 86a of the FI relay or am I missing something. Maybe the cold start valve and thermotime swithes are pulling down the voltage? They are connected to connection 86 on the FI relay.

 

I'm almost tempted to relocate the FI relay . It's a PIA to access.

Ron

[superlen]

Ahhh, my bad on the relay pins. That's what I get for going from memory.

I just bench tested two stock ECUs. As suspected the resistance is large!!!  It's almost 2MegOhms! without any harness or other connections. That's pin4 to gnd measured out of circuit.

When I added a harness it dropped it to 80K which is still very large (This resistance is the mostly resistance of the ECU->Harness connections). Note, because of the test setup/bench I was using the connections are running through two sets of ECU harnesses and two ECU->Harness connectors. I have a breakout board which allows me to flip a switch and run the car with a stock ECU or the HellFire ECU and it was wired through this. As a side note: 80k seems like a bit high for those connections, but doesn't affect the findings for your troubleshooting purposes.

A bit of digression there, but I wanted posterity to know where these numbers came from.

In summary you should get NO voltage drop on pin4. It should either be gnd (when not cranking) and full bat voltage when cranking. The 9.6V you get might be on the edge of the ECU detecting it and adding the extra fuel as well so even though you are starting now, I would continue to figure out why it's not 12V. If you happened to get stranded, jumpering 12v to pin 4 will of course do the same thing bypassing your ignition switch contacts and harness wiring. I need to look at the wiring, is the 12V going to the crank signal the EXACT same wire that goes to the starter solenoid? If so, the solenoid could be dropping the voltage as it will pull a bunch of current. My bench test above would not emulate that.

You could test by disconnecting the wire at the solenoid, hit the starter switch and see if pin4 comes back up to Bat Voltage.

Len

[superlen]

I just pulled my 75 FSM to look at the wiring.  The relevant info is on page EF-47. Here is theory of operation (my words not FSM so anyone jump in if I don't make sense)

1 - Power to ignition switch through fusible link.

2 - Start position on ignition switch  routes 12V to FI Relay(FIR) pin 86A. The wire leading there will  be labeled 76.

3 - INSIDE the FIR pin 86A and 86 are tied together. So 12V will now be on pin 86 of the FIR as well.

4 - Connected to pin86 are two wires. Wire 47 and wire 4. W47 feeds the 12V crank signal to the high side of the Cold Start Valve (CSV). The low side is grounded by the themotime switch. W4 as we might guess routes power to pin4 on the ECU.

W47 and W4 are spliced in the factory harness near the FIR & could very well be a source of your issue.

Here is what I would check.

- Measure the voltage at Pin/Wire 47 at the coldstart valve when you crank. This wire is super easy to get to & should be a solid connection to pin4 on the ECU. If it also reads 9.6V we can assume that splice is fine and the wiring from FIR pin 86 to the ECU is ok & we'll have to look some more. If it's 12V and the ECU PIN4 is only 9.6V, awesome!! We know where the problem is,  it's the splice! These two points should be the same voltage all the time no matter what as they should  be a direct connection.

Len