78 280z Starts then dies (Have searched forum)

[kutukutu1]

Hello All,

I have posted a couple of questions on other issues and i just want to say i will respond to those as soon as they are fixed, but one issue piles on top of another and cant fix one without the other.

So, the issue i am having is the following.

I start the car and it turns on, 10 seconds in, it dies.

This is what i have done as checks

1) unplugged starter solenoid, fuel pump turns on when i put the key to "on"

2) Fuel pressure when cranking is 36 psi, but when i stop cranking, you can see the needle oscillate alot from the injectors firing and i am thinking the fuel pump is not staying on after cranking. then you see the pressure start going down until it dies.

3) Fuel pump relay checks out 

4) Fuel pump control relay check out as well, everything works on it

5) AFM fuel pump pins 36 and 39 have continuity when i move the flap, but when you look at the 78 schematic, that circuit in the AFM it says useless and its not used, but everywhere on the forum on similar topic, they say to check the AFM fuel pump control circuit. If i try the typical check of moving the flap to see if the fuel pump turns on, it does not since that circuit is not even in the stock wiring.

6) I have tested the pump straight and it works and builds pressure

7) When i checked the pump the pressure was a steady 36psi which tells me the pressure regulator is good

What should be my next step?

Thanks

 

[SteveJ]

So, will the car run in the ON position for a few seconds and die, or is it dying when you release the key?

Have you tried having someone spray starting fluid into the AFM when the car is dying?

What is the voltage from the positive terminal of the coil to GROUND when the car dies?

[kutukutu1]

The car will run for about 10 to 15 seconds after you stop cranking. It idles good and sounds healthy, then you see the fuel pressure begin to drop and it dies soon after.

I have not tried starter fluid, but since the car starts every time since when i put the key to "ON" it turns on the fuel pump and builds fuel pressure again, it will start.

i have not checked the coil to ground when it dies, i will do that today after work. Why am i checking this for?

I really dont think its spark since it idles great for the 15 seconds that it does run. 

I should try to apply voltage directly to the fuel pump to make sure it stays on and crank it and see if it runs like that. If it does, then that would indicate that i am not getting any power to the pump after i let go on the key. correct?

[SteveJ]

I didn't say to use starting fluid to start the car. I said to use it when the car starts to die. If it picks back up, that indicates a fuel problem.

Checking the coil voltage to ground would confirm that you have enough voltage to generate a good spark.

I would not suggest hot wiring the fuel pump. I would check voltage at the fuel pump or voltage to ground fuel pump relay to see if it drops when the key goes from Start to On.

 

[Zed Head]

8 minutes ago, kutukutu1 said:

The car will run for about 10 to 15 seconds after you stop cranking. It idles good and sounds healthy, then you see the fuel pressure begin to drop and it dies soon after.

I have not tried starter fluid, but since the car starts every time since when i put the key to "ON" it turns on the fuel pump and builds fuel pressure again, it will start.

i have not checked the coil to ground when it dies, i will do that today after work. Why am i checking this for?

I really dont think its spark since it idles great for the 15 seconds that it does run. 

I should try to apply voltage directly to the fuel pump to make sure it stays on and crank it and see if it runs like that. If it does, then that would indicate that i am not getting any power to the pump after i let go on the key. correct?

Weird that you see the pressure drop while it's running but you said that the pump runs when the key is "On".  The key should be On after it starts.  Did you mean "Start"?  There's no reason to remove the starter solenoid wire if you're turning the key to "On".

The 78 cars are known for a flaw n the system.  If you remove the oil pump sender wire, the fuel pump will run with the key at "On".  If it does stay running when you do that then you can focus on the pump control relay and sender switch.  The sender has two prongs, one is for pressure, the other is a switch.

SteveJ replied while I was writing, hitting the button now......

[kutukutu1]

Yes, i am sorry. i meant "start".

I will try the oil pressure sender trick to see if the pump stays on.

Steve,

I will check voltage at the pump after i go from "Start" to "On".

Ill be post back later tonight with an update.

 

thanks

[kutukutu1]

Update

I unplugged starter solenoid and probed the fuel pump and I got 12v at "start", no voltage at "on".

I unplugged the oil sender and the pump came on with key on "on" and the car ran and idled perfectly with no issues. 

I wanted to confirm it would not stay running when I plugged the oil sensor back in, but it stayed running with no issues.

I probed the fuel.pump again, but with the starter plugged in and key "start", pump came on 12v on the meter, then key to "start" meter went down to .63 then all of a sudden qent back to 12v.

I tried the whole process once again and this time it died and no voltage with key "on" after start. Tried again and it started working again and got voltage again with key on "on". Now it's doing it intermittently so I am a bit lost.

Thanks for the help

[Mark Maras]

I'd check the ignition switch.

[kutukutu1]

Ok, any specific way of checking it or just take it out, disassemble and see if anything looks blatantly wrong?

[Mark Maras]

A  volt-ohm meter is the best tool. Check for switched voltage and repeat turning the key to verify reliability. Or unplug it and do the same thing by reading continuity (ohms).

[Zed Head]

29 minutes ago, kutukutu1 said:

 

I unplugged the oil sender and the pump came on with key on "on" and the car ran and idled perfectly with no issues. 

I wanted to confirm it would not stay running when I plugged the oil sensor back in, but it stayed running with no issues.

 

I tried the whole process once again

You've made some progress.  Might have lost your train of logic though. 

Did you plug the sender back in while the engine was running?  Or with the the engine off, then it wouldn't restart?  And "the whole process" covers too much stuff.  You don't need to do the Start tests becuase you already know that the pump runs and it will start,

Seems like your pump control relay might have problems.  It might be slow to react or defective.  There are two circuits to activate the pump relay, one from the oil pressure switch and the other from the alternator power.  Either will keep the pump running.   If your oil pressure builds slowly and the alternator function isn't working, for example, you could have a problem like you have.

[kutukutu1]

Ok thanks. I'll check the ig switch.

Fyi, I also have the gm alternator upgrade. It could have something to do with it. I wired it per online instructions bUT the alternator light is always on for what I can see, but I haven't had the car long enough I think for it to turn off.