'77 Fuel Problems

[Zed Head]

The fact that your car runs fine when cold suggests that the fuel system can "keep up" with demand. You should have mentioned this in the beginning.

If you can confirm whether or not the fuel pressure drops before or after the engine dies, you'll be way ahead in figuring out your problem.

In post #3 I see that you did remove the vacuum line from the regulator and saw no change, sorry I missed that. You should have seen the pressure increase to 36 psi when you removed that line. 30 psi is fine at idle, but the ECU and injectors expect more when the throttle is opened.

You have many odd bits of information scattered around. The answer is in there somewhere.

[draztik280]

Thanks for all your help.

I will gather all this information and see if I can pinpoint the problem. I am really close to thinking if it is the Ignition Control Module, since I was reading through what the symptoms if these things fail, and it is the same problem on my Z. These are on a different cars, since I cannot find about our Z. They say that when that ICM overheats, the car shuts off with no warning, so they let it sit for about 30 minutes to cool down, and when it does, then the car starts like normal.

This is exactly what happens on my car. But since I don't want to spend $200 yet. I will try the cheapest part to replace first before I can go to that.

Thanks

[draztik280]

I found the problem. After starting the car, when i press the gas pedal quickly, the fuel pump shuts off, basically there is no fuel pump voltage.

What seems to be the problem here?

thanks.

[Z train]

I found the problem. After starting the car, when i press the gas pedal quickly, the fuel pump shuts off, basically there is no fuel pump voltage.

What seems to be the problem here? thanks.

This makes no sense as the two(fp & pedal) aren't connected.

[draztik280]

Yes thats correct. As far as i know it could be a switch in the afm that is going bad? correct me if im wrong. Or the fuel pump relay?

Thanks

[FastWoman]

Draz, the fuel pump relay would have no way of "knowing" whether you were stepping on the accelerator. I would be amazed if that's your problem.

I believe the '77 Z does cut off the fuel pump via a switch in the AFM. (I might be wrong about this, so check an online FSM.) If so, that's quite likely your problem. You can pop the black cover on the side of the AFM to see what the fuel pump kill switch is doing when you hit the accelerator. Maybe it's out of position somehow?

The only other thing I can think of is that you have a bad connection somewhere, and when the engine rocks, or when something flexes or vibrates the wrong way, the connection is lost. I haven't forgotten that this problem is temperature related, and that draws attention somewhat away from the AFM and more towards the wiring (thinking of thermal expansion). Maybe move around your engine bay wiring harnesses gently (and carefully) with the engine running, to see if you can kill the engine somehow.

FAIW, Datsun's fuel pump wiring and electrical connections are marginally inadequate. I finally had to abandon the firewall connector in my fuel pump circuit because it kept arcing and melting. I snipped that wire and connected it through a bullet connector instead. You may need to do some fault tracing at various points in the wiring to see what happens when you replicate the problem.

Let's get a bit creative in our methods here. Try this to see where your problem lies:

First, disconnect the (+) wire to your fuel pump, and attach another long wire. Connect the other end of that wire to your battery. Your fuel pump will then run, irregardless of the state of anything else in your car. Now start up your engine and see if you can replicate the problem by hitting the accelerator. If you can, then it's definitely the fuel pump (for some weird reason). If not, then you know it's something in the electrical supply.

The next step, if it's electrical, is to fault trace. Check numerous points in your wiring to see where the power drops out when you hit the gas:

-- Fuel pump wire at the connector that would be connected to the fuel pump terminal.

-- Fuel pump wire at both sides of the big firewall connector. You should be able to poke your mutimeter probe into the backside of the connector.

-- Fuel pump relay coil wire.

-- The signal from the AFM's fuel pump kill switch

-- Other points I don't know on the '77. (Check a '77 FSM.)

If you keep tracing, you should turn up the fault somewhere.

[draztik280]

Problem Solved!...

Thanks for all your help. I did everything what you all said to check. Everything's working. Turned out it was the Ignition Control Module or Transistor Module. Now it does not stall anymore. I am still observing the car for now.

Huge Thanks to all of you.... Good Advices. I learned alot on my Z now.

FastWoman

SbBlake01

Ztrain

SbBlake

Zed Head

[FastWoman]

The IM? Weird! Anyway, congrats! I'm glad you found the problem. Your efforts weren't wasted checking the other stuff. You freshened a lot of connections and learned more about your car in the process.

Just curious... Did you replace the IM with an OEM unit, or did you do a retrofit? I ended up replacing mine with a GM HEI module.

[draztik280]

Autozone.com I think I got a remanufactured one. Exactly the same as my old one. Same size.

http://www.autozone.com/autozone/parts/1977-Nissan-Datsun-280Z/Control-Module-Ignition/_/N-indg1Z9ci7x?filterByKeyWord=ignition+control+module&fromString=search

[sblake01]

Wow! I wonder why there's such an incredible price difference between the California and the Federal emissions version.

[Zed Head]

The symptoms seem to point to your regulator. If the valve in the regulator is stuck at a certain flow rate, then the pressure reading would be constant and unaffected by the removal of the vacuum line.

You can test your pump by pinching the return hose off the regulator. The pressure reading should go up. If you have an OEM pump it should go up to about 43 psi. But be careful if you have one of the generic pumps the auto stores sell, they usually can go up to 90 psi. If pinching the line causes the pressure to increase it suggests that your FPR is not doing its job (assuming everything is plumbed correctly).

The other possibility is that your new filter or a clogged line somewhere is causing so much back pressure that the pump is maxed out and only 30 psi worth of flow is getting through.

I would pinch the return line (slowly and carefully while watching the gauge) first and see what happens. If it increases, take the return line off and run it in to a container to make sure you're getting good fuel flow. If you are, then your FPR is probably bad. If you just get a trickle, you might have a clogged line.

Fixing old posts - the relief valve in the stock Datsun fuel pump actually has a range of 43 to 64 psi. For future searchers...