'77 Fuel Problems

[draztik280]

:)I need help regarding this problem that I have on my car. Car is currrently stock. This morning when I start the car, it started fine, until the car warms up and when I drove it, the car suddenly stalled.

I was thinking that the fuel filter is clogged/ dirty. So I replaced it and still doing the same thing. So then I inserted a fuel pressure gauge. When running, the reading is around 34 psi. In the OFF position, the reading is 19 psi. I checked the TPS and it seems ok. so as the AFM it opens freely. What seems to be the problem? Does the Fuel Pressure Regulator gives this problem?

Car shuts off if you push the pedal and sometimes turns off when on idle.

I attached a video so it is easier for you guys to tell what seems to be the culprit.

Thanks so much in advance.

[FastWoman]

Your fuel pressure will drop when you kill the engine if your fuel pump check valve is bad, your cold start injector is stuck or dribbles, or your main injectors dribble. It's normal to lose some pressure when you kill the engine, but it's supposed to be a rather slow process, and it should continue until your pressure is 0.

Just a thought: I don't know whether your '77 is wired like my '78, as there was an evolution of fuel pump control mechanisms with each year. With mine ('78), the fuel pump is energized when you're cranking the engine and stays running if there is either output from the alternator or oil pressure. If the fuel pump CONTROL relay (different from the fuel pump relay) were bad on my '78, or if the connections to it were bad, I think I would have fuel pressure only when cranking the engine, and then the engine would die immediately after starting (like yours).

Do you have a FSM for your '77 Z?

[draztik280]

Thanks for the info. I only have the FSM from the 78. I did replace a new pump already. The Pressure reads 30 psi. As soon as I rev the engine, the needle fluctuates. I tried removing the vacuum hose of the fuel pressure regulator, and the pressure remains the same 30 psi.

So far, It has a new fuel filter and new fuel pump.

I will try and return the fuel pump and get a new one. probably a bad pump? Should I suppose to get around 36 psi constant when on idle?

Thanks.

[sblake01]

You really need an FSM for a 77. There are differences between 77 and 78. Some of the realays are in different places, the fuel pump actuates differently, the voltage regulator changed from external in 77 to internal in 78. etc. Try this:

77 FSM

Edited September 27, 2010 by sblake01

[FastWoman]

... but the fuel rail is the same! Something isn't quite right, between your fuel pump, your fuel pressure regulator, and your fuel pressure gauge.

You could test your fuel pump by hooking the pressure gauge directly to the output of the pump (i.e. disconnecting the fuel supply to the fuel rail). You should get a much higher reading, perhaps 40 psi.

Assuming the pump is good...

Your fuel pump should produce a constant output, regardless of engine load (because the excess fuel is recirculated). However, the fuel pressure will be regulated in relation to intake manifold vacuum, so that the pressure difference between the fuel rail and the intake manifold is held constant. As a result, the fuel pressure as measured in relation to atmospheric pressure will fluctuate with engine vacuum. That said, when you pull the vacuum line off of the FPR, it should regulate at a constant 36 psi, not a constant 30 psi. Either the pressure is wrong, or the gauge is inaccurate. (Either is a possibility.)

[sblake01]

... but the fuel rail is the same!

Well, technically, no. The design of the 78 fuel rail is different. But for the purpose you're saying I guess that's true. Still, overall, he'd be much better off with the correct FSM for most all of the testing involved in troubleshooting the system.

Either the pressure is wrong, or the gauge is inaccurate. (Either is a possibility.)

I'll have to think about that one before responding...

Edited September 27, 2010 by sblake01

[draztik280]

Thanks for the huge info. I will open this FSM once i get home and see what could be the problem.

I will check the gauge or get a new one since it's not that expensive anyway.

I'll keep updating...

[Zed Head]

Thanks for the info. I only have the FSM from the 78. I did replace a new pump already. The Pressure reads 30 psi. As soon as I rev the engine, the needle fluctuates. I tried removing the vacuum hose of the fuel pressure regulator, and the pressure remains the same 30 psi.

So far, It has a new fuel filter and new fuel pump.

I will try and return the fuel pump and get a new one. probably a bad pump? Should I suppose to get around 36 psi constant when on idle?

Thanks.

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.

[draztik280]

I did pinch the hose in the FPR. The gauge did not move at all. It seems impossible that the line is clogged because i recently replace all the old hoses in the rear and the inside of the tank is immaculate. no rust at all.

The car starts fine when the engine is cold, no vibration steady 30 psi. But when it starts to warm up and give it a gas, The Gauge reading goes all the way down and dies.

What about the Ignition Control Module? I have heard that those things get hot.

Can Autoparts store check these if they are in good condition?

[Zed Head]

I got side-tracked with the focus on fuel pump, fuel filter, etc.

30 psi is not too bad at idle, it's actually a little high (for idle). But with your car idling, if you take off the small vacuum hose from the center of the regulator that is connected to your manifold, your fuel pressure should jump up to 36 psi. Don't pinch that one, take it off. You'll hear it hissing and the idle rpm should jump a little bit. Plug the end of the hose with your finger to stop the vacuum leak to the manifold. When you put it back on, the fuel pressure should drop.

Anyway, sorry I couldn't be more help. Another video, showing the car actually idling, with a shot of the pressure gauge as you press the throttle linkage, might give some clues.

If your FPR and fuel supply is working correctly, then the next fuel supply component in line is the AFM. When the throttle is closed , the AFM is not adding much, it has a bypass to let air through for idling. When you open the throttle, the AFM flap opens and signals the ECU that more fuel is needed.

The Engine Fuel section in the FSM will tell you a lot.

Edited September 28, 2010 by Zed Head

[FastWoman]

Draztik, I doubt an auto parts store can check your Datsun ignition module. However, you can test your ignition by putting a timing light on the coil wire. Step on the gas to see whether the flashes stop before your engine dies. If everything is working right, you should get sparks/flashes up until the moment the engine stops rotating. If you lose sparks/flashes before the engine stops, then it could be an ignition problem. I say "could" be, because you might also be fouling your plugs with an excessive fuel/air ratio and might be killing your spark that way.

That said, if your fuel pressure drops all the way down when you step on the gas, then you're losing your fuel pump power for some reason. I think that's your problem, not the ignition.

Can you step on the gas very briefly to make the engine hesitate, but then back off and get the engine to continue running? If so, what happens to the fuel pressure? Does it drop and then return to normal?

Remember that the output of the fuel pump doesn't increase when the engine is under load, contrary to what people might want to believe. Fuel is constantly circulated under pressure, and the engine load simply affects the return volume. If there were any fuel flow bottlenecks (e.g. a clogged filter), you'd see them equally under any conditions.

The peak load on the pump would be at peak fuel pressure, when the manifold vacuum is zero. If it can hold steady at 36 psi with the vacuum line removed (not pinched), then it can meet any demands placed on it.

I suspect your problem might be electrical. You might put a mechanical multimeter on your fuel pump line to see what it does. (Mechanical meters show voltage fluctuations faster than digital ones.) Is the power interrupted when you step on the gas? Does the power flutter for any reason?

[draztik280]

I will try this method and see if the ignition cuts before the engine dies.

When the engine is cold, I can drive the car and even rev it up to 5000 rpm with no problem. It's smooth. Then after 4 or 5 minutes when the car warms up, I can see the fuel pressure drops and when I give it a quick gas, the engine still dies, It won't go back to normal. Fuel pressure still goes down.

I will call and ask the autoparts store if they can check the ICM. I researched about the symptoms of this module and it is almost the same problem on the car.

This ICM thing is about $189 in the store.