1978 Datsun loading up - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 5 (permalink) Old 09-27-2018, 08:23 AM Thread Starter
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1978 Datsun loading up

I have a 1978 Datsun 289z that is loading up and will only run at high rpm. It seems like something is heating up and then causing the problem. I have tried everything, frustrated!! Can you help?
When I check the fuel pressure, it is around 28pounds. The manual says it should be 36. I put a new regulator and pump in. Hasn’t done a thing. When you accelerate, the pressure goes down. It should go up shouldn’t it?? Need help!
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post #2 of 5 (permalink) Old 09-27-2018, 12:19 PM
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Originally Posted by Elloyd View Post
When I check the fuel pressure, it is around 28pounds. The manual says it should be 36.
Read the Factory Service Manual. It clearly states that the Fuel Pressure Regulator should maintain an approximate 36.3 PSI PRESSURE DIFFERENTIAL between fuel pressure and intake manifold pressure. That means: If the engine is NOT running and the fuel pump is, the fuel pressure should be ~36 PSI - because there is NO manifold pressure.

While the engine is running you normally have a negative manifold pressure (suction) on an N/A engine. SO, if there's MINUS 8 PSI of manifold pressure, the fuel pressure should be about 28 PSI (36.3 + (-8) = 28.3).

While you haven't provided quite enough information to be absolutely sure, I would suspect that your problem lies elsewhere.

1974 "early" 260Z - L28ET - MS3X full sequential fuel and spark, COP=Ford 4.6 coilpacks w/ Nissan Ignitor, diyautotune chopper wheel - FMIC - T04E .63 A/R - 400cc Injectors (too big!), 14point7 Spartan2 Wideband - C/R 5-speed - Coilovers - Big Brake Kit up front, Porterfield friction all around. Mesa, AZ
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post #3 of 5 (permalink) Old 09-27-2018, 01:51 PM
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Check the Thermo-Time sensor and/or the connections to it. If it's not working, it thinks that the outside temperature is 0 degrees, and causes the cold start valve to keep dumping gas into the mix. A stuck open cold start valve will cause the same. The FSM has procedures for testing everything.

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post #4 of 5 (permalink) Old 09-27-2018, 07:38 PM
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just unplug the cold start valve plug and move on with life

1978 280z - stock L28 engine - Dallas, TX
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post #5 of 5 (permalink) Old 10-05-2018, 06:39 PM
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Originally Posted by cgsheen View Post
Read the Factory Service Manual. It clearly states that the Fuel Pressure Regulator should maintain an approximate 36.3 PSI PRESSURE DIFFERENTIAL between fuel pressure and intake manifold pressure. That means: If the engine is NOT running and the fuel pump is, the fuel pressure should be ~36 PSI - because there is NO manifold pressure.

While the engine is running you normally have a negative manifold pressure (suction) on an N/A engine. SO, if there's MINUS 8 PSI of manifold pressure, the fuel pressure should be about 28 PSI (36.3 + (-8) = 28.3).

While you haven't provided quite enough information to be absolutely sure, I would suspect that your problem lies elsewhere.
First of all your answer isn't entirely correct. The FSM is very confusing for people. I can show you where it gives you 3 different pressure readings on the same page. Which one is right, that's where I come in. I'll make it very easy to understand fuel pressure. At idle, or while cranking, or even at a constant RPM above idle the fuel pressure is supposed to be 28-29 psi. With the engine idling, when you stab the accelerator the fuel pressure should instantly jump to about 36 psi. Not a gradual increase, that indicates a problem. As the rpm's are increasing the pressure should remain at 36, then as the increasing rpm's stabilize and match throttle plate position then the pressure drops back down to 28-29 psi. The pressure will only remain at 36 psi when the engine rpm's are rapidly increasing. If you stab the accelerator and the pressure doesn't rise to 36 immediately then you have a problem. If you're putting a load on the engine (increasing speed) or going up a hill and the fuel pressure doesn't hold at 36 psi then you also have a problem. If it won't hold 36 psi under that circumstance then it's usually either a plugged fuel filter, or a plugged fuel pickup inside the fuel tank, or both. Very common for fuel tanks to be rusty or have an ash like corrosion in them these days, especially if the car has been sitting for any length of time.
The bad pressure scenarios I've listed above are not usually from a pressure regulator or a fuel pump, it's a fuel delivery problem that is caused from a blockage.
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