1976 280z hard start and rough idle - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 24 (permalink) Old 12-05-2016, 02:06 AM Thread Starter
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1976 280z hard start and rough idle

I have a 1976 280z and I am having hard start and rough idle where the car keeps dieing on me. When starting the car it wont start right away. When I crank the engine one or two cylinders fire but then it stops. I have to do this several times (5-10 times) until it will start and fire for a few seconds before it dies. The rpms will get up to 500 and bounce around and fall down until it dies. This will happen a few times and will eventually get up to 1000 rpm, run for a little, and die. After about 5 to 10 minutes it will start to idle rough at 1000 rpm and stay idling. I can then gingerly rev it up to 2000 rpm but if I rev it too fast the engine will hesitate and back fire out the intake until it breaks past 1500 rpm. The weird thing is at 1500 rpm and up it runs smoothly with no hesitation. After 5 to 10 minutes the engine gets up to about 1200 rpm at idle and idles pretty good. The hesitation goes away and I can quickly rev the engine with no back firing from 1200 rpm to 4000 rpm. At this point I can turn off the car and start it again no problem. I think the problem is the car wont start unless it is warmed up. I figure that cranking the car over and over letting one to two cylinders fire eventually warms up the car to where it will kind of run. It eventually fully warms up and idles pretty steady and revs up well. Is there something that would greatly be affected by a cold engine? I'm not sure if my cold start valve is operating right. Could that cause this? I feel like that there might be something more too it. I plan on checking for a signal in the connector to the cold start valve and trying to manually hook it up to the battery to see if it will spray anything.

A few notes:

I have checked for vacuum leaks with brake clean and there are not any. I have cleaned all 6 injectors and I have a new airtex fuel pump and fram fuel filter. New spark plugs and new battery. One issue I do have is the fuel pimp will not turn on in the on position but I have hooked it up directly to the battery so it is not a priming problem.
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post #2 of 24 (permalink) Old 12-05-2016, 09:27 AM
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Do you have the factory service manual (avail on line).
read up on the EFI section.
That being said I would look at 35 pin connector and check the temp sensor resistance reading to see if its consistent with the ambient temps you are experiencing.
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post #3 of 24 (permalink) Old 12-05-2016, 09:32 AM
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also you should have a fuel pressure gauge and hook it up as described in the FSM. This is a very useful tool for making sure the fuel is getting to the injectors at the right pressure, which is critical in a fuel injected car esp the Z without a feed back sensor in the exhaust to correct air/fuel mixtures.


The pump will not operate in the "run" unless there is air flow thru the AFM and the micro switch is on from that air flow.


All this is covered in great detail in the FSM. Read it a few times and it will all sink in.
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post #4 of 24 (permalink) Old 12-05-2016, 12:15 PM Thread Starter
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Thanks Dave M. I do have the FSM and Ive been going through a few things. I guess Ill have to really dive in and go through the whole thing to get everything tuned up.
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post #5 of 24 (permalink) Old 12-05-2016, 03:14 PM
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just read it over and over a few times, its extensive but not complicated. Good luck with it.


We generally tend to look for worse case issues, but most of the time its simple stuff like poor grounds weak battery (from constant cranking), and faulty sensors. Checking things at the 35 pin connector will get you the best quick answer, if the resistance reading are off then you just check the wire harness connectors and then the sensor.
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post #6 of 24 (permalink) Old 12-05-2016, 06:00 PM
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Find the Auxiliary Air Regulator (AAR) in the Engine Fuel chapter. Then find it on the engine, and see if the hose on the side away from the intake manifold is leaking. It sounds like the engine runs better after the AAR closes.
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post #7 of 24 (permalink) Old 12-06-2016, 02:17 PM
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forgot to ask, how to the plugs look?
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post #8 of 24 (permalink) Old 12-07-2016, 08:49 PM Thread Starter
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Update:

I went back to check on the AAR and there are not holes or cracks in the line. I also clamped a few of the lines closed and didn't see a difference. I was able to fix my cold start valve to where I can manually turn it on and off. It helps tremendously with the hard start but the engine still doesnt want to idle. I can start it with the cold start and keep it alive by flicking the cold start valve to rev it up but the rpms drop below 500 and it dies.

The AAR seems like it would be a reasonable cause of the problem since the engine is running lean and after it warms up it doesn't lean out anymore but I cant see any problem with it. Is there anything else that would seal up and stop too much air getting it in after the engine warms up? Or maybe something that allows for more fuel to come in after the car warms up?

Also I haven't checked the plugs, I missed place a tool bag, but I will check them soon.
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post #9 of 24 (permalink) Old 12-07-2016, 09:28 PM
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You could block the AAR off and see if that changes how it runs. It will take some throttle to keep it running for the first minute while cold.

There is one other thing on the Federal model 76's - a second pickup in the distributor and circuit in the ignition module, controlled by a thermo-switch. I don't know if half of an ignition module can fail or not, but there are two circuits there. And the pickup modules have definitely been known to fail. There's a test shown in the Engine Fuel chapter.

You could short-circuit the thermo-switch so that the module runs on the "warm" circuit and pickup all the time. If you just want to go directly to a "fix".
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post #10 of 24 (permalink) Old 12-08-2016, 02:34 AM
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You may try adjusting the idle air control..
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