1977 280z idle issues when cold - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 6 (permalink) Old 10-12-2019, 12:21 PM Thread Starter
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1977 280z idle issues when cold

I can't seem to get my 280z running very nicely when cold. It starts very easily, but then idles very rough around 400-500 rpm, then when it has warmed up runs smoothly around 800-900 rpm. I noticed when I fill the fuel tank all the way, it starts at 1000 rpm then slowly drops to 600rpm in 2-3 minutes running a bit rough, and then when it warms up it's again at 800-900 rpm.

Does anyone have a tip what I could look at next?

What I've done so far:
- New Fuel pump, including new filters
- Fuel pressure: without vacuum 36 psi, with vacuum 28 psi.
- Fuel I use has RON octane rating 98. The only fuel that still is ethanol free where I live.
- Injectors: all cleaned plus new connectors.
- New Temp. sensor. Also tried adding extra resistance with variable resistance but didn't make much differency, 1kohm extra made the car run worse.
- Thermotime+CSV are new, and work
- Air Regulator: opens and closes fine, in about 8 minutes. Connector is new and I read 9volts.
- Vacuum starts at 13 Hg/in and slowly moves up to 17 Hg/in when warm
- New vacuum advance
- EGR is blocked off
- Timing is set to 12 degr, it runs even faster at 20 degr, but didn't want to go too far of the recommended 10 degr
- I've done all the EF checks from the service manual. The only thing that differed was the resistance on the Airflow Meter between terminal 6 and 8 (233 ohms, should be 180 ohms) and terminal 9 and 8 (129 ohm, should be 100ohm)

Last edited by Peter_789; 10-13-2019 at 07:16 AM.
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post #2 of 6 (permalink) Old 10-12-2019, 04:06 PM
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Have you check the pin 13 to ground (emp sensor) at the ECU? does it track the temp curve in the FSM? just because you installed a new one does not translate into everything is correct with it. You could have a wiring issue, or a defective sensor. How cold is it when its not running right? The CSV does not come into play until its in the 60's or below. How about the throttle position sensor (not sure about the name, its the one that connects to the throttle. Its in the EFI check list so I assume it checks out. Did you check for the 12 Start signal to the ECU?
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post #3 of 6 (permalink) Old 10-12-2019, 06:02 PM
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Bump initial lead to 15 degrees BTDC. Report back with cold and hot vacuum readings. Have you set valve lash?
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post #4 of 6 (permalink) Old 10-13-2019, 10:36 AM Thread Starter
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@Dave M
It's about 57-59 degrees here.
Throttle valve switch checks were oke.
Resistance between terminals at temperature sensor:
58 --> 3.1kOhm
(58 --> 3.95 kOhm pin13 at ECU to ground)
68 --> 2.66kOhm
136 --> 0.58 kOhm
(136 --> 1.43 kOhm at pin 13 ECU to body ground)
176 --> 0.29 kOhm

I also measured the coil resistance:
Primary resistance: 1.9 Ohms (between outer terminals of coil, without it being connected. That's a bit high, is that a concern?)
Secondary resistance: 11.97 kOhms

What is the 12 Start signal to ECU?

@Powerglide
I adjusted it to 15 degr BTDC --> 13/14 Hgin cold and 17 Hgin warm (so no change it seems)
I never adjusted the valve lash, I don't think I have the skills to do that, but if it might be the problem I could go to a garage.

The car did start at 1000 rpm, I adjusted the timing, in 5 min or so it dropped to 800 rpm and then raised to about 1000 rpm when completely warm, so that is better than before, but still a bit strange. I adjusted the idle when warm to 800 via the idle adjustment screw, and will see next weekend how it starts when it's cooled down again.

Last edited by Peter_789; 10-13-2019 at 10:45 AM.
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post #5 of 6 (permalink) Old 10-13-2019, 09:07 PM
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there is a 12 signal to the ECU that happens only when the ign is in the "start" position. Don't recall the pin you need to check at the ECU but its in the FSM somewhere. Its a signal to enrichen the mix for a short period. Not the same as the Cold start which has nothing to do with the ECU.
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post #6 of 6 (permalink) Old 10-16-2019, 10:41 PM
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Quote:
Originally Posted by Peter_789 View Post
I can't seem to get my 280z running very nicely when cold. It starts very easily, but then idles very rough around 400-500 rpm, then when it has warmed up runs smoothly around 800-900 rpm. I noticed when I fill the fuel tank all the way, it starts at 1000 rpm then slowly drops to 600rpm in 2-3 minutes running a bit rough, and then when it warms up it's again at 800-900 rpm.

Does anyone have a tip what I could look at next?

What I've done so far:
- New Fuel pump, including new filters
- Fuel pressure: without vacuum 36 psi, with vacuum 28 psi.
- Fuel I use has RON octane rating 98. The only fuel that still is ethanol free where I live.
- Injectors: all cleaned plus new connectors.
- New Temp. sensor. Also tried adding extra resistance with variable resistance but didn't make much differency, 1kohm extra made the car run worse.
- Thermotime+CSV are new, and work
- Air Regulator: opens and closes fine, in about 8 minutes. Connector is new and I read 9volts.
- Vacuum starts at 13 Hg/in and slowly moves up to 17 Hg/in when warm
- New vacuum advance
- EGR is blocked off
- Timing is set to 12 degr, it runs even faster at 20 degr, but didn't want to go too far of the recommended 10 degr
- I've done all the EF checks from the service manual. The only thing that differed was the resistance on the Airflow Meter between terminal 6 and 8 (233 ohms, should be 180 ohms) and terminal 9 and 8 (129 ohm, should be 100ohm)

You say that the AAR opens and closes fine in about 8 minutes but the scenario you're describing has a defective AAR written all over it. ANY Z that idles that low when cold is going to run very rough. It has a richer mixture cold, therefore it needs a higher idle speed to compensate for it. Cold fast idle should be between 1K and 2K depending on the ambient air temp. The AAR is supposed to give you the cold fast idle of course and it sounds like it's not doing what it's supposed to be doing. Or maybe there is some issue with the plumbing for the AAR causing your scenario. Also, try raising your warm idle speed to 900 vs. 800. I've seen some issues in the EFI cars when the idle is set that low. Z Man of Washington

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