1977 280z low idle, gas causes car to stall - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 5 (permalink) Old 08-29-2018, 09:17 AM Thread Starter
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1977 280z low idle, gas causes car to stall

I've replaced a lot of parts because previously my car made popping and banging sounds when pressing the accelerator, I think mainly due to clogged injectors, this is much better at the moment. When the engine is warm after a 20 min drive it idles around 900-1100 and runs almost perfect.
However cold it idles at 400-500 and when I hit the gas and let go the rpms jump around between 200-900. Often rpms gets so low it just stalls. Fuel pressure is also low around 28psi, while I installed a new FPR, when the engine stops fuel pressure directly drops to 0.
What could be a cause of this, where should I look at further?

A few video's:
https://youtu.be/KzmUaDgOSL8 (cold start engine is around 25C/77F)
https://youtu.be/oeSVQ2pN5-4 (bit warmed up, still some popping sounds)

What I've done:
- new temperature sensor, thermo time, thermostat and temperature sending unit
- new cold start valve and fuel pressure regulator
- new fuel/air/carbon cannister filters (incl extra filter before pump)
- new battery and spark wires
- new vacuum advance
- new rubber hoses around air flow meter
- cleaned injectors at professional shop (no leaks, good spray pattern)
- blocked EGR valve with a metal plate and gasket underneith it
- tested for vacuum leaks, couldn't find any
- tested air regulator -> cold open, after 1 min on 12v almost closed
- tested cold start -> works (only in start position)
- tested why fuel leaks away by clamping of hoses: fuel leaks away via fuel return hose, not via injectors or pump, so apparently new FPR doesn't hold pressure
- cleaned fuel pump and tank as much as I could

Last edited by Peter_789; 09-01-2018 at 01:12 PM.
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post #2 of 5 (permalink) Old 08-29-2018, 02:00 PM
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Pretty sure you posted this in another forum also, a few days ago. Did you get any responses there?

You didn't really follow up on the other threads you posted in on this forum either.

https://www.zcar.com/forum/10-70-83-...uestion-3.html

The popping and banging is typical for a lean mixture problem. Sometimes the only fix is to add resistance to the coolant temperature sensor circuit.

http://www.atlanticz.ca/zclub/techti...pot/index.html
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post #3 of 5 (permalink) Old 08-31-2018, 07:44 AM Thread Starter
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No I didn't get responses there, the car seems to have different problem now as well, so therefore I thought a new thread was better.

>You didn't really follow up on the other threads you posted in on this forum either.
I did learn from the video by DaveM a drop in FP after the car shuts down isn't a huge problem (although I don't like it since I just bought a new FPR). I tested by clamping of hoses and the fuel runs away via the return hose to the tank, injectors and pump don't leak. Fuel pressure is around 27 psi now.
I tested the EGR valve that was mentioned with a vacuum pump and the diaphragm moves up and vacuum remains constant, so no leaks, however to be sure I just blocked of the EGR system. At idle there's no vacuum to the EGR.
I've tested the vacuum advance with a vacuum pump and the base plate moves and it holds vacuum, so no leak in the carbon cannister either.
I've tested every vacuum hose for vacuum leaks by spraying brake cleaner on it, but I don't hear any difference. It's almost like it gets too little air when the gas flap closes, when I let go of the gas the rpms drop and the car stumbles a bit and has difficulty in keeping running. Or is that a sign the mixture is too lean? The air regulator was open when I tested it of the car.

I've made an appointment at the local garage to test ignition timing and CO readings (idle adjustment screw) next week, since I don't have the equipment to do that myself. So let's see what that brings up.

Last edited by Peter_789; 09-01-2018 at 04:41 AM.
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post #4 of 5 (permalink) Old 08-31-2018, 09:12 AM
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Quote:
Originally Posted by Peter_789 View Post
I've made an appointment at the local garage to test ignition timing and CO readings (idle adjustment screw) next week, since I don't have the equipment to do that myself. So let's see what that brings up.

Instead of paying the local garage to perform simple tasks, consider using the money to purchase the tools/equipment that might prove helpful - not just in this instance, but on future projects as well.



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post #5 of 5 (permalink) Old 09-01-2018, 07:31 AM Thread Starter
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Quote:
Originally Posted by nkowi View Post
consider using the money to purchase the tools/equipment that might prove helpful
_
Well I can use the tools for free, and since I spent so much replacing a lot of parts I need saving a bit for now. But a timing light can be handy.

I've tested a few other things from the service manual.
Air Flow Meter 2.1
1-2. Resistance between terminal 8 and 6= 233 ohm (is this ok?, should be 180ohm)
3. Resistance between terminal 9 and 8= 129 ohm (is this ok?, should be 100ohm)
4-6. Opening flap causes voltage to decrease from 5V (closed) to 0.6V (open)
Air Flow Meter 2.2 -2.4
All checks ok
Air Temperature sensor
4C = 4.60 kOhm
20C = 2.45 kOhm
Water Temperature Sensor
20C = 2.68 kOhm
80C = 0.29 kOhm
Thermotime
pin 45 and body = 56 Ohm (should be 70-86 according the manual, although it seems it has something to do with the specific part number. I have 2 old thermotimes laying around. Part no A23-600000 (12s): 78 ohms and also a "new"one part no A23-601000 (9sec): 58 ohms)
pin 46 and body at 20C = 0.6Ohms (should be 0 or infinite at 14-20c)
Throttle Valve switch
All checks ok
EF 35-pin circuit tests
All continuity and voltage checks ok

Last edited by Peter_789; 09-01-2018 at 02:29 PM.
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