Issue with idle, also Air Flow Meter and Idling, please explain the logic - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 9 (permalink) Old 06-26-2019, 08:16 PM Thread Starter
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Issue with idle, also Air Flow Meter and Idling, please explain the logic

My car: 1977 280Z with an engine and tranny from a 280ZX

The problem: From a cold start, the car is unable to hold an idle 95% of the time. I need to crank it multiple times before the car is able to hold idle. Once it does hold idle, the car seems to drive fine (I have taken 3 short 5-mile trips without any problem).

What I can immediately tell it needs: The boot in front of the air flow sensor is torn, needs new spark plugs, and a fuel filter.

What I was told by a mechanic: My mass air flow sensor is defective.

Since there isn't any movement of air when the car isn't moving, can someone explain to me logically how a defective mass air flow sensor would prevent the car from initially starting and holding idea.

Also, would any have any suggestions on how I can trouble shoot this or should I go ahead and replace the mass air flow sensor? Thank you
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post #2 of 9 (permalink) Old 06-26-2019, 11:49 PM
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how exactly did the mech determine it was AFM that was the issue?
Get the FSM read the part about fuel injection its very comprehensive and not difficult to understand. Will go thru what to check and how to check it with simple tools like a multimeter.


could be something as simple as the idle adustment not correct. The AFM opens even when just idling, Not sure I follow your commeent about the car moving. The car does not have to move for the AFM to function, it just has to have air flow thru it, as long as the engine is running the AFM is operating sending info to the ECU.


Another possible suspect with low idle on startup of a cold engine would be the Aux Air valve (allows more air to flow when cold). Again all this is covered in the FSM. One thing about owning a car this old is you really need to learn something about how they work. Not a lot of mechanics will be checked out on them, no computer to plug into to get info.

Last edited by Dave M; 06-26-2019 at 11:56 PM.
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post #3 of 9 (permalink) Old 06-27-2019, 12:50 PM Thread Starter
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how exactly did the mech determine it was AFM that was the issue?
Get the FSM read the part about fuel injection its very comprehensive and not difficult to understand. Will go thru what to check and how to check it with simple tools like a multimeter.


could be something as simple as the idle adustment not correct. The AFM opens even when just idling, Not sure I follow your commeent about the car moving. The car does not have to move for the AFM to function, it just has to have air flow thru it, as long as the engine is running the AFM is operating sending info to the ECU.


Another possible suspect with low idle on startup of a cold engine would be the Aux Air valve (allows more air to flow when cold). Again all this is covered in the FSM. One thing about owning a car this old is you really need to learn something about how they work. Not a lot of mechanics will be checked out on them, no computer to plug into to get info.
The mechanic connected a fuel pressure gauge above the fuel filter and when my car was on, he moved the flap on the AFM and it showed no change in the gauge. Then he swapped it with a working one and the fuel pressure changed when he moved the flap. Is that enough information to determine that I have a faulty AFM?
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post #4 of 9 (permalink) Old 06-27-2019, 02:38 PM
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fuel pressure will change with vacuum changes, changing the A/F mix will effect the performance of the engine (and therefore the vacuum). If he had a known good one, then a test drive with it installed with have been the correct course of action. Was that done? Did changing to the known good one resolve the Idling? Its a good diagnostic technique if you have a known good one, as long as changing that resolves the issue.


could have just been a poor connection to the old one, corrosion is a problem. I would have tested the old vs new like this. compare one to the other.


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post #5 of 9 (permalink) Old 06-27-2019, 06:31 PM Thread Starter
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fuel pressure will change with vacuum changes, changing the A/F mix will effect the performance of the engine (and therefore the vacuum). If he had a known good one, then a test drive with it installed with have been the correct course of action. Was that done? Did changing to the known good one resolve the Idling? Its a good diagnostic technique if you have a known good one, as long as changing that resolves the issue.


could have just been a poor connection to the old one, corrosion is a problem. I would have tested the old vs new like this. compare one to the other.
He didn't drive it around. He just did it to show me via what the fuel pressure gauge said and said that I needed a new AFM. I wasn't entirely convinced which is why I'm trying to ask in this forum and just do the work myself.

What I don't understand is why the car would be able to run sometimes and perfectly fine if I have a faulty AFM. Wouldn't the car just not run at all?

Thank you for the video link. I'm going to order a multimeter and try that out.
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post #6 of 9 (permalink) Old 06-27-2019, 07:27 PM
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The best course of action is to start by making sure all the connections for the EFI are clean, no corrosion. Its a big problem with cars this age.


Next you really need to study the FSM on EFI, read thru it a couple of times. The system is really pretty simple. Then go thru and do the diagnostics listed out in the FSM. Your main tool will be the multimeter for various resistance and voltage test.


The AFM is often suspect, and often "adjusted" when in fact it is something else.


His test by substitution is a valid approach, just most folks don't have a spare one to swap in. A test drive should have been done to confirm that is the ONLY problem. Often its several issues, you just have to work thru the test. The AFM is not a complex devise, IF it has not been monkeyed with, the most likely fix will be to simply clean the contacts with some de oxit, and if that does not fix it, clean the trace on the carbon board inside the cover.


The real problems begin when it has been messed with trying to fix another problem. It has a calibrated spring that controls the movement of the flap. There is really NO reason to ever adjust it.


intermittent problems are the most difficult and time consuming problems. If you start with the cleaning of all the contacts and don't have any hack fixes, it most likely will be something simple.


The systems work very well once sorted out. My setup always starts on the 1s key turn and very quickly at that.
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post #7 of 9 (permalink) Old 06-29-2019, 03:43 PM
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One more thing... You say the "boot in front of the air flow sensor is torn." Just confirming you mean the one between the air filter and AFM. Yes, important to fix that, but it shouldn't affect how the car runs. However, if there are any air leaks BEHIND the AFM, that will screw up a lot because it's introducing unmetered air. Fairly common for those accordion hoses to get leaks in the pleats after all these years, so besure to check that out, too.
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post #8 of 9 (permalink) Old 06-29-2019, 04:49 PM
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One more thing... You say the "boot in front of the air flow sensor is torn." Just confirming you mean the one between the air filter and AFM. Yes, important to fix that, but it shouldn't affect how the car runs. However, if there are any air leaks BEHIND the AFM, that will screw up a lot because it's introducing unmetered air. Fairly common for those accordion hoses to get leaks in the pleats after all these years, so besure to check that out, too.

good point you really have to remove the boot and inspect it. Typically it would account for a lean run, but looking for a silver bullet to fix everything is not generally the issue. Typically its a bunch of stuff, corrosion, defective sensors, hack fixes from the past, messing with the AFM, leak like the one described above, missing or incorrect routing of vacuum lines, incorrect hookups of sensors (like swapping the CTS and the thermotime) work arounds like bypassing fuel pump wiring, incorrectly intstalled distributors.. the list goes on.
This is the reason for systematic check off of each circuit. Fixing the little stuff will often get you running. Once the car is running new problems tend to be single point failures which are much easier to deal with.
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post #9 of 9 (permalink) Old 07-05-2019, 07:30 PM
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I’m not so sure I would call that diagnosis definitive. The Bosch L Jetronic like is on your car, both the Z and ZX use a 7th injector mounted directly into the intake manifold. It does the job of the choke on a carbed car and enriches the fuel mixture when cold. Z’s are very hard to start if this is not working, or the temp sensor is bad (ECU not knowing when to turn it on). If it runs fine after it warms up then I wouldn’t suspect the Airflow meter. Also, the fuel pressure will change with vacuum, not by moving the flap on the AFM.
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