Idling Issues - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 16 (permalink) Old 02-11-2017, 06:58 PM Thread Starter
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Idling Issues

Hey Datsun People,

Last attempt to solve before taking it up north and seeing what 5.56x45 can do to Japanese steel.

Vehicle: 1981 280ZX, AT, NA, 45,000 original miles.

Replaced:

Ignition- coil, wires, plugs, cap and rotor.

Fuel- pump, filter, injectors

Emmisions- TVV at ERG, O2 sensor, all vacuum lines

Engine: intake/exhaust manifold gasket

Coolant: water pump, thermostat

Tune up: filters, valve clearance (cold/hot), timing, idle speed, all new fluids

Checked: air flow meter, MAF sensor, cleaned all electrical contacts on temperature sensors, compression, fuel pressure, no vacuum leaks that I could find.

Issue: The car idles rough where the car shakes quite noticeably. The car pulls fine and is fine at driving speeds. No knocking or strange sounds coming from the upper or lower end other than typical L28 sounds. When reving the engine it does sound a little more high pitched than what I am used to hearing with engines. Why is this car idling like crap? Please help been at this for months. Thank you. Located on northwest side of Chicago if anyone local wants to take a crack at this sucker. I have beer.

Feel free to point out any errors in my thread, preferably in a constructive way. Thank you again.

Note: The EGR valve actuates when it shouldnt during warm up. So I plugged the vacuum line entry to the manifold, as well as the vacuum line entry to the EGR valve. Not sure why the EGR valve misbehaves during warm up. The TVV I replaced seems to be functioning properly and I am 99.4% sure the vacuum routing is correct.
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post #2 of 16 (permalink) Old 02-12-2017, 10:52 PM
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No MAFs on these cars.
You note "vacuum lines" and that you are pretty sure vacuum routing is correct. But you don't say whether you have checked to see if vacuum is to specs. So I would do that. You also don't say if you have actually replaced the vacuum lines, so I would do that, too. You CAN have vacuum leaks even with new hoses hooked to all the correct places. There are various rubber diaphragms in, for example, the power brake booster, heater door actuators, etc. I dont remember if the NA ZXs had vacuum advance, but those are a common place where leaks occur in cars that do have them.
Dizzies with vacuum advance also have a plate underneath the breaker plate that rides on tiny ball bearings. This sometimes gums up on these cars and/or the little balls fall out, rendering it useless. So check that out.
I would also disconnect, inspect and clean EVERY electrical contact under the hood, not just to sensors. Including battery terminals and all grounds. Also, seems obvious, but is your battery good and strong?
These cars are very sensitive to tight electrical connects, good grounds, full 12V and vacuum tightness.
FWIW, I have been fooling around with these cars for more than 20 years and have found that poor running is almost never due to any component 'going out.' Its almost always some simple vacuum or electrical glitch.
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post #3 of 16 (permalink) Old 02-12-2017, 11:02 PM
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firing on all cyl? sometimes a weak one will show up more on idle than when RPM is up. Try unplugging on injector at a time and note RPM (also you can use stethoscope to listen to injectors to make sure all are ticking). how does the compression look? even across all? how about a plug reading from some new plugs?
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post #4 of 16 (permalink) Old 02-16-2017, 01:15 PM Thread Starter
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Hey Dave thanks for your time. I checked the injectors and they're all ticking the same, the compression is even across the board and within spec. The plugs do seem discolored, black blotches and they have less than 200 miles on them. I bought new NGK BPRES11 plugs and going to put them in and see what that does, then run a fuel pressure check if that's not my problem (when driving every once and a while the rpm will surge like its downshifting but its not down shifting, this is why I suspect the fuel pump could be on its way out, maybe its not proving proper fuel pressure at idle. I had NGK BPREY11 in there, perhaps that's my problem? There is also way too much ticking coming from under the rocker cover, I set the valve clearance hot and cold and are within spec but still hear quite a lot of ticking. With the cam lobe facing up, I can depress the rocker arm and can get a tap out of it on the lash pad, should this happen? Not sure how loose these rockers should be when dialed in.
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post #5 of 16 (permalink) Old 02-16-2017, 01:36 PM
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maybe a video of the issue at this time.


if all the plugs look the same (which is why you want new plugs) after a short time then I would presume the mix is pretty consistent, but I suppose a bad injector could be ticking away but at idle just not performing.


A fuel pressure check is pretty much SOP for any fuel related issue, so you should def get that. Vacuum leaks are suspect to for idling issues where everything else checks out.
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post #6 of 16 (permalink) Old 02-16-2017, 01:37 PM
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did you try disabling one injector at a time to see if one cylinder is not pulling its load?
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post #7 of 16 (permalink) Old 02-16-2017, 01:48 PM Thread Starter
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So just pulled the spark plugs and cylinder 3-6 have some oil on the center electrode and tip of the threads, 2 has no oil but dark in color and cylinder 1 spark plug looks new. If the compression is good then I don't think the oil is coming in from the bottom end, I changed the valve stem seals a few years back could these have gone already?

I didn't pull the individual injector connectors yet but I will. Should I run the injector test and fuel pressure test before I put the new plugs in?

I am confident there is no vacuum leaks although it sure does seem like a possible culprit. I changed all the vacuum hoses, insured they have a snug fit, new intake manifold gasket and torqued to spec and sprayed starter fluid around all the vacuum line connecting points and around the intake manifold.
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post #8 of 16 (permalink) Old 02-16-2017, 02:35 PM
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Try disabling the plug that looks new. You can pull the injector plug, or pull the spark boot insert a wire and then short the spark plug out while running (I don't like to just pull the plug wire off, prefer the spark has somewhere to go). If disabling the new look plug cylinder has no effect (compare to disabling another where the plug looks normal but disabling the normal one to see how it effect the idle), then you have narrowed it down. Make sure the plug is sparking, if so and the compression is good, then it pretty much has to be a fuel issue, and I would suspect an injector that is clogged. I recently pulled the injectors while still attached to the rail to see how they worked when fuel was supplied and the injector activated.
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post #9 of 16 (permalink) Old 02-16-2017, 04:49 PM Thread Starter
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When I pull the plug on the third cylinder spark plug there is no drop in rpm nor any change in engine idle conditions. Doing the same to the other spark plugs results in a large drop in rpm and increased rough idle conditions. Check the wiring to the corresponding fuel injector and the injector itself? I checked all the wires and the resistance is roughly the same and within spec per FSM.
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post #10 of 16 (permalink) Old 02-16-2017, 05:36 PM Thread Starter
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I'm confused now. Removing the spark plug wire on the third cylinder results in no change, I am getting an arc through that wire so the plug is getting juice. I removed the injector connector to the corresponding cylinder and that resulted in a drop in RPM. Electronically the injector is ok, the dropping resistor and efi harness all check out according to the FSM as well.
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