Just joined, and it's great to be here. In July 2022 I picked up a 1990 300zx with 39K miles. It's a 2-seater N/A Auto in 673 Silver Metallic, and black leather interior.

Like a lot of Zs this one has a slight idle issue. I have purchased the data cable and used both DTA Conzult and Nissan Datascan Consult to look at my engine in real-time. Everything shows as nominal according to https://conceptzperformance.com/wiki/in ... _Reference

Here is a video showing what it looks like on the tach, with the A/C on and in D held by the brake (so it's more pronounced).

https://youtu.be/PP4dWzEY9b4

It happens with the A/C off and it happens in neutral (the tach just doesn't dip as much). It happens when it's cold and when hot. The issue seems to be present only at 2000 rpm or less. Once it's above 2000 rpm, it really runs well and gets normal MPG.

Plugs are NGK OEM Spark Plug, PFR6G-11
Injectors are the upgraded 16600-15v02
EGR is functioning properly.
(IAA / IACV) is set correctly by the book.

Going from the symptoms you see in the video, where would you concentrate first? I know this can be a subject of great debate, but I welcome any help received here. I'm sure you guys have the answer. Thank you, in advance.

*Additional:
DataScan video:
https://youtu.be/qCSLkxe-qyw

ConZult Video:
https://youtu.be/LVD280sXELg

I'm going to test the Injectors, MAF, and PTU this week.

Do some research into the PTU, a most common problem which can affect idle.

Yes, it has the original 1989 PTU in it, so I'm going to replace it.
Injectors and MAF are checking out as within spec.

The new PTU came in a few weeks ago, but I didn't have time to put it in. Went to drive the car today and cylinder 5 was not firing. Verified with Conzult. (Saw plenty of light green corrosion on the original connector.)

Replaced PTU with the revised one and related wiring harness. The car felt peppier and sounded stronger, but the idle issue is still present. MAF tested good.

Time for injectors...

According to the service records:
the Injectors are the upgraded 16600-15v02 and were replaced 10 years ago, which was only 1200 miles ago for the car.
I suppose they could be bad.

Have you checked for vacuum leaks?

Yes took it to the shop 3 weeks ago and they did a full vacuum leak test - zero leaks.

*They also did the 60K mile timing belt, water pump, and seals, etc. service. Since it has 40K miles but 33 years on about everything.

90Z_NA_Auto wrote: ↑

Fri Nov 11, 2022 3:27 pm

According to the service records:
the Injectors are the upgraded 16600-15v02 and were replaced 10 years ago, which was only 1200 miles ago for the car.
I suppose they could be bad.

Age, not miles. Particularly in hot climates.

Please keep us informed on what you find.

What does the readout of the CPS signal look like?

NolimitZ32,
How can I see the readout of the CPS (Crank Position Sensor)? Don't see that in my DTA Conzult or Nissan Datascan Consult software.

I'm not sure, I've never used either of those softwares. I would imagine an oscilloscope would be able to get you what you need. If the video shows the worst of it, it's possible that a coil is on its way out. It's such a small oscillation that a minor electrical inequity could be causing it. I'm not sure if a power balance test would yield any helpful results but you could try running one.

NolimitZ32,
scroll up to see a picture of my most recent power balance test.

Didn't see that in the photos, I'm used to doing a manual test. Swap the coil between 4 and one of the stable cylinders and see if the faulted RPM moves with the coil. Have you reset the ECU since doing all the work? Not sure if there is much of an adaptation memory like on newer vehicles but should be done when swapping components regardless. Since the cylinder was dead for an unknown amount of time, I'd get the physical condition checked with a compression test and warm leakdown. It makes sense that 3 is higher than the setpoint and 4 is lower since 3 is the opposing cylinder.