joshuaho96

No, code 21 is very straightforward. It can only be the things described in that diagnostic flow. In fact it has no way of knowing that the spark plug resistance is out of spec.

Diagnostic flow for a code 21 says check signal at the ECU pins, on a voltmeter of some kind it should read 0.1V at idle, 0.12V cranking, 0.15-0.25V at 2000 rpm. Check each coil such that +1, -3 is infinite or very high resistance, -3, +2 should do the same. (-1, +3), (+3, -2), (+1, -2), and (-1, +2) should all be somewhere between 0 ohms and high resistance. If you trace the path from the coil pin all the way back to the ECU in the diagram the resistance you measure at the coil vs connected to the harness should be pretty much the same.

It's not that straightforward. For example I have had this code because my ignitor/power transistor wires were unplugged. It knows something is wrong but has no idea what it means. If you want to actually diagnose this I recommend following the service manual flowchart for code 21. Ohm out the harness, coilpacks, inspect the ECU, etc. And again, it's entirely possible for there to be no problem at all but if you're running a different coilpack it'll trigger the code because it doesn't like what it sees.

Oil change does not trigger code 21. Code 21 is for coilpacks primary side connection. You can try to clear the code with a battery disconnect, hold down the brake pedal to drain capacitors through the brake lights with the ignition on for 10-15 seconds before you reconnect the battery. I have seen R35 coil conversion permanently cause this code with no ill effects so it might be the resistance it wants to see isn't quite right on one or more coilpacks. Could be inside the ECU, could be the harness, could be a coil. You can test it all if you want or just ignore until the car actually starts misfiring.

I forgot you have a Nistune ECU. Use Nistune to do all the tests I mentioned instead of faffing with 30+ year old electrical connectors. You can read MAF volts off that too, there are reference values in the service manual to tell you roughly what it should be in different conditions.

As MBS suggested if this is happening when the engine is cold you're going to want to remove the intake air regulator and verify resistance + that the shutter is physically opening and closing. At -20C should be fully open, 20C half open, over 60C fully closed. At 20C should be 70-80 ohms. For AAC valve testing using a Consult cable see if you can raise and lower the idle by commanding more or less AAC duty cycle. Hopefully it's pretty linear as well, shouldn't be like 30% AAC valve is barely idling and 40% is 1500 rpm.

Reading through the engine service manual their advice is do a cylinder balance test. Unplug one injector at a time and see if the idle drops a consistent RPM. You can also do this using a Consult cable which is easier. They also call for unplugging the power transistor, then with the engine off and the fuel rail unhooked from the manifold verifying that you have good fuel flow (even injection, no dripping/leaks, etc) when you twist the CAS by hand. Also verify the spark by pulling the spark plugs and allowing the plugs to ground and turning the CAS by hand. I would also start doing the sensor checks and idle valve checks in service manual. Make sure the MAF tests reasonably, the intake air regulator is sane, etc. You may have to get new spark plugs.

Isn't there a PAR engineering gearset that has closer to stock ratios? And they claim they'll do whatever ratios you want.

From the factory on RB26s there's a fitting on the intake manifold collector/"plenum" that provides pre-throttle boost signal. That's how they did it back in the day, I figure it's fine to replicate it.

This is just a dumb way of keeping the wastegate closed to make more boost. Undo it and either follow the factory setup or redo it properly. Manual boost controllers just use a spring preload to determine when to close in response to boost pressure so it no longer vents to atmosphere.

I would put a fuel pressure gauge between the filter and the fuel rail, see if it's maintaining good fuel pressure at idle going up to the point when it stalls. Do you see any strange behavior in commanded fuel leading up to the point when it stalls? You might have to start going through the service manual and doing a long list of sensor tests if it's not the fuel system for whatever reason.

Nothing cheap, but I have seen the AC Hydraulic DK13HLQ which is pretty close to what you're asking for. The problem is the hydraulic cylinder mandates that the back of the jack be a certain height and Skylines have fairly deep inset floor jack points. A more certain and cheap option is to get some nominal 2x10 inch lumber boards and have one board be 1.5 feet, the other board 3 feet. Glue it all together with lots of clamps to maintain pressure. Make two of these. The ramp suggested above is a good idea, it takes quite a bit of clutch slip to get over each of the boards. I only need the first ~1.5 inches for my low profile jack on a standard ride height car but a lowered car you want to go up to the full ~3-3.5 inches. Paint it or store indoors if you want it to last.

Pull codes via the self-diagnosis procedure. As far as I can tell this is just a sign of transmission issues but not a code unto itself.

Sounds like a misfire. Check fuel pressure and don't even think about going into boost while it's like this unless you enjoy engine rebuilds.

The average previous owner for these cars were basically S-chassis owners in the US. Teenagers or teenager-adjacent. I often tell people that neglect is easier to fix than something that was actively "repaired" by previous owners.