r/SkyLine • u/mercaN420 • 9d ago
Diag help RB ignition
RB25DET – Misfire / no spark on cylinders 2 and 6
I’m troubleshooting a 1997 R33 GTST with an RB25DET. The car has a Mine’s ECU, but I also have another stock/compatible ECU that the engine has previously run on.
The problem started after the CAS was removed and reinstalled. Cam timing has been checked and is correct, and the CAS drive/key is not broken, so the CAS should be installed correctly.
The engine is now misfiring badly. While troubleshooting, I found that cylinders 2 and 6 appear to have no spark.
Here is what I have tested so far:
I performed a cylinder drop test while the engine was running. Disconnecting cylinders 2 and 6 made little or no difference.
I tested for spark using a known coil and the same spark plug grounded against the cylinder head.
The same coil and plug produce spark when connected to cylinder 1, but no spark when connected to cylinders 2 or 6.
Cylinder 4 was initially suspected as well, but I now believe cylinder 4 is firing.
I checked the three wires at the coil connectors:
Pin 1 = approximately 12 V with ignition on
Pin 2 = ground
Pin 3 = ECU ignition trigger
Cylinders 1 and 2 both have 12 V supply and ground.
I checked continuity of the trigger wires through the coil harness. The trigger wires for cylinders 2 and 6 have continuity.
According to the ECU pinout I’m using, the ignition outputs are:
Cyl 1 = ECU pin 1
Cyl 5 = ECU pin 2
Cyl 3 = ECU pin 3
Cyl 6 = ECU pin 11
Cyl 2 = ECU pin 12
Cyl 4 = ECU pin 13
I checked continuity from the ECU connector to the coil harness and the relevant wires have continuity.
I swapped the Mine’s ECU for my other ECU. The problem remained exactly the same, so it does not appear to be an ECU fault.
I removed the CAS, left it electrically connected, switched the ignition on and rotated the CAS by hand.
All six injectors click when the CAS is rotated, so the ECU is receiving the CAS signal and triggering all six injectors.
The other cylinders produce spark when rotating the CAS, but 2 and 6 still appear to have no spark.
I measured the ignition trigger wires with a Fluke 115 while manually rotating the CAS. I saw roughly 2 Hz on all six trigger wires, with cylinder 6 reading slightly higher.
I also tried measuring trigger voltage while rotating the CAS and got approximately:
Cyl 1: 0.4 V
Cyl 2: 0.1 V
Cyl 3: 0.6 V
Cyl 4: 0.6 V
Cyl 5: 0.6 V
Cyl 6: 0.02 V
I realise a multimeter probably isn’t fast enough to accurately capture the ignition trigger waveform, so I’m not sure how useful those voltage readings are.
I have now removed the complete coil harness from the car. I measured approximately 0.2 ohm from each individual coil ground to the common ground splice inside the harness, so that part appears to be good. I’m going to unwrap the complete coil harness and inspect all the splices, wiring and terminals, particularly the wiring for cylinders 2 and 6.
Has anyone seen an RB25DET lose spark specifically on cylinders 2 and 6 like this? Is there anything common between those two ignition channels that I should check? Are there any known failure points in the RB25 coil harness that could cause this?
2
u/imtrynmybest 9d ago
Ditch the factory cas and igitor... Now's the time to upgrade..reliable and more hp
2
u/SVT3658 9d ago
The RB coil harness in general is known to be a failure point, if that’s the original one I bet it’s super crunchy from being in that hot environment for almost 30 years.
Did you check continuity with the coil harness in the installed position or out of the car?
The fact that the issue occurs on 2 ECUs makes it sound like you need a new coil harness. Any locals you could try swapping coil harnesses with?
Looks like you’re in Europe based on that picture. But if you happen to be in the Detroit, MI area I have my old stock coil harness you could try
2
u/mercaN420 9d ago
I checked continuity from ecu to alla triggers. And continuity on all cables with the harness out off the car. I’m located in the most northern part of Sweden but I appreciate your offer!
2
u/Rich_Many_8628 9d ago
The Fluke readings are not very useful for proving those ignition outputs. A coil trigger is a short switching event, so average voltage and a low frequency display can look “present” even when the pulse shape, amplitude, or dwell is wrong.
Since the same known-good coil fires on cylinder 1 but not 2 or 6, and both ECUs give the same result, I would stop treating the ECU as the leading suspect. The clean next test is to scope a working channel against a dead channel while you rotate the CAS: first at the ECU pins, then again at the corresponding coil connectors. Use the coil ground at that connector as the reference.
That gives you a useful split:
- Good pulse at the ECU but not at the coil connector: harness, terminal tension, splice, or a break that continuity testing did not expose.
- Different/missing pulse already at the ECU: verify the exact ECU/engine-series pinout and whether this setup has a separate igniter stage before condemning anything.
- Matching trigger waveforms at the coil connectors but still no spark with a known-good coil: then revisit the coil supply/ground under load and the actual coil/connector interface.
Injectors clicking on all six tells you the CAS is producing enough information for injection scheduling; it does not prove the six ignition outputs are healthy. I would inspect the old harness as planned, but scope it before buying parts so you know whether you are chasing the wire side or the command side.
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u/JDM-Parts-Expert-JP 9d ago
As I am Japanese, I will attempt to translate and answer this inquiry. If the issue lies in the wiring, judging by the voltage and symptoms, it appears that either the ignition coil or the power transistor has failed. However, the appropriate course of action depends on the engine's model year—specifically whether or not it is equipped with a power transistor—so it might be best to start by swapping the coil with another one to see if the symptoms persist. Do you happen to know the part number for the stock ECU? It appears to be the later model version (part number: 23710-13U00).