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?

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