Post by
peterpilot9 »
https://forums.nicoclub.com/peterpilot9-u236973.html
Thu Nov 26, 2020 11:23 am
1990 300zx NA Automatic with 310,000 miles; about 90,000 on a rebuilt engine. All stock, no mods.
Issue is a misfire/dead number 4 cylinder. I will apologize upfront for the length of this post but maybe someone who is technically knowledgeable will read this and can make sense/use of the information. This is a description of a fault and the testing, using the on-line service manual provided here as a reference.
The complaint is as follows: Engine starts and runs with no issue when cold. After a period of time, which appears to be temperature dependent, the number 4 cylinder begins to miss, then goes dead. The time to failure appears to correlate to the outside air temp with a time to failure of about 10 minutes with an OAT of 80 degrees increasing to about 14 minutes with an OAT of 60 degrees. The failure occurs in the same manner whether the engine idles for the period of time or if it is driven. The engine is at operating temps (per the gauge) for more than a few minutes before the failure occurs so the failure does not occur at the point the engine gets to temp.
There is no code. I started with the ignition system and ultimately found an ignition related failure. I have tested the issue per the instructions for a code 27; ignition system. All of the tests have been done at the time of failure since the engine operates normally when cold. Coils and plugs from 3 other cylinders were swapped and the results remain the same; the failure occurs only in cylinder number 4.
Diagnostic Procedure 27 EF & EC 118:
All items in Step A test OK.
Step C, using a logic probe, pulse signal between coil connector terminal C and ground exists until the misfire starts then the pulse signal becomes intermittent. After about 30 seconds or so,the pulse signal stops and the cylinder goes dead. If the engine is allowed to cool for a few hours, the pulse signal will again exist and the cylinder fires normally for 10 - 14 minutes, depending on OAT at which point the problem repeats itself.
So, with Step C a No-Go, I proceed to Step D.
Step D(1). The Power Transistor Unit in the car at the time of failure was only 10 months old. I completed the inspection per the service manual and found one circuit that should be open at about 60 Mega Ohms resistance. Not sure that this was a problem but I went ahead and put in a new PTU, tested and passed prior to installation. The failure remains the same.
Step D(2&3). All harness continuity has been checked and found OK. The continuity tests on my DMM defaults to resistance <50 ohms and I am working under the assumption that this is an acceptable resistance. Continuity has been verified between the ECU/TPU including the sub-harness and between the coil pack connector and the TPU.
At this point, the SM testing sequence dead ends as Step D does not flow chart any further. I went ahead and completed Step E and found all ground circuit continuity exists as required.
I made an attempt to measure signal voltage using my DMM, but given the signal is pulsed, I'm not sure of the value here. I did note that the DMM returned a voltage reading of 0.52 on the number 4 input to the TPU and at the ECU output, which falls to 0 volts at the point the pulse signal goes dead. The other operating cylinders have the same .52 volts readout and remain at that value even after number 4 falls to 0 volts and the cylinder goes dead.
I sent the ECU to SIA Electronics who provided a function test and found the ECU to have no faults. In addition, I have installed and ran a known good ECU with the very same failure results.
Sorry for the dissertation, but the issue is a bit complex and I am at best a You Tube Certified Mechanic. Loss of the ground from the TPU to a particular coil along with what appears to be a loss of signal voltage to the TPU from the ECU, with a correlation of some sort to temperature while all components and harness continuity being tested are found to meet standards.
As a caveat, while the harness continuity has been tested and found OK at the point/time of failure, it is in very poor condition. It just seems that if the harness is the cause, it would be demonstrated in continuity failures.
If anyone has had this experience with an issue like this or has any ideas, I would greatly appreciate the input. Thanks