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ECU returned. Repair shop ECU-PRO's whatever. . couldn't find any problems. Also, with the fuel rail put back together, I just put everything back together to test. Same issue. Misfire on cylinder 8. WTF! Getting angry.
So. . here's what i've noticed. I can tell it misfires, but barely. . after 30 sec or so, the ECU turns off the fuel injector for cylinder 8. Then it gets noticably worse. This seems to suggest that the injector is fine.
So, with Air, Gas, and compression all demonstrably proven to be working. I'm back to square 1 again.
Hypothesis . . all of which are dumb:
1. Spark plug igniters are just old and the ECU just can't work all of them. . #8 is the last, so it fails first (although I have tested the coil and plug outside of the engine sparks every time).
2. Fuel injectors are just old and the ECU just can't work all of them. #8 is the last, so it fails first.
3. My code reader sucks and this is the wrong code.
4. compression tests from my AND my mechanic showing perfect compression were both wrong.
5. The engine is not misfiring, but the computer thinks it is. How does the ECU determine this? What sensors?
6. The engine is just barely running and the ECU can't really tell what's not running well, so it just does #8.
These are stupid, but I currently have no decent theories.
Last edited by protonspring; Sep 17, 2023 at 08:26 PM.
ECU returned. Repair shop ECU-PRO's whatever. . couldn't find any problems. Also, with the fuel rail put back together, I just put everything back together to test. Same issue. Misfire on cylinder 8. WTF! Getting angry.
So. . here's what i've noticed. I can tell it misfires, but barely. . after 30 sec or so, the ECU turns off the fuel injector for cylinder 8. Then it gets noticably worse. This seems to suggest that the injector is fine.
So, with Air, Gas, and compression all demonstrably proven to be working. I'm back to square 1 again.
Hypothesis . . all of which are dumb:
1. Spark plug igniters are just old and the ECU just can't work all of them. . #8 is the last, so it fails first (although I have tested the coil and plug outside of the engine sparks every time).
2. Fuel injectors are just old and the ECU just can't work all of them. #8 is the last, so it fails first.
3. My code reader sucks and this is the wrong code.
4. compression tests from my AND my mechanic showing perfect compression were both wrong.
5. The engine is not misfiring, but the computer thinks it is. How does the ECU determine this? What sensors?
6. The engine is just barely running and the ECU can't really tell what's not running well, so it just does #8.
These are stupid, but I currently have no decent theories.
You swapped injectors, and the misfire stayed on # 8. Perhaps you said it, but I could not find it:
1 - Did you swap spark plugs?
2 - Did you swap coils?
I would take a look at the coil connector. If you take pictures of the connector on the pin sides. Then zoom it out to see if the connectors are too open, i.e. they fail to make good contact on the coil side. I do not have the part number for the electrical connector
A similar issue could happen on the ECU side, i.e. the bad luck that the pin for cylinder 8 is not making great contact. Also, you can measure resistance from the ECU side of the harness to the coil/injector connector for two cylinders and compare the values. There may be continuity, but different resistances.
I would also try spraying them with an electric contact cleaner.
ok. . . thanks for the response. I have pulled out the spark plug connected to the coil, grounded it to the motor and cranked the engine. Sparks every time, but I definitely need to repeat some tests since I have to be wrong somewhere.
Used a generic ODB2. . same error. Misfire on cylinder 8.
. . . more data. . . I can start the car and feel the engine. . . it doesn't feel like it's misfiring at all. . very smooth. After almost exactly 40 seconds, it starts misfiring. I restarted like 10 times. Almost exactly 40 seconds each time.
So. . . it's probably not really misfiring, but the computer thinks it is, then disables the fuel injector and it actually gets bad.
The problem seems to be that the computer THINKS there is a misfire and disables the fuel injector after 40 seconds although there doesn't really seem to be any real misfire before that.
How does the computer determine whether there is a misfire or not? Is there an engine torque sensor or something somewhere?
google says: "The OBD II system detects misfires on most vehicles by monitoring variations in the speed of the crankshaft through the crankshaft position sensor."
Hmmm. . perhaps I should put in a new crankshaft position sensor?
I really feel for you. There is nothing more frustration.
Not that you need more opinions, but, when I think of something that runs well when cold but only for a time period, I think of some type of enriching circuit or profile from the control unit. I had a Jaguar that was the opposite, rough when cold and then smoothed out and ran okay.
That was my early thought about an ECU running some cold running profile then switching based upon time or temperature or whatever. The fact that it is only #8 makes me wonder why just that cylinder as my theory would seem to affect all the cylinders.
Is there something near #8 that would cause a vacuum leak primarily on that cylinder? Also, I guess the plug looks okay visually? Not white (super lean) or black (super rich or not firing).
Oh well, trying to help, Have you tried moving the #8 injector connector around while the engine is misfiring to see if you get a change. I am leaning to a signal problem but why not when cold unless the enriching profile is masking the fault. Best of luck.
What puzzles me a bit is the last comment about misfires being reported "exactly" about the same time after ignition.
That means the engine is HOT already, or at operating temperature. If the engine is already at operating temperature and the "40 s" is always present every time after ignition, I would suspect the ECU is always running some diagnostic for a period of time, and it triggers the misfire count if that test fails.
On the idea of a vacuum leak near that cylinder "only", the "spray WD40/brake cleaner/else" technique can be used around cylinder 8 to see if the idle changes/fluctuates.
Similarly, the "Have you tried moving the #8 injector connector around while the engine is misfiring to see if you get a change" is looking for a bad electrical connection, or partially broken wire along the harness.
Fingers crossed, this is a "no parts required" issue, but well-hidden issue that require patience and not a "shower of parts"
The only entry points into the cylinder are rings, valves, and the head gasket. That is why compression or leak-down tests are done for diagnosis.
A vacuum leak at the intake manifold gasket should give a rough idle from the start since there is unaccounted air (measured by the mass air flow sensor). Definitely, it can also produce a misfire, but I expect the misfire not to be so focused on a single cylinder. It's worth keeping in mind.
If there is an intake manifold leak around cylinder 8, the suggested test is to spray around that area and see how the idle changes. Some people use WD40 (I do not like WD40, another story), but you can also try starter fluid, or brake cleaner. (https://www.thedrive.com/maintenance...r-vacuum-leaks )
Maybe the fuel injection hole isn't sealing and air is getting past the injector?
Do you mean the injector seals are damaged? It could be, Did you notice them dried up? When you swapped injectors you kept the existing seals on their own injectors, correct?
So, on the assumption that I've verified that everything else is working, I've decided to order all new ignition coils and plugs. Delphi (OEM) coils, and NGK iridium plugs. Here's to hoping.
As I said, thats what I ended up doing. Figured just do it all and know its good. Injectors, coils, wires, plugs and O2s .I went with the NGK Ruthenium plugs, and don't forget to do the plug wires too.
Yes my mistake. Been a while since I looked at this thread, was thinking we were talking M113. Your M273 would have coil on plug. No wires and single plugs. Much easier to change out thats for sure.
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