Yet Another Idle Misfire Thread - except I've tried all the obvious steps
- Thread starter komrad36
- Start date
I see. Thanks! As I've replaced all vac lines and T's, and sprayed everything from the BBV to the BCS to the EVAP solenoid and not seen any leaks, I suspect that if there IS a vac leak causing this, it's local to cyl. 4 and is likely due to the LIM. I'll be able to do the LIM job some time around late May / early June, and I'll post back with what happens.
Thanks,
Komrad36
Thanks,
Komrad36
Sorry to dig up this old thread of mine, but I finally got time to do the LIM job and solved my problem, and so I wanted to check in and explain for anyone who might read this in future with the same problem.
To recap, I had idle misfires, most but not all in cyl 4 - and only at idle. I also had very negative fuel trims - only at idle - and, though I didn't mention this in the original post because I didn't realize it might be related, I had hard starting (3-4 seconds) EXCEPT when cold.
These hints come together to suggest leaky fuel injectors.
When I did the LIM job, the gasket was in great shape, as was EGR tube, and vac did not change after the job - been 19-21 inHg the whole time. That nice high vacuum combined with negative fuel trims means I don't suspect vac issues.
However, sure enough, I installed reman flow-matched injectors from ZZP (much cheaper than new) and ALL the problems are gone! No more negative trims, fuel pressure holds so car starts instantly, and best of all, no more idle misfires, even at long, hot idle! See Torque screencap below...
So if your symptoms seem to match mine, consider that it may be leaky injectors.
One way to test that doesn't involve removing injectors is simply to check the fuel pressure after the car hasn't been used for a while. If it's zero, you may have leaky injectors like I did! You can use a proper fuel pressure gage, or just push in the valve stem carefully on the fuel rail and see whether fuel sprays out at high pressure. Mine did not just a few hours after ignition off, indicating pressure loss from leaky injectors (note: other things could cause this too. I replaced the fuel pressure regulator in the past to no avail...)
Perhaps you could fix the leak just by cleaning the existing injectors, idk. I went with remans to be safe.
Hope this helps someone!
Thanks,
Komrad36
To recap, I had idle misfires, most but not all in cyl 4 - and only at idle. I also had very negative fuel trims - only at idle - and, though I didn't mention this in the original post because I didn't realize it might be related, I had hard starting (3-4 seconds) EXCEPT when cold.
These hints come together to suggest leaky fuel injectors.
When I did the LIM job, the gasket was in great shape, as was EGR tube, and vac did not change after the job - been 19-21 inHg the whole time. That nice high vacuum combined with negative fuel trims means I don't suspect vac issues.
However, sure enough, I installed reman flow-matched injectors from ZZP (much cheaper than new) and ALL the problems are gone! No more negative trims, fuel pressure holds so car starts instantly, and best of all, no more idle misfires, even at long, hot idle! See Torque screencap below...
So if your symptoms seem to match mine, consider that it may be leaky injectors.
One way to test that doesn't involve removing injectors is simply to check the fuel pressure after the car hasn't been used for a while. If it's zero, you may have leaky injectors like I did! You can use a proper fuel pressure gage, or just push in the valve stem carefully on the fuel rail and see whether fuel sprays out at high pressure. Mine did not just a few hours after ignition off, indicating pressure loss from leaky injectors (note: other things could cause this too. I replaced the fuel pressure regulator in the past to no avail...)
Perhaps you could fix the leak just by cleaning the existing injectors, idk. I went with remans to be safe.
Hope this helps someone!
Thanks,
Komrad36
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I had the same issue, -12 to -15 long term fuel trim on idle. Sometimes I seen it jump up to-25 but only for a second, I only got misfires after bringing the idle up to 1500rpm and letting off the pedal. Misfires would show up, mainly in cyl 3, sometimes the idle got real rough, so I did a balance test. I suspected the injector on cyl 3 since most the misfires came in cyl 3 after letting off the gas from 1500 or higher rpm.. so I connected my scan tool, watched my fuel trims and unplugged each injector 1 by 1 while watching the fuel trims for 3 to 5 min. Doing this let's you see how much each cylendar contributes to the fuel trim.. if you have a leaking injector the fuel trim won't move as much as the rest of the cylinders when the injector gets unplugged..
That's how you check for leaking injectors or injectors not letting enough fuel thru. You can also use a scan tool to prime the fuel system and pulse injector 1 and write down the pressure drop, then pulse it again and write down the next pressure drop, then activate the fuel pump to build pressure back up and move to injector 2 doing the same thing, move on down the line. And look at your data after wards and what ever injectors don't match all the rest are the ones that need to be replaced
That's how you check for leaking injectors or injectors not letting enough fuel thru. You can also use a scan tool to prime the fuel system and pulse injector 1 and write down the pressure drop, then pulse it again and write down the next pressure drop, then activate the fuel pump to build pressure back up and move to injector 2 doing the same thing, move on down the line. And look at your data after wards and what ever injectors don't match all the rest are the ones that need to be replaced