Intermittent misfire at low loads and idle! Pics/Video/Info

[CyFi6]

This misfire has been driving me nuts for a while now. About a year and a half ago when I swapped in the 7mgte everything ran and idled great, this is when I had stock platinum plugs etc. I started to notice a very slight miss at idle after I swapped in some AutoLite copper plugs. The miss got worse and worse over time it seems, and I went to NGK Iridium plugs after the coppers with no noticeable change in the misfire.

This miss happens with no load on the engine (Idle or revving in neutral) only. If I am driving the vehicle it doesn't miss a beat during accell whether it be light medium or heavy. As soon as I let the engine come to an idle or I am on the throttle just enough that the engine isn't accelerating or decelerating the car, I can hear the miss.

Here are some relevant specs on the vehicle
Rebuilt 7MGTE ~10k miles
NGK plug wires replaced during rebuild
NGK Iridium plugs ~couple thousand miles, proper gap checked
550CC Denso Injectors Cleaned and Flow Tested
Aeromotive FPR ~38 PSI w/vac at idle
Speed density conversion (MaftPro)

Things I have done to try to fix it/things I have checked
Checked Primary coil resistance, all in spec
Compression test - All cylinders 170-180PSI
CPS Rewired with shielded 4 conductor cable
Cleaned all oil out of spark plug valley (and replaced leaking VC gasket)
Scoped IGF signal and coil primary signal (Pics at bottom)
Scoped the narrowband o2 and shows lean at idle at times (wideband ranges from 14.5 to 15.5), but with increased engine speed stock o2 switches normal and wideband shows 14.5-15.0 even with the miss.

Verified accurate MAP signal to the MAFTPRO
Checked codes- Only IAT and TPS codes present (NORMAL with my MAFTPRO setup)

Need some ideas of what to look for here. I did not notice any dropping out when graphing IGF or primary voltage. Signals were consistent from one coil to the next and I never noticed a dropped signal at any point (seemed to be firing every time). Assuming I am looking at everything correctly, it seems it is not ignition related. I have yet to scope any injectors, but I want to make sure I am heading down the right path before I continue any more.

Here is a video of the engine running. Keep in mind- this is about as bad as the miss will get. There are times it idles like a dream, not a single miss, and there are times it runs like it is here. There's no relation between when it misses and heat or run time that I can tell.
[video=youtube;n1JrXvVbnng]http://www.youtube.com/watch?v=n1JrXvVbnng[/video]
Let me know if you have any experience with this kind of issue or if you have any ideas. I know this was really lengthy but I figure the more info I can give the better. Thanks a lot!

http://img.photobucket.com/albums/v313/cyfi/Car/Misfire Diag/IGFsignal.jpg
http://img.photobucket.com/albums/v313/cyfi/Car/Misfire Diag/primaryignition1.jpg
http://img.photobucket.com/albums/v313/cyfi/Car/Misfire Diag/PrimaryIgnition2.jpg
http://img.photobucket.com/albums/v313/cyfi/Car/Misfire Diag/PrimaryIgnitionClose1.jpg

 

[Nick M]

Excluding things specific to your car, that sounds like a car that needs to have the cob webs blown out. Maybe you have weak fuel volume at low speed.

 

[CyFi6]

Something else I don't really understand is the IGF signal I got. I thought IGF was a 5v signal, but I am showing less than 10MV on the IGF wire, though besides what looks like a lot of noise, the signal is what I would expect to see.

 

[hvyman]

Have you checked the cps and its wiring?

 

[mecevans]

I have the same issue but i only get it in closed loop. My motor is identical minus the bolt ons and it sounds exactly the same.

Thing i will be checking..

Cps wiring
Igt & igf
Actual Coil ouput per cylinder
Grounds
Valve lash
Vaccum leaks ( its possible for only once cylinder to have it)
Charging system (if i hold window button up it missfires worse)
Power balence test.

 

[grimreaper]

what are the cps air gaps at?

 

[jonahs_supra]

ignitor have a good clean ground?
have your alternator tested to make sure the diode/ripples are normal

 

[jetjock]

IGf is very short. Same as the burn time from which it's derived. And when measuring it you might want to use DC coupling there Sparky

 

[3p141592654]

With regards to the Igf plot and selecting AC coupled, if RC time constant of the scope/probe is short compared to the signal period then

i = C dV/dt

i.e. You are measuring the derivative.

 

[mecevans]

diagnose this yet? i can post known good waveforms if you need.

 

[CyFi6]

Here's my scope patterns for the injectors 1-6. Everything appears to be completely normal. These were taken at idle, but when revving the engine to the point where it is missing consistently they all appeared basically the same.

 

[mecevans]

1. Do they ohm check out ok?

2. Any injector feel quieter or louder than the rest?

3. Did you scope primary ignition signal?

4. Mist water everywhere. Any secondary ignition components arching?(including hood closed) Plugs new?

5. Compression and valve adjust good?

 

[OfnaRcR4]

Mine seems to be doing the same thing

[video=youtube;ynM83ADq_z8]http://www.youtube.com/watch?v=ynM83ADq_z8&list=UUf1XjT2qxx8hMZIgHgSI8Pg&index=47[/video]

New plugs, wires, re-grounded the igniter, new coil harness. Just acquired an old scope though, thanks for the advice mecevans.

 

[Insidious Surmiser]

OfnaRcR4 how does the car feel when you're driving? it sounds like one of your cylinders isn't running on par with the rest. I've had a loose injector clip do this before

 

[Ekmo]

Im still learning, but couldn't a bad catalytic converter cause a miss too?

 

[CyFi6]

1. Do they ohm check out ok?

2. Any injector feel quieter or louder than the rest?

3. Did you scope primary ignition signal?

4. Mist water everywhere. Any secondary ignition components arching?(including hood closed) Plugs new?

5. Compression and valve adjust good?

1. Yes the resistance values are all even from one injector to the other. I don't remember the reading, I think around 14ohm (high imp), but they were all even.

2. No they all appear to be working fine from what I can see/hear. I have noticed however that I almost always lose rest pressure in the fuel rail rather quickly. I do have an Aeromotive FPR though, and from what I understand this is normal. I will further investigate this though by blocking the exit of the regulator and seeing if it holds pressure. If not then I can assume I have a possible leaking FP check valve or injector.

3. I haven't scoped the primary, as the voltage spike even from the primary side exceeds the safe limit for my current scope/attenuation. (Actually I did scope it if you see the pics in the original post, but this was before I realized I could damage my scope doing it)

4. Will investigate this and report back. Wires are relatively new and coils pass visual inspection.

5. Have not checked compression, but before I rebuilt the motor it had good compression (as shown in the video) and after the rebuild I see the same misfire symptoms. I am going to say it is unlikely that it is a mechanical problem with the compression or valves but I will verify. The valves were adjusted to the middle to large range of specs when the engine was rebuilt.

Thought I would add that I recently changed the CPS for a rebuilt unit because of an internal oil leak. This seems to have helped a bit with the smoothness of the engine and misfires at idle and such seem less, but the low load misfire is still there when holding the idle up to 1500-2500RPM.