Random misfire

apsmith49921

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Ok so the car is a 97 cobra. Full mods in signature but fbo e85 NA.

Random misfire at wot. I replaced coil packs and it got slightly better. Data log shows 0 misfire occurrences, fuel pressure holds solid not a single blip. Any ideas? Plugs are about 20 miles old. Coil packs brand new.
 

Venompower

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Ok so the car is a 97 cobra. Full mods in signature but fbo e85 NA.

Random misfire at wot. I replaced coil packs and it got slightly better. Data log shows 0 misfire occurrences, fuel pressure holds solid not a single blip. Any ideas? Plugs are about 20 miles old. Coil packs brand new.
What brand of plugs? How old is the alternator?
 
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apsmith49921

apsmith49921

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Alternator is maybe 3 years old. And plugs are autolite 764s brand new. Battery voltage on the data log looks good. Compared it with another log I have of known good pulls and it's about the same
 

Venompower

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Alternator is maybe 3 years old. And plugs are autolite 764s brand new. Battery voltage on the data log looks good. Compared it with another log I have of known good pulls and it's about the same
What does the log look like when the misfire occurs? Can you post it?
 
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apsmith49921

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Here is the csv. the only thing you can see is a visible dip in the engine speed.
 

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Venompower

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Are these the dips you noticed? At 189 you drop from 4117.75 to 4052.5 until 194 when it goes back up to 4235.5, at 220 you drop to 4610.5 from 4718.75 until 223 when you are back to 4857.5, and finally at 263 you drop from 5525.5 to 5482.25 until 267 when you go back to 5647.5.

There are definitely small dips, but overall it's still pretty linear. I wonder if it could be minor clutch slippage?
 
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apsmith49921

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correct those are the dips i found. They coincide with the misfire or what i believe to be one. im not sure if its clutch or not, rpm is based on crank signal and oddly enough thats where cylinder misfire data comes from too.
 

Venompower

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correct those are the dips i found. They coincide with the misfire or what i believe to be one. im not sure if its clutch or not, rpm is based on crank signal and oddly enough thats where cylinder misfire data comes from too.
I'll keep thinking about this... graphed your TPS and it's flat across the pull so I don't think it's a TPS signal issue.
 
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apsmith49921

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10-4. Also as a note I don't have the ability to log wideband data however I have seen it pop lean to about 1 lambda on these break ups. Which would indicate misfire. And that's in the driverside exhaust
 

Venompower

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10-4. Also as a note I don't have the ability to log wideband data however I have seen it pop lean to about 1 lambda on these break ups. Which would indicate misfire. And that's in the driverside exhaust
How are you measuring fuel pressure, and have you checked the regulator?
 
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apsmith49921

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I hooked a gauge up at the pressure port. I taped that to the windshield and recorded it with my gopro
 

lwarrior1016

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Have you changed the plug wires? I know you said you did plugs and coil packs, but I don’t see mention of wires.

Also, for what it’s worth, we do a lot of performance stuff at work. We run ngk plugs in everything. They are the least problematic plug we have found. They always tune the best. I swapped from Delco plugs to ngk on a procharged vette and picked up 20whp on the dyno.
 

lwarrior1016

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Another thought, does it happen only after it’s hot or does it do it all time? Did you log IAT? What about maybe a bad cam or crank sensor?
 

BHernandez

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I've had random misfires caused by loose plugs and they were Autolites. Be sure your plugs are torqued properly. There isn't much of the plug threaded into the head.
 
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apsmith49921

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Have you changed the plug wires? I know you said you did plugs and coil packs, but I don’t see mention of wires.

Also, for what it’s worth, we do a lot of performance stuff at work. We run ngk plugs in everything. They are the least problematic plug we have found. They always tune the best. I swapped from Delco plugs to ngk on a procharged vette and picked up 20whp on the dyno.
So I did not do plug wires yet.

Also I did have some plug backout on the old Motorcraft but when Installed the new plugs they were torqued to spec and I still experienced the issue.

As far as the timing is concerned it's all the time. I do get lucky sometimes and once it powers past that initial stutter I can make a decent pull.

I had also thought about the cam and or crank sensor but how would I go about checking or troubleshooting them? This just started one random night after a fill up. I did stick the car back on 93 octane last night for the winter. I'm curious if it will run better
 

lwarrior1016

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If you can graph the crank and cam sensor signals, you can look for drop outs. Other than that, I think it’s just a replace them and hope kind of scenario.
 
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apsmith49921

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I'll look into it.

So the car is back on 93 I'm going through it re learning all the KAM stuff so some hiccups here and there.

Did a pull and it stuttered on hitting the gas but came right out. Went hard all the way to 7k in 2nd and 3rd. Now when I stepped on it I was waiting on it to recover the afr. So it was like 1.1 1.05 lambda which is a touch lean. It did that because it was pulling itself back up after leaning out on decel. It seems to be semi relegated to early wot. But when it was on e it broke up 4 or 5 times not just at the beginning

Wonder if I'm having o2 sensor issues?
 
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apsmith49921

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Some more driving today. The Kam has unfucked itself and seem to drive normal for cruising.

I figured out I can get it to stutter just by stabbing the gas to even 50% throttle. I'm wondering if it's an o2 sensor issue but o2 sensors aren't used at wot. It's acting like it's sluggish to catch the fuel change or something of that makes sense
 

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