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Old 11-21-2012, 07:14 PM   #1
Usagi
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Focus misfire under load.

Aargh!

So, saturday my Alternator apparently died with no warning. Not so much as a battery light. Fixed that, and the car's electrics appear to be operating without issue, aside from the fact that the battery is way weak from being blitzed.

Anyhow, I've gotten it back running, and i have a very annoying issue. There's a random misfire under load that appeared out of the blue at the exact same time the car quit on the freeway. The car will run fine if you slowly advance the throttle to the point you want. However, if you suddenly advance the throttle to a value (any value) the engine sputters and loses power. If you LEAVE your foot there, the system regains its composure and pulls just fine.

Also, if you goose the throttle to over 3/4 while it's in neutral, it spins up to roughly 3,000 RPM and then returns to idle. Following some pretty obvious misfiring, it promptly stalls out.

So far I've:

Attempted to datalog, but Livelink is refusing to work >_>

Replaced the Upstream O2 Sensor (DTC code referring to o2 heater ckt. Figured it was worth a shot)

Checked the plugs, noticed they were WAY worn, and replaced them with properly gapped replacements. (Motorcraft Plat's)

Cleaned the MAF with CRC no-residue contact cleaner. It appeared to be mildly dirty, but nothing horrid.

Checked the TPS sensor, and the resistance from pin 3 to the center pin seems to be adequite and changes smoothly with throttle position.

Datalogged with limited capability using just the SCT device. the 'load' (derived from MAF counts) seems to respond immediately to changes in throttle position.

I'm leading to the fuel control sensing system having issues, seeing as it freaks out when the load rapidly increases. However, I'm running out of sensors to check!! Also, given that the adaptive learning history has been thoroughly reset due to roughly 48 hours with no battery, could it simply be that the system is trying to relearn its functions? I'm inclined to think not, because if a new car performed like this while it was learning, I'd take it back!

Any help in this annoyance would be more than appreciated.

Thanks,
Aaron


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Old 11-21-2012, 07:21 PM   #2
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The alternator connector and the fuel rail pressure sensor have the same connector if I remember correctly. Double check that you got the right connectors plugged into the right things. I have seen similar issues when they were swapped.
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Old 11-21-2012, 07:26 PM   #3
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I've never disconnected the fuel rail pressure sender, so i doubt that's the issue. Good call though!
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Old 11-21-2012, 07:32 PM   #4
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No problem. I have seen it before when people disconnect it to gain more access to the alternator and then plug them in backwards. Was hoping for an easy fix for you...
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Old 11-21-2012, 08:32 PM   #5
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Depending on how far off the tune was, or more correctly, what parts have been added since it's last revision; the tune may not be up to par and it may take longer to learn, if it can at all.

But not being able to log might relate to the issue at hand. make sure all the fuses are secure and good. Might just get lucky.


I can come up with no less than 5 situations that would cause similar problems.

Low fuel pressure, or lack of flow to the injectors.
Damaged TPS.
Damaged MAFS.
Damaged plug wires.
Low voltage from the alternator,,, pump DC has a scalar for voltage, it's pretty tough to tune the correct flow vs voltage and most just go by what it was stock, no clue if that works well or not.


Update drivers and whatnot and see if you can get a good log. That'll really help here.
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Old 11-22-2012, 07:20 AM   #6
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Iminhell,

Here's a datalog. (LINK)
If you're inclined to look at it, just hit download at the top of the pastebin, and save as a .csv file. Livelink will open it right up.

Nothing seems amiss (heh) here, not at least with the sensor readings. MAF and TPS seem to follow the throttle input, and each other, fairly well.

The only thing is, it seems to have cut off before it started missing real bad, so I'm going to try again. However, i did notice it seems to get worse when the car is at full temperature.
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Old 11-22-2012, 07:38 AM   #7
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I thought misfiring under load was typically a cracked coil. Is the CEL flashing when you experience the misfire? How about new plug wires?
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Old 11-22-2012, 08:26 AM   #8
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I'm leaning toward a fuel delivery error. It looks like the system slams lean when the throttle is opened rapidly. IDK if this is an expected condition, but it looks "off" to me, esp. since the mix stays lean for quite a bit.

Next datalog: http://pastebin.com/0uXKwKMN

EDIT: the system is NOT detecting a misfire. at all. No misfire flags were set during this most recent datalog, despite the fact that they very definitely occurred. Looks more like flame-out than an out-and-out misfire.

Also, iminhell, the tune is exact to my configuration. No changes have been made to the car (aside from some paint about a year ago) since the tune was put in. Besides, aside from an intake I'm stock engine wise. The same condition exists on a stock tune.
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Old 11-22-2012, 08:58 AM   #9
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I wonder if your knock sensor is working right. I've heard that spark advance under load is kind of dictated by knock sensor readings, and the PCM will keep spark advance right at the boundary of knock for maximum performance/efficiency. It's curious that you got zero misfire counts in that data log but you feel it happening. If you can feel it one would think the knock sensor would be feeling it also.

There are also spots in that datalog above (if I'm reading it right) where your spark advance is up to 40 degrees before TDC and that seems like a lot, my car doesn't go beyond 32 or 34 and I though that was generally as much advance as any car runs.
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Old 11-22-2012, 09:43 AM   #10
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Good catch 6spd. I'm tuned for 93, which means the spark advance is more aggressive. Thanks for the input though!

The next time i do a log I'll talk to the knock sensor directly and see what's up.
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