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Old 09-11-2013, 07:07 PM   #1
dbspcl
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Default Hesitation (misfire?) 2007 GT help!

Okay, I am struggling with my 2007 GT in terms of it hesitating/bucking on strong acceleration. No problem if I gradually increase RPM's, but if I stomp on it (or start going up a hill for instance, under load), it sounds like one or more cylinders is misfiring and the car has no power to accelerate.

To make things worse, on a couple of occassions it has just died (twice at freeway speeds 80mph, once at 10 mph on the street) and will not restart if I pop the clutch back out while still moving. It comes to a complete stop, I turn off the ignition, wait about 20 seconds, turn it back on, and it starts right up with no problem and runs fine.

I have a JLT CAI, SCT X2, Brenspeed tune and 4.10 rear (it's a 5-speed). It will not reproduce the problem at rest, so I'm not sure how to proceed. No codes are thrown at all. While trying to pull injector connectors to see if the engine runs rough and try to find the misfiring cylinder, I ended up breaking the stupid tab on one of them and then it really ran like crap all the time. So I replaced that and it is now back to being okay when at rest or in neutral but running like crap when under load. I think it happens more when the car is warmed up - don't seem to notice it as much when the car is cold.

I replaced the fuel filter, no change. I tried different codes (I have a different Brenspeed one with the O2 sensors turned back on that I use for smog checks), no change. I am thinking the fuel pump may have a problem (although I think the TSB was only for 2005-2006, and it doesn't only do it after freeway driving), but don't want to change it if it won't help. Perhaps putting a fuel pressure gauge on it might help. Still doesn't explain dying at 80 mph on the freeway, though...

Incidentally, the number 6 plug has twice now had its electrode vaporized somehow and I've had to replace it (the champion one-piece 7989), don't know if that is related as well, think it might be a result of the misfiring.

I'm going to probably try datalogging next to see if that shows anything. I can't understand why no codes are ever thrown, though.

I've also heard engine grounds etc. might be a problem, but my connections all look solid. I do have some manual wiring - I bypassed the stupid high/low radiator fan resistor/coil since the connectors got corroded, so that is now just a standard Bosch-type relay hooked up through a switch in the cabin to turn on and off the fan. Similar problem with my ignition - it would not start sometimes and I traced it back to the ignition switch so I have a momentary engine start button as well going to a relay and then the starter. I don't think any of this would cause what I am experiencing, though.

Any suggestions would be very welcome.
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Old 09-11-2013, 09:50 PM   #2
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My 05 has been doing the same thing lately were if I drive when it's still cold runs normal but once the cars fully warmed up and I try to jump on it then it will start to bogs down as if there's no gas in the tank. I did get a check engine light and when I pulled the code it was 02 sensor bank 2 rich. So I'am going to change both my header O2's this weekend and see if that puts everything back to normal. If not I guess it's off to the stealership for me.
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Old 09-11-2013, 09:53 PM   #3
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How many miles do you got on yours, mine has 85,000 miles which from what I heard is when most O2 sensors should be changed.
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Old 09-12-2013, 07:44 AM   #4
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Check simple stuff like your air intake. If it has slipped a little bit and is letting unmetered air in it will cause your car to run exactly as you describe. Take the CAI off, check it and reinstall.

Good luck. Don't throw parts at it. Find the issue first.
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Old 09-12-2013, 10:30 AM   #5
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How many miles do you got on yours, mine has 85,000 miles which from what I heard is when most O2 sensors should be changed.
I am at 107K or so. Have not ever done anything with the O2 sensors. Wouldn't they throw a CEL or code at least (my SCT X2 shows no stored codes). Also, would the Brenspeed tune I generally use (where the O2 sensors are "turned off" even though I can't switch them out being in CA) still cause bad O2 sensors to affect driveability?

I think I will change them anyway, wish there was some way to verify that there may be something wrong with what I have now.

Thanks for the suggestion.
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Old 09-12-2013, 10:32 AM   #6
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Quote:
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Check simple stuff like your air intake. If it has slipped a little bit and is letting unmetered air in it will cause your car to run exactly as you describe. Take the CAI off, check it and reinstall.

Good luck. Don't throw parts at it. Find the issue first.
One of the first things I did (going back to old school of air-gas-spark) was to take off the JLT, clean the TB, clean the MAF, put it all back, ensure it is sealed. I will check again, though, doesn't hurt.
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Old 09-12-2013, 10:35 AM   #7
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My 05 has been doing the same thing lately were if I drive when it's still cold runs normal but once the cars fully warmed up and I try to jump on it then it will start to bogs down as if there's no gas in the tank. I did get a check engine light and when I pulled the code it was 02 sensor bank 2 rich. So I'am going to change both my header O2's this weekend and see if that puts everything back to normal. If not I guess it's off to the stealership for me.
Please let me know if that solves your problem, it sounds just like what is happening to me. That's why I went after the fuel filter, thinking that must be it, because it seems like the mixture is tweaked incorrectly when I stomp on it. But the filter was clean. The pump was to be my next step along this troubleshooting route, but performance pumps are expensive
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Old 09-12-2013, 11:46 AM   #8
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Please let me know if that solves your problem, it sounds just like what is happening to me. That's why I went after the fuel filter, thinking that must be it, because it seems like the mixture is tweaked incorrectly when I stomp on it. But the filter was clean. The pump was to be my next step along this troubleshooting route, but performance pumps are expensive
Yeah that's what I did was change my fuel filter then check for air leaks around the intake then cleaned my MAF sensor and got all of my stock parts back on with the stock tune. Engine lights went back off warmed the car up went for a drive seemed to run better but after driving around for a hour or so it started hesitating again when I tried to jump on after slowly taking off then going up to speed like it was starving for gas it never die on me but it felt like it was. So when I change them O2's if runs normal again I be sure to let you know.
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Old 10-02-2013, 11:16 AM   #9
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I've been trying to datalog to find the problem with my lack of power under load. The KNK_SEN_ADDER PID goes up (to 20?) each time I try to accelerate hard. It correlates almost exactly to the LOAD PID, and makes sense because I can hear the engine knocking when it happens. Question is why? The STFT drops (to .85 sometimes) also whenever the KNK_SEN_ADDER goes up, which seems like the right response. Still stumped though, and will continue to look at logs. Any suggestions very much appreciated.
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Old 10-02-2013, 06:26 PM   #10
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Make sure your TCS is "OFF". I had issues going WOT; didn't matter if from a dig or roll; that I thought were fuel pump/delivery related but turned out to be the TCS. Hasn't happened once since I remember to have TCS off.
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Old 10-02-2013, 06:26 PM
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