Thread: misfire on hwy

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  1. #1 misfire on hwy 
    justin10morrissey
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    Default 01-18-2013
    have an 04 gtp it throws codes p0420 and p0300. 0420 is from the hi flo cat dont know why p0300 but when its getting on hwy its misfiring and runs very poorly. need help or suggestions
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  2. #2 Re: misfire on hwy 
    blueguy
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    Default 01-18-2013
    Plugs or wires?

    Coils or anything done recently?

    Details, watson...details.
    Sold WBody's: '03 Blue GTP/'98 Green GTP/'98 Silver GT/'05 GXP
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    '89 Ford Mustang LX Notchback - LM7 5.3, 4L80, 9", HX40
    '04 Chevrolet Corvette MRM A4/LS1 - TSP LT's, 3"O/R X, AFE S2 CAI
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  3. #3 Re: misfire on hwy 
    nos4blood70
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    Default 01-18-2013
    ICM too perhaps? How many miles you got?
    -Carl
    "Penelope" - 145k Miles 2003 Bonneville SLE
    ​

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  4. #4 Re: misfire on hwy 
    Explicit_Spade
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    Default 01-18-2013
    Dirty Maf, Bad o2 sensor, wrong plugs?
    Quote Originally Posted by W-Body Store
    Remember, GM engineers didn't take into account your need for speed.
    Daily Driver: 06 Grand Prix GT / 9.4:1 CR / IS3 Heads / Ported Gen V / ZZP Headers / HPT Pro / Pulleys: 4.25, 3.8, 3.5, 3.4, 3.2, 3.0, 2.9, 2.7
    Projects: 86 Monte Carlo SS / 98 Sonoma / 74 Honda CB750
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  5. #5 Re: misfire on hwy 
    GTPpower
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    Default 01-18-2013
    Are you sure the cat isn't plugged?


    2001 GTP Drag Car - XPZ, Tischler heads, Upsidedown M90 IC'd, e85, Gen V, 2.3 w/ 5%OD
    2005 F350 6.0 - studded, deleted, tuned
    2001 GTP - cam, headers, nitrous, stock trans - 11.83 @ 116 <$2k in mods - sold
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  6. #6 Re: misfire on hwy 
    justin10morrissey
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    Default 01-21-2013
    recently changed the plugs and wires cant remember if it was doing it before i did this but it is still doing it. havnt replaced coils yet thats next on my list. plugs are autolite 606's and wires are napa beldan 7mm car has the zzp downpipe and throws a p0420 code bc of the highflo cat. when it misfires the mil flashes and pulls code p0300 random multiple misfire. maf is newer had on the car for about 7months. the car has 135,xxx miles. i believe i answered all of your questions. also the mil only flashes on highway it seems
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  7. #7 Re: misfire on hwy 
    Fivefingerdeathpunch
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    Default 01-21-2013
    Try the coils, but I almost think its the ICM.

    SMGPFC Member #1
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  8. #8 Re: misfire on hwy 
    blueguy
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    Default 01-22-2013
    You can test the ICM for free at your local auto parts store...do that.
    Sold WBody's: '03 Blue GTP/'98 Green GTP/'98 Silver GT/'05 GXP
    '99 Chevrolet Silverado Classic Z71 4x4 - K&N Intake/Gibson Exhaust #TRUCKTHINGS
    '12 Buick Regal Turbo - ZZP CAI/20% Tint/HID's
    '89 Ford Mustang LX Notchback - LM7 5.3, 4L80, 9", HX40
    '04 Chevrolet Corvette MRM A4/LS1 - TSP LT's, 3"O/R X, AFE S2 CAI
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  9. #9 Re: misfire on hwy 
    Fivefingerdeathpunch
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    Default 01-22-2013
    Quote Originally Posted by blueguy91 View Post
    You can test the ICM for free at your local auto parts store...do that.
    It can test good and still be a bad unit. Just a FYI.

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  10. #10 Re: misfire on hwy 
    blueguy
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    Default 01-22-2013
    Quote Originally Posted by Fivefingerdeathpunch View Post
    It can test good and still be a bad unit. Just a FYI.
    Well FYI your face, I've never had a false reading on these cars.

    So: blah.
    Sold WBody's: '03 Blue GTP/'98 Green GTP/'98 Silver GT/'05 GXP
    '99 Chevrolet Silverado Classic Z71 4x4 - K&N Intake/Gibson Exhaust #TRUCKTHINGS
    '12 Buick Regal Turbo - ZZP CAI/20% Tint/HID's
    '89 Ford Mustang LX Notchback - LM7 5.3, 4L80, 9", HX40
    '04 Chevrolet Corvette MRM A4/LS1 - TSP LT's, 3"O/R X, AFE S2 CAI
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  11. #11 Re: misfire on hwy 
    justin10morrissey
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    Default 01-22-2013
    Quote Originally Posted by blueguy91 View Post
    Well FYI your face, I've never had a false reading on these cars.

    So: blah.
    thanks ill run up there and have it tested
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