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5.0L (1979-1995) Mustang Technical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.

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Old 11-16-2014, 08:38 PM   #1
89turbofox
 
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Vehicle: 1989 ford mustang
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Post 1989 mustang cylinder 4 misfire HELP

I have a 1989 mustang gt (mods listed below) with a misfire at dead idle in cylinder 4. My mechanic has went through his checklist to fix this misfire with no luck. Compression test was done and all cylinders at 150. Like i said he has checked mostly everything that causes misfire besides tearing apart motor. he has done everything to swapping the computer, checking plugs and wires, checking roller rockers, to blocking all VACs off and no luck.

he states the last two things he could check are the intake manifold gasket and the head gasket (but he says its not the head gasket because there is absolutely no sign of that)

Intake manifold gasket causing skip in cylinder four with out any external leaking from the gasket? does this sound correct? anybody else had this problem ?


I need to get this misfire out so i can tune my baby. I have already spent 800 dollars looking for this dam misfire problem


Mods:

Motor: 
306 trick flow twisted wedge heads cobra intake 42 lb injectors 90mm mass air intake spacer 70mm throttle body ford racing valve covers, 1.7 roller rockers, pistons, anderson motor sport cam... everything powder coated.

Cooling: 
Griffin radiator with black magic fan

Suspension: 
UPR tubular k member kit with coil overs sub frame connectors upper and lower control arms UPR camber plates 5 lug conversion 410 gears new rack and pinion

Power Adder: 
On3performance turbo kit 70mm with full exhaust 2 chamber flow master and 2.5 flow master tips

Interior: 
All auto meter guages short throw shifter interior is in immaculate condition
Custom 5.0 floor mats

Wheels and Tires:
5 lug conversion cobra wheels nitto tires
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Old 11-17-2014, 07:52 AM   #2
mjr46
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so when the plug wire is pulled for number 4 cyl while running there is no audible change/feel in how car runs?? I assume it's a dead misfire, meaning present all the time at idle?
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Old 11-17-2014, 08:02 AM   #3
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yes. misfire present all the time.
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Old 11-17-2014, 08:15 AM   #4
88 orangepeel notch
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I'm a little confused, your mechanic found cyl 4 has compression, spark and fuel ?

One of those has to be missing.
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Old 11-17-2014, 09:59 AM   #5
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yes. misfire present all the time.
inline spark checker verified spark present? if so, plug checked? if good... injector pulse verified via noid light checked??
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Old 11-17-2014, 11:58 AM   #6
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all cylinders have 150 compression. Cylinder 4 has a misfire. new plugs new wires installed. all vacuum was bypassed problem still persisted, put a chip in the computer and misfire continued. Mechanic says im down to intake manifold gasket or head gasket (but there is no sign of head gasket failure) My mechanic hooked up his computer to the car's and couldn't shut the motor off (which he should be able too), meaning the car is still getting air from somewhere.... hope this helps
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Old 11-17-2014, 12:00 PM   #7
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noid light was used on injectors everything was running properly. Cylinder 4 was half the temperature and very cold compared to all other 7 cylinders due to some heat gun my mechanic uses. any leads??
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Old 11-17-2014, 01:38 PM   #8
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stuck injector or clogged , dripping fuel instead of spraying it.

I wonder are you getting a rich code on that bank?
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Old 11-17-2014, 06:34 PM   #9
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did a leak down test and nothing came back.. mechanic is forced to believe its head gasket because cylinder 4 is a lot colder than the rest and cylinder 3 has less compression. Also spark plug for cylinder 4 is exceptionally cleaner than the rest.. sound legit?

injectors checked out fine he said.
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Old 11-18-2014, 02:03 AM   #10
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I assume there are no bent push rods on that cyl?? and was a leak down test performed on that cyl?? Also, if compression is okay on that cyl,as well as no leakdown, you can often rule out a head gasket issue as well as internal issue, also if push rods have been inspected and valve springs inspected for any breakage and injectors verified all in working order via swapping suspect cyl to another and noid light verified circuit integrity and all secondary ignition components relative to that cyl are good then I'd say you need to verify the tests yourself and or make sure the mech didn't assume one of the tests or skip one........because dead misfires specific to 1 cyl are not hard to find through the above tests........my next question would be to him, how is he verifying that is the problem cyl?? just because heat gun says it's a little cooler?? pulled wire while it's running on #4 and no audible change?? or electronic verification through an ocilliscope by either watching the firing voltage for that cyl or the raster pattern?? Usually, if it's a dead misfire and it's a result of a head gasket, a compression test would pick it up......if you think it's a blown intake gasket on that runner, pull the upper plenuim and shine a light down that runner and see if a piece of exposed/blown gaskets shows around the port
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Old 11-18-2014, 02:03 AM
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