Still trying to figure out this 99 GT

99GTbaby

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Mar 30, 2021
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So I did have an issue with the car stalling out at idle, I now have two brand new o2 sensors plugged in correctly and now the car doesn’t stall anymore, it back fires on deceleration and tonight it started tapping something awful that can only be heard in the exhaust, standing up by the engine it’s silent, no unusual noises, not even a belt squeak, walk back the car and about right at the rear seat level the tapping starts, when I drove it at a lower speed around 10mph it revs up to and sits at around 1.3k when the clutch was pressed in, and when the clutch was fully out and foot off the gas and brake it felt like it was lightly accelerating. I know there’s an exhaust leak, likely multiple exhaust leaks due to poor welds on the exhaust, I can’t imagine that causing all those issues, or can it?
 
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GT4Lyfe

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Feb 21, 2021
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Check fuel pressure, run quality 91-93 octane, next check spark plugs. Make sure all are torqued properly and not too loose or that one hasn't stripped. Exhaust leaks are not good, inconsistent performance.
 
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McFly2

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Nov 21, 2020
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You might have more then 1 issue. I would check your fuel trims: o2 sensor voltages and other live data. OBD scanner think 100 for $35 will give you the data. You can then read and catch up with what the data info means. If you have any air leak up stream from o2 sensors (1); that will send a signal to your PCM to add fuel (lean condition because your o2 sensor seeing to much air in the air/fuel mixture). That also can be caused by leaking exhaust manifold connection at exhaust pipe just before 02 sensor (1). The idle issue: replace IAC. They are known to go intermittent (work sometimes and bad sometimes) that can cause mixed symtoms. They are cheap and easy to replace. A scanner will show you any hidden codes you might have.
A backfire can be caused by a combustion or explosion that occurs when un burnt fuel in the exhaust system is ignited.. If you have lean condition your PCM is going to try to compensate by adding to much fuel (backfires possible). You will also notice the smell of unburnt fuel at idle situations. That is not good for your catalytic converters. Check fuel trims. That will show you how much extra fuel is being added by PCM. A local like Midas should be able to diagnose (scan) and smoke test for vacuum leaks for $89.
 
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99GTbaby

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Mar 30, 2021
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You might have more then 1 issue. I would check your fuel trims: o2 sensor voltages and other live data. OBD scanner think 100 for $35 will give you the data. You can then read and catch up with what the data info means. If you have any air leak up stream from o2 sensors (1); that will send a signal to your PCM to add fuel (lean condition because your o2 sensor seeing to much air in the air/fuel mixture). That also can be caused by leaking exhaust manifold connection at exhaust pipe just before 02 sensor (1). The idle issue: replace IAC. They are known to go intermittent (work sometimes and bad sometimes) that can cause mixed symtoms. They are cheap and easy to replace. A scanner will show you any hidden codes you might have.
A backfire can be caused by a combustion or explosion that occurs when un burnt fuel in the exhaust system is ignited.. If you have lean condition your PCM is going to try to compensate by adding to much fuel (backfires possible). You will also notice the smell of unburnt fuel at idle situations. That is not good for your catalytic converters. Check fuel trims. That will show you how much extra fuel is being added by PCM. A local like Midas should be able to diagnose (scan) and smoke test for vacuum leaks for $89.
I have an OBD II scanner and have plugged it in numerous time and the only codes that ever popped up were for the O2 sensors that have been replaced.
 

McFly2

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the o2 voltage (sensor 1) should fluctuate between 0.1 and 0.9 volts on your live data scanner. Any voltages out of normal
range (0.1 - 0.9 volts) sends a signal to the PCM that fuel trim at 02 sensor is lean or rich. The PCM will react to control fuel trims (add fuel or subtract fuel input to engine). This change in fuel being added or taken away is called Fuel Trim. Fuel trims should be plus or minus 5% - ideal fuel trims at idle and 2500rpm. There are exceptions for the idle fuel trim (plus or minus 20% -25% at idle) on 2001 mustang Bullitt because of the modified intake. Their are other sensors the PCM reads to maintain 14.7:1 fuel / air ratio.

Read more at: https://www.obd-codes.com/faq/fuel-trims.php
Copyright OBD-Codes.com
 
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Nightfire

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Dec 1, 2002
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As you previously mentioned, you most likely have a :poo:ty exhaust job. Perhaps a broken baffle in the muffler.
 
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99GTbaby

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Mar 30, 2021
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Update: still running rough but not as rough, tightened down the battery terminals and it actually did something and it started running ever so slightly better, the tapping isn’t constant any more when standing behind the car. We’ve discovered that there is a coil pack retainer bolt that is broken and needs replaced and I’m wondering if that’s the cause of the misfire I’ve got now, the coil pack isn’t being held down solid.
 

McFly2

Member
Nov 21, 2020
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Cincinnati
Are your fuel trims the same for bank one and two? That will give you a direct clue to where your problem is (passenger or drivers side of the engine).