l recently bought a 1999 cobra with 17,000 original miles on it, now. One morning, after a previous night of normal driving, l started it and it sounded like the exhaust had a leak in it. l could hear exhaust gas coming out in rythm with the engine. l checked it out and the bolt-on exhaust had come loose from the manifold on the driver is side. Shortly after, l got trouble codes that the fifth cylinder was misfiring and the exhaust was running lean. The second was completely understandable and the first, l reasoned, might have been a problem with the EGR or something related. So, l got the exhaust all sealed b/w the manifold and pipe, but it feels like the engine is missing a cylinder. l had cleared the DTC before l sealed the exhaust back up and it is not coming on, now. Also, the exhaust sounds pretty disappointing compared to before. Please help. l am worried about my car and unsure what could cause this. l never race it or dog it or push it hard, and the previous owner hardly ever drove it.
Oh, and l checked the plugs on one side of the engine (drivers) because l figured l might as well check them all while l look at the fifth one. Also, l have GMS Coil Packs installed. Could one of them have gone bad?

l kept looking for another explanation after spending all that money on aftermarket coil packs but l replaced them with the stock ones today and that immediately fixed the problem.l am so angry and hope that l can get my money back.l guess next time l should go with Occam is razor and just stop being stubborn, lol

Possible that one of the COPS went bad, but it would not be related to the exhaust issue. O2 might have gone bad due to the lean condition. O2s will throw a DTS eventually. The car should be fine, as long as you corrected the lean condition, You need to join Corral(dot)net. It is a free Mustang forum and the guys there are super helpful and friendly with any problems. For now, replace the plugs.

whenever u have an exhaust leak before the o2 sensors will trigger and check engine light will come on.the o2 sensors regulates the exhaust flow and pressure..try testing fuel injectors by unplugging the # 5 if it same no change then it that injector might not be working, if changes then the coil might be bad.
let me know how it went.

Try switching the cop is with another one and see if the problem changes to that cylinder and if it does then you have a bad cop. From what ive heard is that the stock cops and the coil packs(96-98) are the best ones you can get cause the aftermarket ones tend to fail alot.

Come over to svtperformance.com great site since you have a cobra and there are alot of people that offer tons of help.