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To check for plugged cats remove the pre o2 sensors use an adapter to fill the hole where the o2 sensor was removed and hook up a pressure gauge, I usually use a old valve stem that I cut/grind down with the core out because I am to cheap to buy an adapter ( obviously the vehicle is cold) start it up and rev it to 2000 rpm you should have less then 1 psi.

Also if you have a scan tool look at the fuel trims if the converter(s) bad the fuel trim should be on the neg side (more exhaust getting left in the cylinders leaves less room for O2 which means less fuel) this may also help you determine which bank is bad if you do have a converter problem.

Another thing I would check is your fuel pressure, as your pump may be getting lazy and only supplying a fraction of the pressure, an indication of this can be seen in fuel trims as well, near 0 or slightly pos at idle then the further you get into the throttle the numbers will continue to get more pos may be somewhere +30% at WOT.

Long term fuel trims is what you want to look at LTFT these are corrected #s
 
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Yeah, that would make life easier for sure. I had the exact same thought when it happened to my Mach 1. I posted up on the Mach 1 forum and a member said he had the same problem and it ended up being an ignition coil pack. I took the chance, and it worked out.

I don't normally subscribe to swapping parts without testing them, so I'm not suggesting you do that either. I'm not entirely sure how you would test the coil packs? Maybe there's a specified resistance range?
I have seen this happen aswell, good at idle and cruising then at higher throttle positions it is to much of a load and the coil craps out, correct me if I am wrong but the 5.8 is coil over plug? for some reason Ford is one of the worst when it comes to detecting misfires. GM almost immediately, Dodge will take a few times, but Ford 3 weeks later then you'll know lol.
 

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I have seen this happen aswell, good at idle and cruising then at higher throttle positions it is to much of a load and the coil craps out, correct me if I am wrong but the 5.8 is coil over plug? for some reason Ford is one of the worst when it comes to detecting misfires. GM almost immediately, Dodge will take a few times, but Ford 3 weeks later then you'll know lol.
Yes, the Ford Mustang engines are coil over plug.
 

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Discussion Starter #25
Without codes, it makes for difficult diagnosis. I'd pull the plugs and inspect them to see if anything looks out of the norm. If you suspect cats, you might have someone rev the engine up to 2-3,000 rpm and see if you notice any change in exhaust volume. The cross-over pipe might compensate.
Cat, thinking I’ll just go w/ a new set of OEM coil packs and might as well change the plugs while I’m at it. What plugs do you recommend? 19k, never tracked/rarely driven, stock pulley, 67mm TB, JLT 123 and tune.
 

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The PCM is pretty good at identifying a bad coil pack, even which cylinder it is in. But boots can tear and sparks can arc and not fire the cylinder. I think I'd put in a set of NGK TR7IX plugs. Look for signs of oil or other contaminants in the spark holes before you pull each plug. Also inspect the boots for rips and the wires for corrosion.
 

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The PCM is pretty good at identifying a bad coil pack, even which cylinder it is in. But boots can tear and sparks can arc and not fire the cylinder. I think I'd put in a set of NGK TR7IX plugs. Look for signs of oil or other contaminants in the spark holes before you pull each plug. Also inspect the boots for rips and the wires for corrosion.
A popular and reliable plug on these cars. Hope the OP finds something obvious.
 
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Discussion Starter #28
Thanks for the suggestions. I swapped out the plugs and coil packs, running normal again. However, no smoking gun...the coil packs & plugs look great. Maybe one of the coils would crap-out under load.
 

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Coils can separate when heated. There's a very good chance that is what it was. Glad it was a fairly inexpensive fix.
 

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Thanks for the suggestions. I swapped out the plugs and coil packs, running normal again. However, no smoking gun...the coil packs & plugs look great. Maybe one of the coils would crap-out under load.
Glad you found the fix! That's always a relief!

When my coil(s) went bad, there was no visual indication. Admittedly, I didn't replace the plugs, just the coils.
 
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