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Discussion Starter #1
Hey guys, I put in Mac longtubes in my former GT (now my brothers). Its running fine, even at WOT now(reset the computer and that helped), but past 4500rpm its hesitating and backfiring. My brother broke the #7 sparkplug and just bought a cheapie from Autozone, didn't gap it or anything. Would that cause anything? I double checked the wires and they're in the right spot. I didn't put back that tube that runs from the h-pipe to the smog pump, I just plugged that up, but I don't think that would do anything. Anyone have any ideas? It was running fine before the install.

Aaron
 

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You may want to check and see if any wires are close to the headers and possibly burned on them. The first time I put on a set of headers I melted my passenger side o2 sensor wires on them. You may want to buy the right plug and gap it, just to be sure. Also check all the plugs to see if any of them have hairline cracks from the install. good luck. eric
 

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That line that you capped off, did you make any other changes/modifications to the emissions system? Basically what you're doing by plugging that off is shutting the path for the upstream air that the air pump is creating and therefore you're forcing all that air back into the heads/intake/air pump, you may want to hook that line back up or disconnect your emissions system completely. What a lot of guys do, including myself is purchase the cylinder head downsize threads that insert into the front of the stock cylinder heads and screw those into the back where the crossover pipe goes, thus blocking the path at the heads and then disconnecting the air pump all together. Another thing you may want to do is if you're doing this, you'll want to get an intake spacer, so you can block off the exhaust rising into the EGR plate, otherwise the EGR valve is still going to try and open and you're going to get HOT air running all through your intake manifold, not to mention a clogged EGR valve.. This is the thing about modifying these cars.... especially the off-road h-pipe thing, you have to make changes to multiple things in order to "trick" the car into thinking everything is fine, otherwise you're screwing up all the hard work of the Ford Engineers... :) Good luck.
 

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I had the exact same problem with my brothers mustang. Put longtubes on and started backfiring like a s.o.b. I cleaned out the throttle body and the IAC and it got rid of it.

Trevor
 

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Discussion Starter #6
It turned out to be the spark plugs. Some were dented in after the longtubes were put in. THanks guys for the help.

Aaron
 
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