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So I’m currently at a loss with my car. I have a 99 Cobra with an 04 Terminator swap done and it’s damn near perfect. The car idles and drives fine without a sign of any issues but when I go to open it up in WOT, the car falls flat on its face at 4000 rpms. It reaches 4000rpms and begins to buck and jerk until I let off the gas. So far I’ve changed the air filter, cleaned the MAF, changed the spark plugs, and changed the fuel filter. Has anyone had any similar problems and if so how did you fix it? Thanks.
 

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What MAF is in it? Is it the stock MAF from an 04 Cobra, your 99 MAF, or something different? I remember back in the late 90s and early 2000s there were foolish recommendations on the web saying to cut the post out of the stock MAF housings to reduce restriction. This completely throws off the calibration. The result on a lot of Cobras was they would die out at 4000. I'm going back a LONG way here but I do remember that being a common problem. Cobras that couldn't get over 4000 would come into the shop where I'd hang out. They all had the post cut and ground out of the stock housing.

Outside of that, I'd need a lot more data on exactly what has been performed and what datalogs look like.
 

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Has it always done this? If not, check out the air pipe from the filter all the way back and make sure there are no obstructions and the filter is clean. If it's not that it's something in the way the air is being metered (MAF) and compensated for (tune).. Of course it's always easy to say.. :)
 

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One thing is for sure your A/F ratio is off. Now start with simple **** like coils, plugs, (again) leaks in vaccums, as loud as these engines are, sometimes hard to hear them, then move onto iac valves, maf, ecu calibration, fuel starvation from a hole in your in tank fuel pump, or bad fuel pump all together. If all that fails, check into ecu. Yes multimeters are required for some of this testing. Good luck.
 

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I've wrung my hands for years wondering if mine had an air leak and frankly unless it's pretty big or you get lucky it's very easy to miss. How is a guy supposed to sleep at night? Over and over again you wake up thinking you're hearing the flutter of a connection as air is sucked past it. Then my life changed. I bought a smoke generator. This takes all the worry, the pain, the second guessing, its like suddenly your world is right once more. You can see so clearly, you can wiggle fittings and see if a connection is marginal, you can even pump in pressure to what your adder is pushing and see if marginal fittings are leaking only under boost. I haven't enjoyed a new tool as much since my Uview Airlift Radiator leak checker and purge tool (when you have cars with multiple radiators these are a must! Heck, my wife is jealous again, she made me promise after the Airlift that there would be no more sexy tools.. And that was after I was caught cheating with my Ozone generator (pssst.. these things really do get the stink of the last owner out of the car).. I feel lightheaded..
 

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So I’m currently at a loss with my car. I have a 99 Cobra with an 04 Terminator swap done and it’s damn near perfect. The car idles and drives fine without a sign of any issues but when I go to open it up in WOT, the car falls flat on its face at 4000 rpms. It reaches 4000rpms and begins to buck and jerk until I let off the gas. So far I’ve changed the air filter, cleaned the MAF, changed the spark plugs, and changed the fuel filter. Has anyone had any similar problems and if so how did you fix it? Thanks.
More than likely your fuel pump or regulator.
 
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