04 gt misfire odd problem

parthos

Founding Member
Sep 21, 2000
529
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Bismarck ND
I'm working on a friend's '04 gt, manual transmission, BBK headers, off road exhaust, and that's about it for any modifications. It's got a real strange problem. It develops a miss while driving. Not just a small miss, but it damn near sounds like a jake brake. It happens under somewhat of a load, say cruising and going up a hill. Suddenly, like a switch has been turned, it goes into big rig mode and drops all it's cylinders, it runs but runs real rough. Pulling over and shutting the car down, and then restarting the car, the miss is gone. It will stay gone for a few blocks, or few weeks. But it never goes away for good. I've changed the plugs, on it, and pulled the codes, pretty much every cylinder is misfiring, but no lean cylinders. Anyone care to enlighten me as to where to search? The car runs great when this problem is in remission. Like I said, it's like a switch has been turned on and things go all haywire. :shrug:
 
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Sounds electrical. Make sure the battery and alt are in good shape. A voltage regulator could be dropping load and causing it to run like this.

Were there any mods or work done just prior to what you are describing?
 
You need to have data stream recording capability to see what changed as the car gets stupid. You can scroll back a bit and see if voltage droped, cam signal, crank signal, ign timing and fuel trim are doing before and after, the scan gauge will even tell you fuel pressure.


A simple, yet effective cure can sometimes be as simple as a fuel filter.

The computer controls injector ground. Perhaps make sure all connections at the computer are clean and free of green fuzz.