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Discussion Starter · #1 ·
06 Grand Prix, base model SFI 3800, begin intermittent misfire/rough idle. First malady, was no start a couple days after a 80-95 mph 5 hr road trip. (I was pushing it pretty hard the whole trip, has over 200,000 on this motor) and then a 60 mile round-trip. It sat for a few hrs after the short drive but would not start. (Crank -no start) Did nothing, and it started the next morning just fine. Stop to fuel, during the return trip home, (not pushing it as hard) and same issue. Waited about an hour, and it started, and I drove all the way home. Did not manifest over the several days, but now as of a couple days ago, there is an intermittent miss, and rough idle. Today, at a stoplight, it was manifesting the problem, and then quit.
After the trip I ran engine scan which sited a P0305. Live scan revealed in fact cylinder five was missing periodically. Check for coil, spark was good checked the injector, working fine, pulled the plug, which was wet. I cleared the code, checked all the ground connections, and it continue driving the car. The latest scans show no code, but the car experiences the intermittent miss, and rough idle. Today was very odd, as at a stoplight after exiting the freeway, The issue was manifesting, but then quit. In the car idle fine, accelerated fine, all the way back to the house. O2 sensors are reading fine, and I suspect it may be an exhaust manifold issue. Looking for some feedback, as the issue seems similar to my past experience that revealed the rear header was cracked. Does this sound familiar to anyone?
 

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Discussion Starter · #2 ·
This is an update on previous notes.

under hard acceleration going on the freeway the car begin to manifest the same problems, and the P305 code returned. Pull the car into the shop, pulled all the plugs, to see if number five was wet again, which it wasn’t. Plugs all look good, purchased a coil, with the intent of replacing it, but decided to look at the plugs first. Put everything back together, and I forgot to mention earlier but the engine will only rev two about 3500 RPM and no further. cleared all the codes, it runs fine, with an occasional misfire. The limit to the throttle may be related to crankshaft sensor. So looking for some feedback on that.
 
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