My 2001 C5 has long tube headers, 6-speed, K&N Dual Cone air intake, Corsa exhaust but is otherwise sotck. Engine stumbles and multiple backfires when accelerating WOT at around 4000rpm. Also dumps smoke at that point. Any ideas appreciated.

 

Sounds like she's running rich @ 4,000 RPM under WOT. Is it only 4,000, or from say 3,800 until 4,300 or something like that? If it had stumbled & no smoke, i'd say it's running lean. With the smoke I'd say rich. Secondly, if what you are describing is happening when you lift off of the gas @ 4,000, then I'd say your plug gaps may be too large & may be corrected by going to a smaller gap if they were recently changed. Could also be an ignition or emissions related problem. Is the SES light on? Don't mean to be all over the map on this, but, there's not enough info. about the problem to know for sure. How many miles are on the car?

 

Car has 77K on it, I am the original owner. Tends to stumble in 6th gear when cruising at 80mph and you mash the acelerator as well. Problem is becoming worse. I had MTI dyno tune the car about 8 months ago and she ran great. Do not know if an EGR valve is sticking or something similar.

The 3800 - 4300rpm range is a good approximation on when the problem rears its head. I can keep the pedal to the metal at that rpm, continue to accelerate, and the engine will backfire multiple times with smoke out the tailpipe. At lower rpms and just driving around engine is fine. Occaisionally stumbles of the line but always stumbles at WOT accelerations.