HELLO I HAVE A MISS IN MY 3.8. IT FINALLY SEEMS TO GO AWAY AT HIGH RPM'S. I HAVE TUNED IT UP WITH WIRES, NEW FUEL FILTER, EGR WAS CHANGED 45,000 MILES AGO. REPLACED SOME ROTTEN VACUUM LINES. RAN GREAT UNTIL I HAD A TRANNY MOUNT AND BALL JOINT CHANGED. IT DEVELOPED A EXHAUST LEAK AT THE DOGNUT IN THE CROSSOVER PIPE, AND THE MISS. I BROUGHT IT BACK AND THEY SAID IT WAS NOT SOMETHING THEY DID. THEY DID SCAN FOR CODES BUT COULD NOT GET ANY CURRENT, THEREFORE NO CODES. IT HAD A HISTORY OF BURNING IGNITION SWITCHES SO I REPLACED IT. HAVE NOT CHECKED PORT FOR POWER YET. I THINK MY WIFE'S PHONE CHARGER WAS CAUSING THE IGNITION SWITCH FAILURE.I TOOK IT AWAY 5 MONTHS AGO AND HAVE NOT HAD THE PROBLEM. DID NOT KNOW IF THIS WOULD DROP CURRENT FROM THE COMPUTER PORT. IT DID SOME STRANGE THINGS WHEN IT DID GO BAD. NO POWER WINDOWS,AC BLOCK COMPLETLEY DEAD,TRANNY WOULD NOT LOCK INTO LOW OR O.D. WIPERS WOULD NOT TURN ON AT TIMES. ANYWAY, WHEN IT FIRST STARTS UP THE FIRST FEW SECONDS OF RUNNING IT SEEMS OK AND THEN IT GETS VERY ROUGH.ANY HELP WOULD BE GREATLY APPRECIATED!!! THANK YOU

 

The power to the DLC issue sounds like a fuse or wiring problem due to the overload at the cig lighter.
It sounds like you may have some other vacuum lines that are suspect or even still have an EGR issue. The exhaust leak can cause issues with the valves that will have these same symptoms.
What is the maintenance history on this vehicle?
Hope this is helpful to ya,
Billy

 

OK, first thing, shut off the caps. No need to yell.
The first step will be to determine which cylinder is causing this. It appears you don't have OBDII or it would have set a misfire code. Once you know which cylinder, thoroughly check the ignition for that cylinder. Once you know ignition is NOT the problem, have you fuel injecters professionally cleaned. There is a TSB about dirty fuel injecters causing that cylinder to go into "lean misfire" on that and othe GM engines.

 

Hello Desi 501 and Billys68ss. Thank you both for your help. Icould not find a vacuum leak, so I pulled all the plugs. The 1&4 were fouled so I replaced that coil and a set of plugs and it runs great. Thanks again for your time and advice and have a great year!!!! Robert

 

Glad to hear you got it. That was some good diagnostic work on your part.