90 28,000 miles backfires on hard accell

Got a 90 LeSabre down from the city. Wasnt driven most of its life. Has a very bad backfire and stumble on hard accell. If driven normally it runs like a new car. At take off from stop or when passing at highways speeds it will back fire through the air filter. Car has had new plugs, wires, fuel filter,cleaned MAF, seafoam 4 cans,catalytic converter and professional Motor Vac twice. We have repaired 3 vacuum leaks and unhooked the EGR valve. The car has no codes and with scanner on car every thing show normal. I cant believe it would be a timing chaim with this low of miles. Car just make a trip 1000 miles from florida because they wanted to sell him a 2200 dollar trans to fix the problem? Car had ecm changed a month ago for a iac problem but problem was there before ecm was changed. Car will run smooth untill you stand on it and will run 90 mph. Fuel pressure is fine. I would think a blocked screen in tank would keep it from running 90 mph.

i would take a look at your spark. make sure all cylinders are firing properly at all times, i seems like a cylinder is malfunctioning. have your coil packs tested etc. i am sure someone else can elaborate from this point.

It should be scanned to look for fuel trims out of whack, confirm proper o2 operation, check fuel pressure under load, do this first. remember a improperly installed wire or a cracked plug can do just this.

Believe it or not fuel trim looks good. Just got back from driving it with fuel gauge hooked up and it runs around 35 till you accell then it goes to 45 and stays steady. Wires are routed good and it is not just one cylinder it is leaning out on all. Just took of ground at bracket, cleaned connection and added star washer. It acts like bad MAF. Disconnected the ect to see if it would flood it out and it flooded it so bad it would not even start. It has the availability to get fuel but it just is not getting it.

Let you all know that a stock replacement coil pack fixed this problem and I dont understand it but it is fixed.

Let you all know that a stock replacement coil pack fixed this problem and I dont understand it but it is fixed.

Because the coil packs on the original 3800's overheat. Heavy acceleration will cause this especially in older moldels. If anybody else needs help with this, i hear that you can swap the coil packs with the coil packs off of a series 2.