Mabye timing issue

[mrnickleye]

Originally Posted by f00g00

Since its happening at 4000 rpm and up it could have something to do with the ACIS system. The little flapper valve on the intake manifold that opens when you hit that rpm. You could take the vacuum line off of it so it doesn't work and hit the rpm's again and see if you still get the backfires. Also you can see if its even working by watching it as you run the throttle up manually. Ive never heard of it causing a backfire problem but its worth a look.
Check the dashpot system too by blocking off the little vacuum line, revving up past 2500 rpm and letting go of the throttle and see where the dashpot stops the rpm. It should be around 2000 rpm. It should also if working properly
show a slight hesitation for the throttle as it comes down. You can check that with the engine off. If your shifting quickly enough though the dashpot system wouldn't even come into play at high rpms.

Neither of these components is causing the backfire problem.

And....if the timing is set properly, it will not cause the backfire either, as it is completely controlled by the computer.