Hello, I am new to this vehicle and probably got ripped off.
Have stumbling backfiring fall flat problems when I accellerate hard.
Goes away when I let off.
Any ideas where to start ?

Thanks

What year is this? Miles?

When was the last tune up?

oh ya

04 with 265000 kms
just did plugs and wires.

Did you do plugs and wires in response to this issue or did the problem start after plugs and wires or . . . ?

Did it do this before you paid for it?

Is the Check Engine light on?

Is the air filter fairly clean?

Is it supercharged?

I'd check fuel pressure. If fuel pressure isn't good, change the fuel pump and the fuel filter.

If fuel pressure is good, change the fuel filter and try again. If the problem is still there, plug into the PCM and check TPS movement.
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Check to see if the supercharger belt is still attached.

it did do it before i changed the plugs.
i did not notice it on the test drive or the next couple times that i drove it.
it is getting worse and worse.
it makes the engine light flash during the condition, then it stays on for p0300 and p0303.
the air filter is clean and it is sipercharged with the belt still there.
i will check fuel pressure next
i noticed that the long term fuel trim sits around -17% as well.
next time i drive it,i will plug in and watch tps.

thanks