ding ding, anyone check the wires/plugs? If wires havent been done I would put money on that being a culprit.

 

i would check the fuel pressure first, other then that, mine did the same thing when the 02s were bad.

 

I doubt it's a broken valve spring. Wouldn't just act up only under load.

 

Thanks guys for all the input this is greatly appreciated. Plugs and wires were done around 40K miles ago and I will be checking them this weekend. The wires all still look great and they are NGK TR55's. I will post up what I find out about the fuel pressure and the regulator. Any insight on why to switch for a different tune. I have had the wait4me for almost 5 years now with no issues at all.

 

Wires can look fine. Checking the resistance is the only way to see if they are good or not.

 

All of those things may cause missing and stumbling but if it really is backfiring that has to be valvetrain issue I'd think....