I have a 03 5.3 ext cab LS with 109k. Today i was on the highway had to punch it to get around traffic, at high rpm I lost power and the engine light started flashing. Later that day I was driving and got on it again same thing happened. Then on my way back it just started missing out like crazy on normal throttle and the engine light was flashing again, at stop lights it was trying to die. I went to a parts store and ran codes, got MAS code and random misfiring code. So in the parking lot I replaced the $80 MAS. Didn't change a thing, cleared the codes and let it run till the light came back on. This time it was only random misfiring code.

So I limp it home, then check plugs and wires. They were original and had planned on changing them any way, but now only had plug money. New plugs ddn't help.

Well I continued to investigate, Noticed a noise that kinda sounded like a diesal engine from the center of the engine, used to only make this noise when it was cold. I sprayed carb cleaner all over the intake looking for leaks. No luck there. Please help! Need my truck for work.

PS I recently replaced the door speakers, not a bose system so I don't think ths maters but I don't know. Anyway yesterday got in my truck and the radio wasn't working. Could change stations but no other buttons worked and no volume. Later it was back and working fine, but the clock reset. No probs since.

 

distributer cap? check if it has any cracks.

or instead of spending money on parts you're not sure will fix the problem, get your truck scanned.

 

Sounds like bad gas. I got bad gas and it caused misfire on multiple cylinders.

 

Sorry, 2003 had distributerless ignition. When it first happened it had 87 octane, filled it with 92 then it got worse.

 

And it has 53psi of fuel pressure.

 

did you get the dreaded P0300 multiple random misfire code? it could be several things..........wires, plugs, one of your ignition coils, or the ignition control module............ my guess is it is probably a coil going out

 

Is your miss fire constant? If so pull your vavle covers and i bet you'll find a bent pushrod, broke rocker arm or broken vavle spring.