PO300 MISFIRE on my 2000 chevy with 5.3
 

I'm having the same problem with my 2000 Chevy with 5.3. I've changed fuel pump,map,idle,idle control valve,plugs,wires,Knox,and all 4 o2 sensors.You can hardly see the misfire at an idle.But when you hit 2000 rpms its all over the place.And check engine light start to flash. I've checked the injectors,swap out coils and still the same.Oh yeah I even had A muffle shop open up my cats to see if clogged.Ready to run it off a hill. Their is a lot of post on this po300. But no one ever tells their out come if it was ever fixed. PLEASE HELP. Any one.:mad: Also had the computer flashed as well. Time for smog and dame check light is on. Some one out has the answer. Please tell us. PS truck has 159000 miles on it.Replaced transmission and rear end.I,ve got air aid cold air and throttle body.Hyper Tec Programer.Cat back system on it

Since you seem to have replaced all the usual suspected components,I would say a head gasket problem between cylinders.

I say this because it happens at 2000 RPM and above,plus I suspect the truck has/is being driven rather hard due to programmer,throttle body and rear end replacement and trans replacement with only 159K miles.

Try a compression test and /or leak down test....if 2 adjacent cylinders read low you will know that's it.

Could also be as simple as a bad battery.

Good luck :)

How did you check the injectors? Very common. Crank sensor is also common but crank variance must be set with a scan tool after replacement. What brand plugs and wires did you use? What brand o2 sensors?

Thanks for the response. AC Delco everything.. O2s are bosch. I bought a Nome light kit. Checked them my self. Then I had A local shop check them. They said I might have A stuck exhaust valve. Compression test was 180lbs for all except cylinder 7.Its was 190lbs. I have over 1200 misfires on #7 next highest is #4 with 500 misfires. Can the EGR valve have anything to do with this?

What is a nome light? Do you mean noid light? If so a noid light only tests the signal to the injector not the injector itself. In other words it proves the wires are good to the injectors, the power supply to the injectors is ok and that the computer is telling the injector to fire. You can measure the resistance of the fuel injectors, compare # 7 to a few others. Then actually remove the fuel rail and check for trash inside the rail/ injectors. It is common for #7 and #8 to get trash in them as the are at the end of the rail. 10 psi difference would not cause your issue. A valve problem on this engine would be very very uncommon. EGR would not cause a single on double cylinder misfire. I just re read your post you may want to try taking the hypertech off to see if it is contributing to your problem.

i have seen cracked intakes cause this code too

OK I uninstalled the Hyper tech and bought A new battery.Replaced the wires and plugs again. When changing my plugs I noticed that a bolt missing from exhaust. Looks like it broke off. And its right under #7. Going to put it in shop and get it fixed. Thanks for help guys.

Got it fixed. Knox censers were bad. And changed intake gasket. Also swopped the #7 and 4 injectors with 1 and 3. Both 7&4 on scanner were missing really bad. Went for test ride and Check light went out by its self. Checked it with scanner and no codes.

Glad to hear you got it fixed. I had an 01 5.3 I was fighting a misfire. Finally found it was an intake gasket after it got bad enough that it pulled oil up through the pcv system and hydro locked and broke #5&6 rods.

hydrolocked 5.3 liter
 

intake was slightly leaking, run fine , didnt know it was leaking . couldnt tell by the way it was running. what happen was the hood seal leaked rain water right on #6 injector seeped into the intake gasket,i found straight water in the cyclinder #6 free up by cranking motor over with spark plugs out. then it cleaned out . sat outside it rained , i pulled it in with a tractor pulled intake found # 6 had rain water on top of the intake valve again !
intake gasket leaked rain water in on the valve.
gaskets done deal .
i pulled the head but probley didnt have to.