hey hows it going i have a 2004 chevy silverado and my check engine light came on and the code is a PO300 Random multi cylinder misfire so i change the coil packs with MSD 82868 coils for the LQ truck series i also have accell plugs and MSD 8.5 mm plug wires bully dogg programmer k&N cold air intake jet mass air flow sensor 160 degree thermostat electric fans summit racing shorty headers and my truck is still running like **** after changing the coils plug and wires i de programmed the truck to see if that was causing this and still doing the same thing a friend was saying i need to have the computer or ecu re flashed to fix the problem any suggestions?

 

Check your data coming from the oxygen sensors.

 

Check to make sure all your plugs are in good shape and gapped correctly. Could be a vacuum leak, maybe. Or possibly a bad cam position sensor.

Misfires are one of those issues where there's a host of possible causes. It could be no spark, spark at the wrong time, not enough fuel to ignite or not enough oxygen; all of those require a couple different checks.

Make sure there aren't any sucking sounds coming from around your manifold to eliminate any bad vacuum leaks. Have you changed your fuel filter ever? If you aren't getting enough fuel pressure, it can cause a misfire. You could also run some fuel injector cleaner through to degunk your injectors if one or a couple is blocked.

I wouldn't suggest flashing if you haven't been doing anything with the computer before. They typically don't go bad without completly breaking to begin with.

Outside of that, you might be looking at a sensor fault. Maybe O2 sensors, but the computer should have told you if that was giving bad data. Some of the intake sensors (MAF, IAC, TPS, MAP, etc) could be bad. Unless you had diagnostic software and knew what sort of signal they should read, there's not a whole lot you could do on your own.

 

ok well thanks for the imput by so far I've change cam positions sensor and crank potion sensor and all new coils plugs and wires that's also what I was think injectors or possibly fuel filter I though that I had a inline filter. But hey thank you very mch any other thoughts please let me know thank you

 

Be sure to let us know what fixes your problem, that way we can share that information with other users.

 

Will do the next thing I did was order a new computer my mechanic buddy said that on the computer side it's not firing 2 part time and not firing 7-8 at all so misfire on three cylinders hopefully the computer will work he said I could also have it reflashed so ill will stay I touch with everyone with all possible updates thanks again for your guys help

 

Can you see what cylinders are misfiring? If all on same bank first o2 sensor for that bank could be bad. Would miss when it goes into closed loop (warmed up). I had one today misfiring on 1,3,5,7 code p0171 system lean bank 1. No o2 sensor codes bad bank1 sensor 1 02. Was fixed at 30mv on scan tool.

 

I would be very skeptical with replacing the computer. What testing was done to prove the computer is bad?

Do you have any kind of custom tune or wiring harness made for it? If you put shorty headers on it, do the new shorties have 02 bungs welded in the collectors or the runners? Your engine will run ****ty if the sensors are in one of the runners. You gotta have them 02 sensors in the collectors, or in the Y-Pipe right after the flange at the very least. Honestly, this is what I did for the headers on my Firebird when they got installed. Hole saw and my buddy's welder.



I'm still kinda leaning towards it being an 02 sensor issue, but you're gonna have to either share some datalogs or have someone go over the computer data with a scan tool to know what's going on. Trowing parts at it most likely isn't gonna fix your issue ...

 

yeah i know what you mean bro i have a bully dog program and i also have shorty headers on it but the o2 sensor is back by the h pipe and so far the only thing ive done were common issue for miss fire, coil packs plugs wires and crank and cam shaft sensors ive had all the test done to it my mechanic he did compression check i have almost perfect in each cylinder but i will double check with what your saying he check it will his tools at his shop in my town really good mechanic said that the computer isnt sending something right he check to see if i had power to it he grab my header on the number 8 cylinder and never got hot 2,7,8 are miss firing

 

Okay as long as he already knows what component of ignition, compression, and fuel isnt being given to the cylinder, sounds like he's on the right track. Because that's all it takes, ignition, compression and fuel. You need all three of those things to make each cylinder go boom.

So as long as he's done testing to know which one it isn't getting on those cylinders for the missfire, then let him do his thing. The computer should control your coils and fuel injectors. If the coil isn't getting a signal at the coil connector, check to see if the PCM is producing a signal right at the computer's connector to see if it's a wiring issue. If you have signal at the computer and not at the coil, then it's wiring. If you don't get signal out of the computer, then the computer's bad -etc, etc, etc. Same goes for fuel. If your noid light doesn't light at the fuel injector connector, check it at the computer. If you don't get a signal from the computer, replace the computer.

I'm just concerned that he is just throwing a computer at it ... just make sure he is checking things out to make sure it isn't individual components or wiring. Wiring can cause some pretty funky stuff sometimes.