Chevy 350 5.7 ltr #8 cylinder misfire

Possible fuel injector.

how do you know that number 8 is misfire. By a code P0308 or else.

did you try a different spark plug wire on cylinder 8? if cylinder 8 has good compression i would guess either plug, wire, or injector. another possibility could be no injector pulse on the injector plug in.

The P0141 means O2 Sensor Heater Circuit (Bank 1 Sensor 2)
The P0121 means Throttle/Pedal Position Sensor/Switch A Circuit Range/Performance Problem
The P0200 means Injector Circuit
The P0308 means Cylinder 8 Misfire Detected

Hope them can help you! Good luck!
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car key chip replacement

P0308 is cylinder misfire detected, cylinder #8. Causes can be: bad spark plug, bad coil pack, faulty fuel injector, poor compression, faulty computer..I generaly recommend changing spark plugs first. Then test the coil pack, after that I would look at the injector, I have never seen a bad computer throw this code.

Code P0308 - Cylinder 8 Misfire Detected

Possible Causes:
- Improper spark plug at cylinder #8
- Insufficient compression at cylinder #8
- Incorrect fuel pressure
- The injector circuit is open or shorted at cylinder #8
- Fuel injector #8 may be faulty

Possible Solutions:
- Perform Injector Service
- Replaced Spark Plug #8
- Replaced Injector #8
- Replaced Ignition Coil #8

misfires can be caused by faulty ignition components, but they can also stem from other components. Such as faulty injectors, or improperly timed or unseated valves. Sounds like you have touched on all of your ignition suspects.
Are there any codes present other than P0304, P0305 and P0306? If so they should help guide you in the right direction. If not, You might want to make sure your valves are seating and if so, go ahead and swap an injector from a misfiring hole with a good one from a hole with no misfire. (like switch #2 and #4 injectors for example) if the misfire moves to cyl 2, you know the injector is at fault.

Fuel injector cleaner commonly has a low success rate, and will not help if it's more than one cylinder that is missfiring. The fact that you have a "rattle" noise, leads me to beleive the engine is not receiving fuel on one or more cylidners. If it is all the cylinders, than the fuel pump may be the culprit, or the fuel pump circuit. But if only one cylidner is missfiring than it is a possibly an injector. A code P0300 refers to all cylinder missfires, and any code between P0301-P0308 distinugiushes a particular cylinder.

Misfiring
1- a general electrical tune-up (spark plugs, wire set, cap & rotor) is a basic start to cure this issue. Wiggle the dist shaft while you are in there, if you have one.
2- Then a good carbon cleaning / throttle-body cleaning.
3- Vacuum leaks at any of the hoses that connect to the Intake manifold; especially the MAP sensor connection.
4- If that does not help then the O2 sensor could be bad or drifting.
5- Funky coil/coil-packs
6- Theoretically a microsecond glitch in either the cam or crank shaft sensors would cause this, but I am not aware of this as a typical cause.
7- Lastly a funky PCM but again it is not common for this to cause miss-firing.

Note for the V8 then also:
a) bad erg valve (this dilutes the combustion gases and hence raises the combustion temp
b) a leaky plenum/Intake manifold gasket. Leaking Intake gasket symptoms

Also can check the TSB 18-48-98 Ignition System Cross Fire/Secondary Ignition Wire Induction (Complete Version )

Hope help with this (remember rated this). Good luck and have a nice day.

It's detected a Misfire in Cylinder # 8
Any of the following could cause a misfire:
faulty spark plug or wire
faulty coil
faulty fuel injector
poor compression
defective computer

These have a real problem with the distributor caps, i would try that