Had my '96 Blazer hooked up and found out that I was getting a random misfire in number 4. Do I need to replace all of the injectors or just number 4? Also what should I expect to pay to have them replaced for me?

The P0304 code means that the cylinder 4 is misfiring or is randomly misfiring. Start by checking for intake leaks an if no leaks are found the next step is to replace the spark plugs on cylinder 4. If the problem persist more tests needs to be done to diagnose problem, see "Possible Causes"

- Faulty spark plug 4
- Clogged or faulty fuel injector 4
- Faulty ignition coil 4
- Fuel injector 4 harness is open or shorted
- Fuel injector 4 circuit poor electrical connection
- Ignition coil 4 harness is open or shorted
- Ignition coil 4 circuit poor electrical connection
- Insufficient cylinder 4 compression
- Incorrect fuel pressure
- Intake air leak
most likely change your spark plugs these should be done every 60,000 km

have you cleared the codes between fault reads
clean the maf first .....replace the maf if faulty
coil packs ?
injectors

An intake vacuum leak can cause this as well as other issues, read about it below.
Check Engine Light On, P0300 Random Misfire Code A random misfire means your engine is misfiring, but that the problem is not isolated to one or two cylinders. It is jumping around in a random way from one cylinder to another. A random misfire code usually means the air/fuel mixture is running lean. But the cause might be anything from a hard-to-find vacuum leak to dirty fuel injectors, low fuel pressure, a weak ignition coil, bad plug wires, or compression problems. Even a dirty MAF sensor can cause a lean code and/or misfire to occur. The engine may be stalling because it isn't getting enough throttle opening. The cause is often a problem in the idle air control system. The first thing to check is the intake vacuum with a vacuum gauge. On most vehicles a normal reading is 17 to 21 inches Hg. If the needle is lower, is jumping up and down or steadily dropping, you have a vacuum problem. Look for possible vacuum leaks by checking vacuum hose connections, the throttle body and manifold, and PVC valve and plumbing. An EGR valve that is leaking can also act like a vacuum leak and cause a random misfire. The next thing you should check is the fuel pressure with a gauge. If it is not within specifications (refer to a service manual for specifics because fuel pressure is critical for proper engine performance), the problem may be a weak fuel pump, low voltage to the pump (check the relay and wiring), or obstructions in the fuel line (like a plugged filter). A bad fuel pressure relay can also leak pressure and prevent an otherwise good fuel pump from delivering full pressure to the injectors. Dirty injectors can also restrict fuel delivery and cause a lean fuel condition. Many regular grades of gasoline do not contain adequate levels of detergent to keep the injectors clean. Frequent short trip driving accelerates the buildup of injector deposits. Cleaning the injectors with a good quality fuel tank additive (or having them professionally cleaned) can solve this problem. Look at short term fuel trim (STFT) and long term fuel trim (LTFT) with your AutoTap Express DIY. If the numbers are high, it tells you the engine is running lean

usually this code is followed by another number 1-4 or 1-6 or 1-8 depending on how many cylinders your vehicle has ,, what the code you got is a basic P0300 Random/Multiple Cylinder Misfire Detected A code P0300 may mean that one or more of the following has happened:

• Faulty spark plugs or wires
• Faulty coil (pack)
• Faulty oxygen sensor(s)
• Faulty fuel injector(s)
• Burned exhaust valve
• Faulty catalytic converter(s)
• Stuck/blocked EGR valve / passages
• Faulty camshaft position sensor
• Defective computer
• I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.
  Random misfires that jump around from one cylinder to another (read: P030x codes) also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck ope
  

Hello your po135 is o2 sensor bank 1 your po141 is O2 sensor bank 1 sensor 2 and your pO300 is random multiple cylinder misfire these problems are your oxygen sensors and engine misfire pO300 is you engine missing o2 sensors are switches in the exhaust system the engine misfires are the engines cylinders missing.

I would check the injector. Pull the plug and check for corrosion. Use on ohm meter to check the injector. The resistance should be 11 to 15 ohms. Check it against another injector. they should be close. You can also check the injector plug. best to use is a noid light, but you can use the meter. set it to dc volts to check. the numbers should jump everytime it fires. since you got a 300 and 301 misfire it tells me that you have an intermitant misfire in cylinder 1. The other sensors are still good don't replace them. The coil is still good, because when it starts to go, it will have misfires on all cylinders or will not work at all. The crank sensor is the one that detects the misfire, again don't replace it because it's not bad, it's just doing it's job. If the injector and harness check out good then I would start check the cylinder for compression and mechanical problems

If you already replaced the wires, I would suggest replacing the plugs, distributor cap and rotor. They are also very important parts of the spark. Fuel injector problems can also cause misfires.

The 98 chevy blazer has a one spider injector that runs to six poppit valves where the fuel is injected into the cylinders.It is possible that the poppit valve may become plugged. Before looking into this more I would verify that the spark plug or wire is not the cause of your cylinder #5 misfire.

these were bad for the spider injectors to fail. they are hard to diagnose with out the scan tool. the poppets in the end of the injectors get clogged and will need to be replaced. pull the plug and see if its oil fouled. they also have a problem with the #1 cyl getting out of round and oil fouling the plug. compare it to another plug in the engine. if its oil fouled the engine is no good.

my 96 blazer would have random missfires.... sometimes more than one cylinder at a time... there are a few things to try.... 96 vortecs still used distributor caps... but they had such a high intensity spark that they were well noted for frying distributor caps and ignition coils... i would start there... also a few times i had to replace the crankshaft timing sensor.... it could be something as small as a spark plug wire... i would start with the ignition before spending money to check the fuel system