i have a 02 blazer with a 4.3 it has a miss fire on cyl. 3&4 at idle, as soon as you take off the miss fire goes away. i have replaced the timing chain, wires, plugs.dist. cap and rotor. I was wondering how hard it is to change the injecters on those cylinders. also my distributer is making a noise sometimes but it runs no different with or without the sound,sounds like the shaft is vibrating inside the distributer, can your distributer give you miss fires on just a couple cylinders. any help would be appreciated. Thank You

 

how many miles on it, have you had the codes pulled, checked the fuel pressure?

 

Sometimes I get code 0300-0303-0304 at different times it has 120,000 miles. I put it on a scanner and cyl.3-4 miss fire at idle give it gas and it quits, has a new fuel pressure regulator and intake gasket

 

If you have to replace 2 or more injectors, you really should look at doing the MFI conversion (search "MFI+conversion").

P030x - Random/Multiple Cylinder Misfire Detected will get you the diagnostic chart as well as a link to a thread where the P0300 code is discussed to the n-th degree.