This is driving me nuts! I''m getting code 304 at 2500 rpm inn a 97 s-10 4.3 vortec. I have sent this car to a shop who say they cannot recreate the misfire in the shop. I've replaced the plugs, wires, dist ,fuel spyder, maf sensor and at this point i'm stumped. l should add the motor has about 210000 miles on it. Has anyone had this issue? Also done extensive vacum checks.

 

Perform the fuel pressure test. It's in the sticky topic.A random misfire is also a symptom of a dying fuel pump.
If the spider is missing the fuel for injection You'll have a random misfire. Also, bear in mind, that 2500rpms on "Park" are not equal to the 2500rpms while Your ride is rolling (on "D"). The difference is within the engine load. The higher the load is, the bigger the fuel consumption is. That explains the problem with re-creating the misfire in a shop, if the transmission is on "park",

 

Mike thanks for the reply. Didn't mention, but also replaced fuel pump. Haven't done the pressure test, but will look at that. Again thanks for the reply. will comment on what I find as soon as I can. Out for the moment { Knee surgery}

 

Have a speedy recovery - best of luck .....

 

Is this a repeatable code? Is SES light flashing?

Is this a felt misfire? If not, do a functional test on the misfire detection. Display misfire counters on a scanner and a pull plug wire on #4 (PO304) and then do same again on another cylinder that is not setting a code. Each case counter should increase rapidly on just that cylinder. If it doesn't, then misfire detection is malfunctioning and cannot be trusted.

FYI, excessive bearing clearance can cause false misfire codes. I have experienced this first hand. How's the oil pressure when engine is fully hot off the highway? Any lifters ticking?

 

To amplify Les’ point, a detected misfire is a detected momentary change In crankshaft velocity near the point where that cylinder should be firing. It’s not an actual electrical measurement.

George