I got a 2001 Chevy blazer 4x4 4 door with 106k miles.

I’m having misfires verified by my scan tool with misfire counter. There is no P0300. But I’m having random misfires only in idle and light throttle inputs, on cylinder 1,2,3, and 4. The misfires are very mild, only 2-6 current counts, and about 40-60 history counts in 3 minutes on cylinder 1 and 3. I almost can’t even feel the misfire, but I know it’s misfiring because I can feel it skipping when I pay attention.

Camshaft retard is reading -4 degrees, and it seems to stay the same when I’m driving.

Here’s the things I have changed on my car over the past 4 months:

- Fuel filter (Wix)
- Lower intake gaskets (Mahle)
- Spider injectors (Acdelco MPFI)
- Spark Plugs and wires (NGK spark plugs and Acdelco wires)
- The whole distributor Assembly. (Acdelco)
- Ignition Coil (Acdelco)

All parts were brand new Acdelco parts except the fuel filter and spark plugs.

The crazy thing is, I’ve had this misfire even before I changed all these things. And I still have the same misfires even after changing all these. I have cleaned the EGR, IAC, MAP, MAF, and the PCV. Compression on all cylinders are also excellent and even.

What is my issue? I’m lost at this point.

 

If it is not setting even a pending misfire code, then you have normal operation. 2-6 current counts and 40-60 history counts are completely normal IMHO. Mine has done this much anyways for 60K miles and doesn't set misfire codes.

The purpose of misfire detection is not to accurately detect every misfire. It is to determine when misfires have become enough of a problem that the catalytic converter will be damaged. It just isn't that accurate. So if yours isn't turning on the light or setting a code, it is fine.

Misfire detection is performed by the PCM comparing the current electrical waveform of the crank sensor to a stored waveform from the last time a crank sensor relearn was performed. It looks for variations in how long it takes for each cylinder to be triggered and if it takes longer than normal it calls it a misfire. It knows which cylinder because of the cam position sensor in the distributor. When counts become excessive (spec unknown), it flags a misfire and turns on the light so the driver is warned that the catalytic converter is in danger.

Note that anything attached to the crankshaft can cause a disturbance in the waveform. Also disturbing or replacing the crank sensor will change the waveform (some more than others). Transmissions and belt driven accessories are sometimes culprits. Since this are low speed random recorded misfires that are not causing a trouble code, if you want to prove things to yourself I would recommend pulling the accessory drive belt off and see if that makes a difference. Sometimes a bad tensioner letting the belt jump around is enough variation in the waveform for a scanner to see a few misfires.

Spec for the cam retard is zero +/- 2, so it should be between -2 and +2. You might get a few less misfire counts if you correct this to zero. Other than that, I wouldn't sweat things so much.