Ok its a 1998 Blazer LT 4x4 with the W engine code.

It is missing at idle only, and only on cylinder #4. at 1800-1900 rpm the misfire disappears, and the misfire counter on the scan tool for cylinder 4 stops detecting it.

There is actually some type of miss there because I can feel the truck shaking as it misses every compression stroke at idle. When driving the truck has loads of power and toque and no problems except the rough idle. Sometimes the miss can be felt stronger than other times. Sometimes its barely noticeable. It has not been relative to coolant temperature in any way (FWIW).

Now let me say this, Sometimes this code will reset? itself (or possibly disappear). When the miss is barely noticeable the SES light will turn off on some occasions. It normally comes back on when the misfire has become more prodominant and you can feel it big time.

So far I have done some basic troubleshooting. I replaced the plugs as they were the origionals (168k), wires, cap and rotor (NAPA). I used the delco platinum as recommended. This didnt change the misfire at all. I then swapped the spark plug from 2 to 4 with no change, and wire from 2 to 4 with no change.

At this point I checked for intake leaks with carb cleaner, found leaking lower intake gasket. Replaced the gasket, and while I was in there I swapped the #4 injector and poppet assembly onto my metering body from an extra 350 spider I had in the shop. Then when I pressure tested the coolant I found my water pump leaking, replaced it...

Ok so after doing all this the misfire is still there. I did a running compression test on cylinder 2 and 4 and found 145 for 4 and 155 for 2. Both of these readings are plenty high for getting combustion, so I dont believe the small difference should matter too much.

Now im getting kind of frustrated... sorry for the novel...

1 more thing... the genesis evo scan tool reads p0304 only when I scan it in obd-II universal mode, but it reads p0300 only when I read it by entering the vehicle information.

 

Is the #4 spark plug fuel fouled or dry? If it's fuel fouled, the fuel pressure regulator might be leaking and fouling out the plug at idle.

You have eliminated the possibility of a bad fuel injector, poppet valve, spark plug, and plug wire. Next you need to make sure the ECM is commanding the #4 injector to open and close. The fact that it runs good off idle could point to a poor connection at the #4 injector, or at the ECM.

At the injector electrical harness connector: Make sure all of the pink wires at the injector terminals show battery voltage when the ignition is in the RUN position. Then check for a ground pulse on the light blue with black trace wire, (#4 cylinder) while cranking.

 

Try the distributor.

 

can i just stick noid lights in each injector hole and watch for flashing?

 

Yes, all of the pink wires should show battery voltage while cranking. The other 6 wires are supplied with a pulsed ground from the ECM, the noid light should blink on each one while cranking. The light blue with black trace wire, (#4 cylinder) is the one you should pay extra attention to.

 

I thank you for the tips... it appears that my cheapo noid light kid did not include the correct one for my application. I was wide open at work so I didnt find time to check anything yet...

This morning it was running great, after running poor yesterday. I made a couple trips to the store during work and it was doing great each time. (Yes I was at full operating temp.) At some point the CEL turned back off and was running noticably better with a "slight miss" at idle only. So I jump in to drive home from work and it starts up and is missing heavy again. It accellerates well and is getting decent mileage if my fuel computer is working properly (how likely is that). It only misfires at idle and decreases misfires until around 1800 rpm at which time they disappear completely.


How much free play is normal in the distributor rotor? I grabbed the rotor on a vortec 350 core sitting in our shop and it had little, possibly no play. had a very tight feeling to it... when I grab mine it moves a good deal from side to side and up and down. I did glance at the distributor gear while I was doing the intake gaskets but Im kicking myself for not looking closer... I guess ill yank the distributor soon to check the gear.

Does anyone know if these engines with the worn distributor gears show anything funny in the CMP sensor? Id imagine it must if its giving the vcm a bad enough of a signal to cause a misfire... Should be able to lab scope it and watch for irregular waveform...?

 

Shouldn't be any side to side play. You will get slight up and down movement and it will rotate slightly, but no side to side movement is allowed. Dorman makes a complete assembly, with an all aluminum housing and comes with a new cap & rotor. I wouldn't trust anything but an AC Delco cap and rotor on the 4.3L though.

The Camshaft Position Sensor is a Hall effect switch and there is one window in the rotor, (one pulse per distributor shaft revolution). The CMP sensor tells the ECM when the camshaft is at TDC compression for #1 cylinder. So even if the distributor shaft bushings are worn, the cam sensor won't recognize it or be affected by it. CMP data is used in conjuction with the crankshaft position sensor data for determining which cylinder is misfiring. The CMP sensor is adjusted by turning the distributor and the readout is listed under "Camshaft retard" on a capable scan tool. The desired spec is zero degrees, with plus or minus 2 degrees being acceptable.

The crankshaft tone ring has 3 "points" spaced 120 degrees apart from each other. The crankshaft position sensor sends 3 pulses per crankshaft revolution to the ECM. Crank sensor data is used for determining ignition timing and fuel injector timing. The crank sensor is calibrated when performing the "crank sensor relearn procedure", sometimes referred to as CASE relearn. Both the cam and the crank sensors must be adjusted/calibrated correctly for the ECM to make accurate adjustments to ignition timing and for accurate misfire detection.