Hello Everyone...
I have a very complex question that I need your help with. I have a random misfire (P0300) on my 2004 Chevy Blazer. the engine light blinks only when I reach the 2100 to 2400 rpms, but it never stays on. I have changed the spark plugs along with the wire and also, replaced the distributor and crank shaft. But, the misfiring still doesn't go away. I really appreciate any help I can get. By the way, the misfiring started after I replaced the wires and spark plugs. Wires are good, they are new and I also inspected every single one of them... by the way, I also replaced the ignition coil just in case...

Thanks in advance!

 

You replaced the coil, but what about the ignition module just beside it?

 

No, I did not. You thiNk that could be the issue? Is there a way to test the module to see if in fact is what it needs to be replaced?

 

Can you actually feel a misfire, or does it seem to run fine? Did you say you replaced the crankshaft position sensor? Did you relearn it?

 

It seems to run fine. And no, I did not relearned the sensor. How did you do that?
sorry for asking such a dummy question...

 

Not a dummy question at all. Unfortunately, the relearn procedure only takes about 5 minutes, but it requires a fairly high-end scanner that's capable of two-way communication. That's not something most home mechanics own. The procedure has been described here a few times, so it's probably faster for you to do a search than for me to type it all out again.

If the sensor is replaced without doing the relearn, it can cause exactly what you're experiencing.

 

I wouldn't rule it out.

When I worked at Advance Auto Parts some 13-14 or so years ago, we had equipment to test ignition modules if they were brought in. Pretty simple little device that used an adapter plug specific to the modules and plugged into the tester computer box, would run the diagnostics and turn on lights as it did the test at various RPM ranges, and let you know if it was functioning properly or not. Not many of us knew how to use it or read the results, but I did lol. Most modules I tested were Ford ones it seemed... Not sure if they still carry that kind of equipment in most of the stores any more, but it's worth asking one.







This is also a good possibility. I only skipped over that since the same issue was there before replacing the sensor according to the description in the original post.

 

This!!

Crankshaft sensors display a waveform (as reluctor passes the magnet) which is specific to the sensor under normal operation conditions. If you have a misfire, engine slows down for a incredibly small period of time because that cylinder does not contribute to the rotation of the engine. Your PCM monitors the crank sensor waveform all the time and compares it to what has been "learned" as a normal waveform. If it deviates, then that deviation is interpreted as a misfire. It uses the cam sensor signal to determine which cylinder the malfunction has occurred on.

If you change or disturb the crank sensor, the "learned" waveform is no longer valid and all bets are off for correct misfire detection. Some people will tell you that their crank sensor didn't need recalibration after messing with it, but that is not what the factory manuals say.

If you do happen to have a real misfire, you will never get the crank sensor learn procedure to complete as they have somehow programmed the PCM to recognize a bad waveform. (So don't take the bearings out revving the engine again and again).

I have two scanners that will do the crank sensor relearn. One is HP Tuners. The other is only $150 or so. Vident I-link 400

 

How did the truck run before any of this was started? We’re there any codes at that time?

which parts were replaced before the misfire code appeared?

Which parts were replaced after the misfire code appeared?

And to confirm, the truck still runs fine but now you have a misfire code? Any other codes?


George

 

George,
Thanks for the comment...
There was a P0016 code to which I had to replace the distributor and the crank sensor. The distributor was very worn out and after that the P0016 code went away. Then I replaced the cables and sparkplugs, and that's when the random misfire began at 2100-2300 rpms.