PO300 code without actual misfire

[cameronc]

My 2003 2500hd with 6.0/4l80 is flashing a PO300 code but the truck isn't actually missing. I thought maybe I just couldn't feel the miss so I replaced all the Injectors, plugs, and wires, pcv valve. I checked compression on all cylinders and they are all good. I had it hooked to a tech II scanner and it shows #1 & #6 have a dead miss on live data. They both show equal continuous misses per cycle. The truck rubs and drives great so I just overlook the flashing light but it's killing me. My question is what tells the ecm the misfire. How does it calculate a misfire? I can't figure it out. It has to be a false reading. I've already tried have a crank relearn done also.

[Justin101714]

You tried to have a crank relearn done? Did it actually complete?

[Jsdirt]

It uses cam and crank signals to calculate - it expects the crankshaft to be turning a certain speed after each firing event. It detects a misfire if the crank is turning a bit slower on that firing event than it should be. Little tabs on the crank reluctor tell the sensor exactly which cylinder is which.

 

Like Justin was alluding to, when you have misfires on 1 & 6 without an actual miss, that normally means a crank relearn is needed.

[cameronc]

Well they told me it did but I didn't actually see it myself. I guess I could try it again

[e_schott]

Will agree with others and say that relearn is most likely your problem. If you do not have access to a tech II to use on your own, and want to avoid paying the dealer, try to ask around your town and see if you have any friends, or friends of friends that are technicians. You do not need a tech 2 to do a crank relearn, most of the major brand names will be able to do that function. I have a snap on and an older autel unit that can both do crank relearns

[cameronc]

Well the 3rd crank relearn attempt didn't end in success. We used HP tuners to turn the po300 code off to get rid of the light but Its still in the back of my mind and driving me crazy.