Misfires and codes P0300 and P0302

[Fast4drss]

My truck is a 4.3L W\5speed, 235K miles. The truck is a Throttle body with central sequential fuel injection. The truck runs fine and normally starts easily. I was at a light and the MIL light came on. Cked the codes and found P0300 engine misfires and P0302 misfires on the #2 cylinder (only). This was the first humid day in a long time. That night I listened to the motor and it sounded like arching was occurring. I changed the plugs and wires (approx ~90K on them).

In a few days the codes came back. Cked and changed the cap and rotor.
Mil light \ codes returned again.

I noticed that the misfires only happen when the truck is at idle. If you normal and spirited driving no misfires occur.

Questions:
1. How does the computer determine a misfire has occurred? Is it a delta in the cam and crank position?
2. The O2 voltages are between .2 and .8V.
3. Could the idle air intake valve stall the motor and make the computer think a misfire has occurred?
4. Bad coil or ignition module?

Any suggestions are welcome to hunt this down.

[thej]

My truck is a 4.3L W\5speed, 235K miles. The truck is a Throttle body with central sequential fuel injection. The truck runs fine and normally starts easily. I was at a light and the MIL light came on. Cked the codes and found P0300 engine misfires and P0302 misfires on the #2 cylinder (only). This was the first humid day in a long time. That night I listened to the motor and it sounded like arching was occurring. I changed the plugs and wires (approx ~90K on them).

In a few days the codes came back. Cked and changed the cap and rotor.
Mil light \ codes returned again.

I noticed that the misfires only happen when the truck is at idle. If you normal and spirited driving no misfires occur.

Questions:
1. How does the computer determine a misfire has occurred? Is it a delta in the cam and crank position?
2. The O2 voltages are between .2 and .8V.
3. Could the idle air intake valve stall the motor and make the computer think a misfire has occurred?
4. Bad coil or ignition module?

Any suggestions are welcome to hunt this down.

hi well i have the same problem to . i have a 96 sub tryed everything . mechechanices tell me that it can be the injectors.to me i am not sure about that. i have the same code come up for about 6 months now i get those coed and p0175 p0172 wich is fule system too rich bnank 1 and 2 . like you i do not know what to do . is there anything else with happening with your turk ? like the way she start or the way she runs while stoped at a traffic like ?hit me back if you get any info .. the guys on here are realy helpfull you just have to give it some time . jay

[Fast4drss]

Jay

My truck had a hard starting problem at first but, that is no longer an issue. I think that was the plugs.

Currently it runs fine but sometimes stumbles at idle with the ac on or off.
Checked the truck with an AutoTap and found the wierd misfire problem.

Otherwise no issue. BTW the SES light went off tonight.

[Cop_Dodger]

You wanted to know how your truck knows you have a misfire so here we go.

Every time your truck fires a cylinder the crank speeds up momentarily. The PCM (computer) looks at the crank sensor and expects to see the cranks speed to increase suddenly. If it doesn’t see the increase in speed, it thinks the cylinder is misfiring. It's can see it really good on an oscilloscope. It's like looking at a heart monitor. You’ll see a strait line then a big spike when a tang on the reluctor wheel passes by the crank sensor. The signal should be fairly consistent. Some engines have a odd notch on the reluctor wheel (the 5.3does). This is to determine when the #1 piston is on top dead center. Now, with the odd notch, the PCM doesn't know if the #1 piston is on compression or exhaust. This is where the cam sensor comes into play. The only thing the cam sensor does is tells the computer when the #1 cylinder is on top dead center on compression stroke. That's all it does. Once the PCM gets it's signal from this sensor the PCM knows which cylinder should be the next to fire. If it doesn't see the speed increase on the crank sensor it assumes that the cylinder that was expected to fire is at fault. That's HOW IT WORKS.

[thej]

Jay

My truck had a hard starting problem at first but, that is no longer an issue. I think that was the plugs.

Currently it runs fine but sometimes stumbles at idle with the ac on or off.
Checked the truck with an AutoTap and found the wierd misfire problem.

Otherwise no issue. BTW the SES light went off tonight.

now the the hard time starting, is that a cold start like first thing in the morning or is it after the truck runs for a while then try to start a few hours later . how does your truck run with new pluges ? what kind of pulges did you use ? i might do the same thing this weekend.

[jackelope]

Both Of You With Misfires Should Find A Shop That Has A Motorvac Fuel Injection Machine And Let Them Run It For A While. At The Dealer I Used To Work At We Fixed Lots Of Misfires By On-car Injector Cleaning Machines.
P0300 Is A Multiple Cylinder Misfire By The Way.

[chevanater]

Try changing your fuel filter. If that doesnt help have a dealer do an injector cleaning like someone said above. It does wonders no little fuel injector cleaner bottle could ever think about doing