2000 GMC Jimmy 4dr
sunday moring i drove home from work and didnt have a problem, then sunday night i went to start my jimmy to go to work and it would NOT start, cranked no problem but just wouldnt fire, sounded like it wanted to but wouldnt. I disconected the battery for a bit and the hooked it back up and it fired up but was running rough. It threw codes P0300 AND P0303, Random miss fire and Cylinder 3 misfire.

Ive changed cap and rotor, wires and plugs, checked compression, checked the fuel pressure, changed fuel filter, but still misfires on #3.

Could my timing be off? but if it was woulndt it affect more than just one cylinder?
Bad Camshaft position sensor?
Bad ignition coil? but again wouldnt more than one cylinder be affected?

Any idea what it could be, and why it would only fully start up only after reconecting the battery?.... smileys amuse me...

 

Well, starting after battery disconnect could be because of the clearing of KAM. KAM is a volatile Random access memory (volatile because it is cleared with power loss; IE: batt. disconnect). KAM is where adaptive strategies are stored (program that compensates faulty sensor inputs, allowing vehicle to run (not well though)). If KAM is cleared, the VCM starts with a fresh slate, compensating where it can, up to a predetermined point.

for P0300

First off, what do all the cylinders have in common? (because random miss= most to all cyl. are missing.
1. Coil-check resistance on both the prim. and sec. winding (compare to manuf. spec)
2. spider-check with a power balance test
3. Fuel pressure-check with fuel press. tester
4. Rotor-thorough visual inspect. melted, corroded, burnt.
5. Cap-visual inspect. look for carbon tracking, melted electrodes, etc.
6. And last,last, last, last!!!! I emphasize this because all over this forum people are told to "replace the cap, rotter, plugs and wires," all the time, and then find that it wasn't that. It is easier to visually inspect the cap, rotter, and plugs, and ohm out the wires then it is to pull them out, run to the store, fork out $130, drive back and install them. You don't waste money and time blindly throwing parts at you rig.


P0303

this is going to be an issue on the secondary side, plug, wire, cap or rotor, but more then likely on the plug and wire(not to say that the rotter or cap couldn't have an issue, but it would bother another cylinder) I would ohm out the wire and inspect the plug.

wires-usual spec of resistance on a wire is about less than 10,000 ohms per foot.
Plug- check gap, color, electrode, high side and low side. all edges should be sharp, not rounded off. look up a spark plug condition chart online for pics. Also, these 4.3 HEI engines are particular about the plugs they use. Stick with AcDelco or Delphi parts.

another sec. ignition syst. check can be to spray a mist of water on the ign. components and see if it results in any arcing to grounds or another wire. this could cause a miss.

if all checks out, you may have a mechanical issue.

 

Greetings, I had a similar Problem. I bought a 96 s10 with a 4.3 that supposedly had a Blown Head Gasket. I was going to Drop an LSX 5.3 I have into it but thought what the hack why not try to see if it runs. I paid $400 for it so a couple hundred more or so to see if the motor was shot was a good investment. I had my idea that it was just an intake gasket as I've done about 2 dozen of these mid 90's V6's that had water in the oil as well.
Turns out it was the Intake Gasket so I removed the Intake and Plenium, disassembled it and Cleaned everything out. I was very careful not to damage the Spider assembly however when I had it back together I to chased the P0303 checking Everything out. I should have pulled codes to start with but didn't so I first thought maybe the P0303 was an existing Code.
Compression was 180lb, had spark but it was intermittent, a Conventional Timing Light was the Best Tool for me to see what was going on. My Miss was at Random RPM's Never Steady at one RPM, in fact holding one RPM it was very smooth.
The Problem ended up being something I probably did to it. When I was Plugging the Connecter back into the Fuel Regulator I somehow bent one pin over. This pin was in the middle on the drivers side(#3). I took a screwdriver and bent the pin back straight, cleared the codes and now it has power beyond what I thought the 4.3 would have.
Best of Luck.

 

This thread fixed my 97 blazer after 2 weeks of issues after an engine swap. 1 of the pins for injector 3 was bent over on my new set of mpfi injectors causing the cylinder 3 misfire. Had i not found this thread i never would have looked at the pins.