Oso Blanco

Hey guys, I've got a 1999 Express 1500 w/4.3L and 198,000km. I bought it about 6 months ago. At that time I replaced cap and rotor, plugs, wires, PCV, air and oil filter. It's been running like a champ until yesterday. I stopped at a light and it was sputtering and missing badly. The check engine came on solid then started flashing. I was near a part store so I stopped and they scanned it for me. Just a PO300 nothing else.

I pulled the cap off and everything looks good. Checked the wires and everything is tight. Found a vacuum line that had some minor cracks so I replaced it. Still ran badly. The fuel pressure is 55psi at idle and jumps to 60psi when I snap the throttle (seems odd it increases with rpm). I'm reading a absolutely steady 14psi of vacuum which is just below the "late ignition timing" area on my gauge. Since the timing is auto adjusted, I'm guessing (after much reading here) that the distributor gear is a good bet.

Am I missing anything?

Cory

Oso Blanco

The distributor gear ended up being fine.

I took the van to a friend who works at a shop. He scanned it and all the misfires were from #4 and #5. Having a look at the cap he noticed that there was a carbon trace about 1/8" long between the 4/5 contacts (crab style cap). The spark was jumping between those 2 and not firing the plugs. We swapped the cap and it was much better but still had a rough idle. We rescanned and found that #5 was now firing but #4 was not. We assumed #4 plug was either fouled from the misfiring or went bad before and that's what took out the cap.

I drove home (shop was closing for the weekend) and grabbed a plug. The original one looked good but I installed the new one anyway. Fired up the motor but still had a mis. Checked the plug wire and found it had no continuity. Replaced the wire with a new one (checked it with multi meter first). There is still a solid mis on 4 even with a new plug, wire and cap. I'm stumped.

Thoughts?

Oso Blanco

After much messing around the miss remains. The compression on #4 is 120psi, not great but shouldn't cause a miss. When the plug came out of the hole it was wet with fuel. When I put my finger over the hole and cranked the engine, there was definitely fuel present.

With the plug grounded and snapped into the wire, I cranked the engine and a nice bright blue spark was visible.

So, I have fuel, compression and spark. I can't see anyway that the timing of the fuel or spark is off when the other 5 cylinders are good.

What am I missing?