Astro 4.3 P0304 Intermittent
2000 Astro 4.3, P0304 intermittently. New plugs, cap/rotor and intake manifold and plenum gaskets less than five months ago. Sometimes it runs just great - other times sets SES and MT2500 finds random missfires or #4 has a lot of misfires.
Seafoam seems to help but not as much recently.
Plugs are double-plat NGK and less than 30,000 miles. New wires, etc. Snap-On DIS tool finds 13KV +/- 2KV per plug wire at cap. Last oil change @3,500 miles ago, Castrol 5/30W black cap w/ZDDP - not CARB-Low Emission version. New coolant w/gasket change; new radiator and new 195ยบ t-stat.
I don't find any arc-over nor does the plug look unlike all the others, even swapped plugs for kicks. No engine noises, no backfiring - just chugs sometimes. Fuel mileage is in low 20s most of the time around town - better at higher speeds.
Cleaned MAF element and tested TPS and cleared all codes. It may go for 200 miles or 10 miles before it resets P0304.
Any ideas?
No - no compression test yet. It runs very well @ 60-75% of the time. So I don't suspect compression would be both good and bad at the different times in the same cylinder. I've run NGKs since 1970 or so and never had a failure. They also always passed MVPC Cal inspections, as I am a retired Cal installer/inspector and I know they work well in GMs and Japanese. I suspect the FI system the most. Since Seafoam seems to not make much/any difference now - that's where I'm leaning
Guess its beter than champions or autolite. Get a compression test while good and bad. It may be in the fuel but it may also be a valve worn into a comfortable spot that occasionally rotates and let's air pass. Spider injectors are very pricey.
Check fuel pressure a weak pump can give you the code for random misfire. Be a hundered percent on problem before throwing parts at it as some are in the two to Three hundered range. Fuel pressure needs to be 40lbs to 53lbs a call to the dealer will place you in correct range, I personally do not recommend using after market (Carquest,AutoZone,napa, ect) ive had really bad luck w parts new out of the box being utter crap. Buy GM on injection parts, its engineered for your specific application. Good luck.
I too hate tossing money/parts at something. That's why I'm trying to home in on this problem first. I've got a new and in-the-box spare injector, and although I could drop it in, I still want a better definition of the problem before I do that. It's just the 'sometimes it's there - sometimes it isn't' that bugging me. I'll run the FI pressure test - I kept my Snap-On Tester and even though the new AC- Delco pump is less than two years old (and the tank NEVER gets lower than 1/3 full) - I guess it could be acting badly. I just wonder how it could only be picking on #4 cylinder most of the time when it acts up. I dunnow. But the fuel pump is a new thought however ---- although not a happy one.
Its still early here give me a couple hrs to pick a couple brains and see if I can get a better answer for.you.
Thanks for the timeliness of your answers. I'm into the third month of this problem now although I think I noticed an occasional burp in the engine for a while before that. It's primarily my wife's vehicle, but we use it for longer trips when I don't want to drive my K5 or Amigo. We've moved into the 20s with highs near 60 now, and the engine can start cold and be fine or missing and it can start missing at any time it feels lie it. So I don't believe engine nor ambient temps make any or much difference.
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