Astro 4.3 P0304 Intermittent


Asked by Oct 27, 2012 at 11:08 PM about the 2001 Chevrolet Astro LS Extended RWD

Question type: Maintenance & Repair

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 195º 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?

14 Answers


Why did you put junk NGK in your Chevy? Have you done a compression test yet?


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

2 people found this helpful.

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.

1 people found this helpful.

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.

2 people found this helpful.

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

1 people found this helpful.

Is it fine cold, and becomes more prominent as it warms up?

3 people found this helpful.

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.


Can you move the injectors to another cylinder to see if the # 4 injector creates a problem where it was moved to? You indicated your testing equipment shows or the codes show the # 4 cylinder is miss firing or "burping"? That is where you start. Even exchange plug wires and plugs to eliminate less costly issues. Is the spark actuator good and can it be swapped out with another? If you have a distributor, I'd look at the cap for cracks or evidence of arcing or sparking. It could be electrical, or more costly mechanical. Why not replace the injector with the new one. (Who really cares what plugs you use? If the plugs weren't good, they'd soon go off the market.) Have you taken it in for a fuel rail, injector cleaning? It is costly, but deposits could be causing a problem. If you take it on long trips, it is a good idea to do it every 25K miles or so. (I don't myself, but I'm also a cheap bas---d.)

Save time ohm test specs mfg specific

Warm and cold have different ohm readings

1 people found this helpful.

Got replies in In-box. Seems some feel swapping injectors around would work - but this is a single injector system with differing delivery tubes to individual cylinders inside the intake runners, not individual injectors. Plugs (new) were changed, wires swapped and tested, cap/rotor is new and yet the single result is erratic, defies testing and only shows in #4 cylinder when/if it feels like it. The inconsistent nature makes me want To let it finally did and then go and perform an autopsy - but the wife feels - most incorrectly - that it will die when she is crossing a rail crossing and become cow-catcher fodder. 'Cruise-mode' seems to bring out the worst semi-failure, with the SEL- SOON' firing off and scaring her more than anything else I feel. It's livable - to me anyway - but annoying. Fuel mileage with it acting up verses being normal, do not seem to vary a fraction of 1 MPG at all. BTW a new injector is quite expensive --- over a few hundred dollars for the newer styled (aftermarket) improved version that purportedly gets hundreds of miles to the gallon --- so the ad copy says anyway.


here is what I found: There is another possibility. I wrote a lengthy post under the hard to start discussion because mine had degraded to the point of not starting at certain times. Mine was the cam retard angle was negative 12 or 13 degrees. Combined with an aftermarket cap and rotor and wet mornings I could get misfires and sometimes not start at all. What I think was happening and I saw this on some other forums that are for automotive techs was that when the cam retard angle is that far negative, you can get crossfire on the rotor to the wrong post on the dist cap. Wet weather aggravates it. That is why some people had results going back to oem dist cap and rotor and why some had to go back to a completely new distributor and also why wear on the distributor can affect it. Mine had no wear but would still misfire or not start. So I made the distributor adjustable by modifying the hold down bracket hole into a slot. You can check the cam retard angle with a scanner that shows the engine data. I then adjusted it to show near zero cam retard. Because this has the potential to mask other problems, I do not recommend it until you have ruled out everything else but after over two months of frustration, mine appears to be running. Good luck. and: Well guys i have an official reason that will cover about 95% of you guys. The injectors on the newer Astro vans were bad from the factory a flawed design. It never was made a recall but GM recognized it as a problem you can go to the dealer and give them the vin number of you van they will put it in there system and run to see if your van has the bad injectors. If it does and it is 10 years old or less and has 100,000 miles or less they will change them out free of charge. My mechanic did some research and found this out i went to the local Chevrolet dealership the got my vin took the van for a spin determined that the injectors were the issue via a scan and test drive and 2 days later i got it back and it is running stronger then ever and the new injectors are a whole new design so you will not have to worry about this happening again. Anyone who is having a misfire issue i highly suggest you try this route and see what comes of it, what do you have to lose??

3 people found this helpful.

Bottom line is it could be the injector issue, an after market distributor and or cap issue, or cam retardation issue. It is a place to look into and start.

1 people found this helpful.

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