New to forum - Have a misfire
 

Hey all,
This is my first post on the forum and I'm glad to be here. Anyway, I have p0303 code that came on today. The check engine light will blink for a few seconds, then remain lit for a while, and then will blink again for a few seconds. There is no noticeable effect on engine performance at any speed. I've already tried clearing the codes and seeing if it comes on again and it does. So the next step I want to do is to check the spark plugs and ignition wires but I can't seem to find a timing diagram for the 4.8l engines. Does anyone have one? Also, if I check the spark plug and wire for the #3 cylinder, and everything checks out okay, what should be my next step. Any help will be much appreciated.

Side note: The truck has 123,XXX miles on it and I purchased it a year ago with 116,XXX miles on it. Not sure what the maintenance history has been on the truck.

What do you mean by timing diagram? How are you going to check the spark plug wire? My recommendation would be to switch #3 plug with a different cylinder and #3 wire with a different cylinder (other than the one you moved plug to) unless there is something obviously wrong when you look at them. Which ever one the misfire move too then there your problem. If it is still on #3 then you need to look for an intake vac leak,a bad injector or internal engine problem.

Thanks for the reply. What I meant by a timing diagram is the firing order / order of the cylinders. I was planning on doing the swap you mention but I'm not sure which cylinder is #3. Also I guess I should ask if the spark plugs need to be gapped on these engines.

If the current plugs are out of the spec for gap then they need to be replaced. You should never have to re gap any plugs. Make sure to check/gap the new ones. If you use the correct plugs they should come with correct gap but always check.
Drivers side front to back 1,3,5,7 passenger 2,4,6,8 I would recomend only using AcDelco plugs

Great thanks! That's exactly what I needed. I will post back with my findings tomorrow.

Bad injector on number 3 or if you have random misfire. Engine runs good on load but misfire on light load then you could have a bad gear on the distributor.

Alright so here is an update. Swapped the #3 and #5 spark plugs and wires and still got the p0303 code. After that I swapped the #3 and #5 coils and the same thing happened (code p0303). I have noticed some strange things though while driving. The CEL will either blink and shut off completely, continue to blink constantly, or stay lit and then blink randomly. I have reset it multiple times and always with the same results.... No pattern. I have noticed that the blinking is more likely to happen while I am going up hill.

Not really sure where to go from here. I read that is could be an intake manifold gasket leak. I remember once hearing that if you have a vacuum at the oil filler cap while the truck is running that this is a tell tail sign of it but I am not sure ( I do have a slight vacuum so that could be my problem). Any suggestions will be appreciated. Thanks

Same frustrating problem
 

Have you figured out the problem? I am having the same issue (2003 Silverado 4.3 L). I've changed all plugs, wires, cap, rotor, fuel filter, O2 sensor (bank 1 sensor 2). This helped it run a little better but still misfiring and backfiring under light load. After clearing code 0303, check engine light blinks again, then comes on steady. I'm trying not to replace any other good parts. Any idea's?
Thanks.

Sorry, unfortunately I have not. Last night I looked into some problem spots with no luck finding a solution. For one, I tried finding an intake leak by running the engine and pointing a small bottle of propane that plumbers use at different spots. I read that if you do this, and you see a rise in rpm at a specific location, then you have a leak there (the vacuum caused by the engine will suck the propane in). I couldn't find any spots where this would occur. Secondly, I used a cardboard insert for wrapping paper (the tube) and put it next to the #3 injector to listen to it tick. From what i heard it had the same rhythm and strength as the other injectors, so that seems to be ok. Lastly, I cleaned the throttle body and the MAF with MAF cleaner and still no change. My next step is to get a bottle of fuel system cleaner (I've read chevron techron is the best) and give that a go.

I asked a mechanic that was in the parts store with me about the situation and he said that he has heard cases of the timing being off a bit. He said that the timing is set by the PCM on the 04 model so it would have to be reset by the dealer. I'm sure an 03 is the same. That's a last resort for me though.

Good luck.

Before I go and buy a bottle of techron does anyone know if there is a benifit/detriment to have Pep Boys or another place do a fuel injection service where they pump the cleaner directly into the lines. I'd rather pay $50 for the service and have it over with then spend $15 on a bottle that might not help that much. My fear is an effect on the cats/o2s as I've heard it produces quite a lot of carbon smoke. Any advice?