99 SILVERADO P0304 HELP

[xIIPHOENIXIIx]

Need help with this code. I’ve taken it to a couple “mechanics” and the issue seems to still be there.

 

I have a 99 Silverado 4.8L 4x4; I have the check engine light on (currently). However, I don’t feel anything wrong with the truck. No rough idel or acting weird. I have already replaced; fuel pump, fuel filter, spark plugs, wires, #4 injector and air filter. All Acdelco brand, yet this light is still on. 
 

One thing I did notice different, my battery voltage  drops. Which is weird since I have bigger alternator then 105 amp, optima battery, and big 3 wire upgrade. 
 

Any help is appreciated! 

[CamGTP]

Did you swap the complete coils/W bracket side to side to see if the issues follows?

 

What about doing a crank/cam relearn just for the hell of it with a scan tool?

 

Or what about a compression test and leak down test on that cylinder??

[xIIPHOENIXIIx]

Tried swapping coils; seems there’s still an issue. The light goes off when driving it since I usually just use freeway. 
 

If I start the truck and let it sit there like warming it up. The voltage fluctuates and makes the light come up with that code.

 

Haven’t tried the cam re-learn or compression test. I’ll do that next thanks. 

[carkhz316]

Do you have any freeze frame data with that code that you can provide? Otherwise, what are fuel trims or O2 data. This will indicate if it's actually a misfire, and if it's fuel or ignition related.

[CamGTP]

What is the voltage at the battery and at the back of the alternator?

 

A loss is power could make it possible to have poor performance at the coils/wire/plugs.

[davester]

Yeah, you want a nice, stable, voltage first.  Bad power can cause all kinds of problems with your truck.

[xIIPHOENIXIIx]

Thanks for the help guys! Really. 
 

I have a new Optima Yellow top battery, and upgraded alternator from 105 to the bigger one the 05+ models have can’t remember if it was 145. It has a Big 3 Upgrade. Yet voltage still drops, when idel. Also cleaned all the grounds the truck has.
 

I don’t have freeze data, Just have a cheap scanner to check codes. I’ve taken it to a couple mechanics and they can’t seem to find anything. 

 

Check engine light is off right now; but If I start it and let it idel for a bit. It will start dropping the voltage and that’s when the light comes up. So I’m guessing it has to do with the power. Just cant find what is dropping the voltage 

[davester]

Does the new alternator have the right pulley?  When I upgrade my truck, the 145 alternator had a bigger pulley, and I needed a longer belt to make it work.

 

If you did need the longer belt to make it work, then next I would suggest using an actual voltmeter to test the voltage from the alternator, and then if it is fluctuating, diagnose why that is.  You may need a more advanced tool like a techii to assist, and/or access to the full service manual to do this.

[xIIPHOENIXIIx]

Yes, I did have to buy a new belt to make it work. Alright I’ll check the voltage from the alternator & then the battery. I will update when I do

[Black02Silverado]

One other thing to check is remove all your grounds that you can find and clean them up.  Put some dielectric grease on them and tighten them back down.  A bad ground can cause all kinds of problems.  Also check you cables.  Some corrosion inside the casing can cause issues as well and isn't noticeable from the outside.  Slight chance of that happening but possible.

[riverbanks]

Anything is possible,guy on here a few days ago,removed and reinstalled his fuses and got some interior lights back working,check as much as possible

[xIIPHOENIXIIx]

Already cleaned off all grounds and added grease. Terminals to battery are clean as well. One of the grounds comes from frame; which was sanded and threaded with a bolt. Second ground comes off the alternator bracket. Power comes straight from alternator. The factory connections are still there on side post.

[riverbanks]

The question is,who is making it fluctuate, check the ground on back of heads,2 and strap to firewall?

[carkhz316]

What about the ground thats near the bottom of the block on the left hand side? It's the main engine to battery ground. I just found one recently on a friends 1500HD that was finger tight and causing low voltage issues. A simple voltage drop test showed about 1.5V being lost between the engine and battery. And yes, this is the ground from for the alternator as well, since it goes through the bracket to the engine block and then out to the battery.