My 2003 Silverado 5.3 has an intermittent CEL. I plugged in the reader and found code P0304, misfire #4 cylinder. I changed the coil, swapped the spark plug and wire from #2 cylinder to see if the code would follow. Nope, didn’t follow. I swapped the injector also, same results. My daughter decided to borrow the truck and drove it so low on fuel that the low fuel light came on in the drive way. I added about 4 gallons of fuel and now the light went off. I started the engine after a jump start, and the engine really runs rough and now won’t stay running. I am thinking that the fuel filter is probably needing changed because of how low the fuel. I haven’t checked the fuel pressure yet and will do that today. Now, what really puzzles me is the intermittent part. Sometimes it will run with no problem, but then the CEL will come on and the fun begins. The Chevrolet dealer says it’s caused by the valves turning in the head and when it reaches a certain spot, it detects the misfiring #4 cylinder resulting in the CEL. If I pull in the driveway with the engine missing, go back out in a short time, start the engine and it runs fine. Same thing if it’s running fine and I can start it and the CEL blinks and it begins missing, but may stop and run fine. I am stumped and not so sure about the turning valves and the $6000 repair cost. HELP!!
I will tell you I don’t know what’s causing the code and the miss fire. I can tell you that $6000.00 to do a valve job is way over the top. You can get a rebuild motor for less than $2000.00 ($2,380 dealer here in MI 3 year 100,000 warranty) and I would think you could get it put in for less $1200.00
If the valve is turning that much it would not stay in that spot for very long. I call BS on this.
Here is a thought. Have you checked the coolant ? The intake gasket could be leaking and causing this.
The $6000 was twice what I can buy and install an engine with the 3 year 100,000 mile warranty. Right now I can get a used engine with 71,000 miles for $750 and can R&R it myself. I saw where there was a problem with some cracked heads on these engines made by Castech and I do have an unexplained coolant loss. I think I’m going to pull the intake off and do a close inspection. At the same time I’ll see if these are the ones prone to cracking. They are aluminum, Any idea what area I would look at for the intake leak? I am thinking probably a gasket. Thanks for the response.
Yes. My 1997 305 intake gasket leaked. It was easy to see because I had used a stop leak to get home. I use a product called Dike made by the Conklin co. It’s the only stop leak I will use. It’s blue in color so it makes it easy to see were a leak is. I would look close at that intake port that giving you the miss. You should see a evidence of the leak. You are right. It could be a cracked head too. I read that Dex Cool cause the intake gaskets and head gaskets to leak.
Oh and if can find Dike in a store by you. I would recommend keeping some on hand. When I worked repairing oilfield trucks. They had it in all the trucks. I have seen 1/8 holes stay fixed for 2 weeks Till I could fix them. Oh and it wont plug up the heater cores. I also run it in my Jeep. It’s keeps me from having fix it on the trail. But I check the cooling system often for damage. Good Luck and please let us know what you find.
Well I’m going to pull the intake off and see what I have. I’ll also be able to look at the casting on the heads to see if they are the one that are prone to cracking. The fun begins. Once again, thanks for the response.
Coolant does not flow through the intake on the 4.8, 5.3 6.0 liter engines. Most common coolant leak is water pump gasket, and the intake can be leaking vacuum due to wrong gaskets. If this is a flex fuel vehicle then it should have the teal colored gaskets and it most likely has the orange ones which will not handle the ethanol.
Well I pulled the head off and will take it to my shop for some clean up and check to see if it’s cracked. I didn’t see any evidence of a crack, but I know that they can be hard to detect at times. I’ll do the dye test and if that doesn’t reveal anything, I’ll get it checked at a machine shop. I’ll let you know what I find. The cylinders look good though which is a plus!!
Would a broken valve spring on the missing cylinder cause this problem?
It certainly will. It’s a shame you pulled the head before finding the broken valve spring.
Completely normal. 
I pulled the valve and it looks like it was seating just fine. I think I’ll get the spring and gaskets and put it back together.