Code 303

I’ve been working on my truck for over a month trying to fix it. I was driving the truck after having it parked 3-4 months and I got an 1/8th a mile down the road and the check engine light came on and started blinking. I turned around and went back home. I’ve changed the battery, the spark plugs, coils and fuel injectors. When I drove it to test drive it today it’s still blinking if I get over 35mph. I have no idea what it could be at this point and really need some advice. I’m also not a mechanic. Thanks for any and all input.

 

Try swapping the coil pack from cylinder 3 to cylinder 5 next to it. Does the code change to P0305? If yes, replace with genuine Denso brand coil DO NOT BUY on scAmazon or fleaBay (counterfeits are rampant).

If not, swap spark plugs between cylinder 1 and 3. Verify plugs, both look good? Does the code change to P0301?

If not, report back with results.

 

^This 1st.

 

We swapped the coil to cylinder 1 and spark plug to cylinder 5 and it’s still saying misfire cylinder 3.we just put all new fuel injectors in this morning and it’s still saying cylinder 3 misfire.

 

Swap injectors next to eliminate #3 injector and/or #3 injector wiring as the culprit then.

Edit: be sure and put a very light coating of oil or Vaseline on the injector o-rings and grommets to help them slide into place and prevent damage.

 

Sad to be the one to tell you this, but unless you have an electrical issue, it's totally possible your intake gasket is breached at cylinder 3 and is letting more air into the cylinder than the ECU is expecting, and you're misfiring.

When that happens, you need to replace both gaskets at the same time, DO NOT USE AFTERMARKET PARTS, this is what it looks like when you blow off one cylinder. If it doesn't jump to the right place, its 5-6 posts down on that page.

@bmf4069 used aftermarket gaskets on his. Twice. Both blew out again within a year, as you see. Then he installed OEM. To date, those haven't blown. Seriously. You don't want to be up under that intake manifold again in the near future.

There's a way to tell how to orient the OEM gaskets, based on tabs, but I don't have the non-VVTi V8 in my truck so I've never done it before. Plenty of others have, and have posted pics. Best to get it right the first time.

Also, be DAMN SURE there's nothing funky down in the injector seat. Old o-ring, boot, similar. Because anything interrupting flow of gas would also cause misfire, and if I had a dollar for every time someone double-o-ring'd or had something in the seat over the last couple decades, lunch would be paid for tomorrow.

 

White tabs up and out