2014 Sierra 5.3, 200K miles, code 0305 (misfire cylinder 5)

CrawlSlow · September 24, 2021

No pressure is good. Back pressure could cause symptoms you are experiencing.

Try cleaning your MAF to see if it helps.

KARNUT · September 24, 2021

It’s getting out there now. Sounds like a quality shop is the answer.

Roger Rogers · September 24, 2021

17 minutes ago, KARNUT said:

It’s getting out there now. Sounds like a quality shop is the answer.

I hear you. I did take to a shop and just looking at the code, and the miles (220k!) they immediately went to dropping in a new engine. I'm prepared for that possibility, but the truck is almost all highway miles and in excellent condition otherwise. The only problem is this P0305 at idle and goes away when accelerating / driving. I'm afraid I'm going to get similar response wherever I go, or a lot of parts swapping and then drop a new engine, so trying what I can myself before then.

Maybe I'm at the end of the road with this and I don't want to run too much if it's going to damage my catalytic converters, etc.

Any other things to try I'd really appreciate. Was thinking to pop the valve cover to see if anything is sticking for that cylinder.

KARNUT · September 24, 2021

28 minutes ago, Roger Rogers said:

I hear you. I did take to a shop and just looking at the code, and the miles (220k!) they immediately went to dropping in a new engine. I'm prepared for that possibility, but the truck is almost all highway miles and in excellent condition otherwise. The only problem is this P0305 at idle and goes away when accelerating / driving. I'm afraid I'm going to get similar response wherever I go, or a lot of parts swapping and then drop a new engine, so trying what I can myself before then.

Maybe I'm at the end of the road with this and I don't want to run too much if it's going to damage my catalytic converters, etc.

Any other things to try I'd really appreciate. Was thinking to pop the valve cover to see if anything is sticking for that cylinder.

I have an 02 with 178K miles. It idles a little rough in the morning, worse when cold. It doesn’t throw a light. Runs fine warm. I just go with it. I bought a hot rod truck 14 years ago that used oil. Thought about replacing the engine. It ran very good. 12 years later I sold it. It still runs very well. Same engine. If it’s passing inspection I wouldn’t worry with it as long as it runs smooth driving. If you have a dead hole you definitely would know it at any speed.

BigBadSierra · September 24, 2021

Try switching the coil pack and see if the misfire moves to that cylinder.. You are at the mileage when thy can start to go.

Actually that would have been the first thing I checked for a miss fire.

Roger Rogers · September 24, 2021

4 hours ago, KARNUT said:

I have an 02 with 178K miles. It idles a little rough in the morning, worse when cold. It doesn’t throw a light. Runs fine warm. I just go with it. I bought a hot rod truck 14 years ago that used oil. Thought about replacing the engine. It ran very good. 12 years later I sold it. It still runs very well. Same engine. If it’s passing inspection I wouldn’t worry with it as long as it runs smooth driving. If you have a dead hole you definitely would know it at any speed.

Yeah, I'm kinda thinking of just doing that. If it dies then I drop a new one in. It just feels like something small is going on. The cylinder has great compression - same as the others in the bank.

Roger Rogers · September 24, 2021

3 hours ago, BigBadSierra said:

Try switching the coil pack and see if the misfire moves to that cylinder.. You are at the mileage when thy can start to go.

Actually that would have been the first thing I checked for a miss fire.

Hey @BigBadSierra, as you say, that was the first thing I tried. Coil pack, wire, plug. No joy. Next compression test, which was solid. Last thing I did was injectors.

The intake ports had some carbon build up, but didn't seem bad at all. Have a hard time believing that is the cause, but I'm thinking to take the manifold off and doing a cleaning and walnut shell blasting the intake ports. Just can't believe that would be the problem.

KARNUT · September 24, 2021

48 minutes ago, Roger Rogers said:

Yeah, I'm kinda thinking of just doing that. If it dies then I drop a new one in. It just feels like something small is going on. The cylinder has great compression - same as the others in the bank.

I use to go crazy over absolute perfection. Sometimes you have to pick your battles.

Roger Rogers · September 27, 2021

Well, I had to try one more thing... I took off the manifold and walnut blasted the intakes. The actual intakes didn't look bad, but where the valve seat was gunky and I used seafoam to clean that up as best I could. No more misfires!

I didn't realize these engines were so finicky about carbon build-up. It really didn't look bad, definitely not as bad as many pics I've seen, but apparently it matters! To think the solution suggested was to change the engine. Definitely got me thinking about installing a catch can (though, I've only heard about them and need to read up a bit more). Clearly oil vapor going through the intakes without the gas acting as solvent is not helping things.

One bummer in this whole process is that after I changed my fuel injectors the MPG has dropped I would say 2-3 MPG. I'll start a separate thread on that... not sure if they need to settle in for MPG to rebound or if the compatibility chart I looked at isn't accurate.

Thank you everyone on this thread for giving advice. Much appreciated!

Edited September 27, 2021 by Roger Rogers

diyer2 · September 27, 2021

Good Job!

NE18 · September 28, 2021

Great thread and thank you. I think you are the first I have heard to have a problem with the direct injection with these trucks. Sounds like it should be a maintenance item at 200k.

Roger Rogers · October 1, 2021

Well, I spoke too soon. The misfire is back. So strange, because it entirely disappeared for about 100 miles. I could idle with no misfires, but it's now back to misfiring at idle.

To recap:

At idle I will get about 150 misfires per minute (using OBD2)
At highway speeds the misfires disappear entirely (OBD2 still connected)
When idling the misfires appear to grow in intensity over time. Check engine light will come on eventually and I've had the traction warning appear as well.
Replaced coil packs, plugs, wires, injectors
Compression test on the misfiring cylinder is excellent and same as the other cylinders
Cleaned the intakes carefully (I cleaned with walnut shell blasting and then seafoam with a brush on the valve stem and valve opening), and after doing so the misfire disappeared entirely... for about 100 miles, or about 10-20 minutes of idling... I idled a lot just because I was so amazed that the problem was gone

I was (happily) surprised that the cleaning had an effect, sadly, it didn't last long. This makes me wonder if I'm having a problem with the lifters/springs? Maybe the valve in this cylinder isn't sealing tightly/quickly? I'm probably talking gibberish, as this is by far the most I've ever worked on a modern 4 stroke engine.

As KARNUT says, I should probably take to another shop, but the high miles (220k) seems to make them go towards replacement and I feel this engine has a lot of life left on it. It runs great at highway speeds and under load and the cylinder compression tests are all basically at factory specs.

Edited October 1, 2021 by Roger Rogers