Code p0308 cylinder 8 misfire

[shadeitplease]

Hey everybody! I just joined the forum as a new owner of an 04 Avalanche. I'll begin by saying I dont know a whole lot about vehicles and use a combination of Google and YouTube to attempt to teach myself. My check engine light came on after buying the truck saying cylinder 8 misfire. I changed the cylinder 8 coil and all the spark plugs and wires yesterday and cleared the codes. The light came back on pretty quickly. It stays on all the time but flashes when I get above about 30mph and then stops flashing when I drop below 30. Any help pointing me in the right direction would be much appreciated. Thanks in advance!

 

[EXT4ME]

Leaking intake manifold gaskets have been an issue with these engines for quite some time.

Research that and see if you can find some Youtube videos to help with your diagnosis and repair.

FelPro intake gaskets seem to be recommended a lot of the time.

That is what I had installed on mine.

 

[ygmn]

FLASHING means it is missing so much that exhaust is so rich it is HARMING cats and could damage them.

Need to get it looked at ASAP else your repair bills will continue to grow.

So do not drive until fixed.

Misfire can be many things.
you changed some of them but you need to check fuel injectors.
Valve sealing
Cylinder compression etc.

 

[2004Slickside]

Hey everybody! I just joined the forum as a new owner of an 04 Avalanche. I'll begin by saying I dont know a whole lot about vehicles and use a combination of Google and YouTube to attempt to teach myself. My check engine light came on after buying the truck saying cylinder 8 misfire. I changed the cylinder 8 coil and all the spark plugs and wires yesterday and cleared the codes. The light came back on pretty quickly. It stays on all the time but flashes when I get above about 30mph and then stops flashing when I drop below 30. Any help pointing me in the right direction would be much appreciated. Thanks in advance!

If this is a felt misfire (can you actually feel it missing?), begin by checking static, idle and snap compression.  https://support.alldata.com/article/tech-tip/running-compression-tests  This will help insure it's not a mechanical problem with the engine.  Read and understand the link or ask questions please.

 

[shadeitplease]

Thanks all for your replies. I do not feel the misfire at all. The truck runs and drives just fine. It seems like there is a pretty long list of things that could possibly be wrong with it so I may just take it in and have it diagnosed and determine if I can do the repair myself from there.

 

[2004Slickside]

Thanks all for your replies. I do not feel the misfire at all. The truck runs and drives just fine. It seems like there is a pretty long list of things that could possibly be wrong with it so I may just take it in and have it diagnosed and determine if I can do the repair myself from there.

Note that even if there is no actual misfire, there can be a number of causes of false misfire codes.  Best way to quickly check for false misfire detection is to do a injector balance test, but you need to have a capable scanner to trigger and read the test results.  The PCM looks at the RPM drop when each injector is taken out of the system.  No RPM drop = 100% misfire on that cylinder.  All cylinders drop equally = no misfires.

I have a 2001 Blazer that had the flashing SES light with a P0304 and no actual apparent misfire when I bought it.  It had a very worn thrust bearing and some other excessive bearing clearances causing the false misfire detection.  I fixed the SES light which had tape over a wire at the instrument cluster and then fought the diagnosis for weeks before I happened to pry the crankshaft forward and backwards and saw the excessive thrust clearance and the clunk.  It was apparently a victim of the failed intake gaskets allowing coolant into the oil - taking out the bearings.  The intake gaskets were fixed but the damage to the bottom end was done.  Apparently previous owner disabled the SES light and traded it in at a new car dealer.  Mint-appearing 2001 Blazer with 60,000 miles.  No noises unless completely hot, right off the highway. 

Factory info also indicates that disturbing or changing the crank position sensor, bad belt driven accessories, broken flex plate, slipped harmonic balancer, etc can also cause false misfire codes.  It's the same for the Avalanche.  It also can simply need a new crank sensor relearn - again done with a capable scanner - someone may have been jacking around with it.

My point is that you can go down the primrose path of a lot of expense (shotgunning parts) very easily if the technician doesn't realize if the misfire code is false or real at the beginning of the diagnosis.  Make sure you get a good experienced technician and know that these are sometimes quite difficult to diagnose. 

 

[shadeitplease]

After doing some more research based upon the replies in this thread I'm really hoping the mechanic tells me it's just a bad fuel injector as that looks pretty easy to swap out.

 

[shadeitplease]

I took the truck in for a diagnosis and as the compression was good, the plugs and wires and coil were good. Turns out it's a bad fuel injector on cylinder 8 and the one on 7 is starting to go out. I'm going to buy 2 injectors and try and do it myself. Is there anything I should know before I go to do this? Besides depressurizing the fuel system by taking the pump fuse out while the truck is running? Maybe like some common problems people run in to that might not be explained in a youtube video? Thanks again everyone for yalls help.