Po305 Misfire Cylinder 5

[Dantheman-2003]

2008 GMC Sierra 1500, 5.3 Vortec. On the clock about 180k miles. Started running a little rough then check engine light comes on. Run the scanner, po305 misfire cylinder 5. I pulled the plug and it looked ok. Replaced it with a different one, no change. Moved some wires around, no change. Replaced coil pack, no change.

Any thoughts on the next move?

 

[Dustin Jackson]

@Dantheman-2003 Is it making any concerning sounds?

 

[Dantheman-2003]

@Dantheman-2003 Is it making any concerning sounds?

@Dustin Jackson - No concerning sounds in terms ok knocking etc. This truck has ran excellent since ownership never had an issue. This morning, Engine turns over just fine but with rough idle and flashing CEL.
1. I'm suspect of the plug because I cheeped out and got the whole plug and wire set off eBay "AC Delco". As others have mentioned maybe a fake set. I already had one fail on cylinder 8, cracked porcelain. Now on Cylinder 5 same thing and same symptoms. I'm headed to O'reilys to get a new plug then work backwards from there.

2. My next move if it's not the plug is the fuel injector.

 

[OR VietVet]

You could move plugs too.

 

[Dustin Jackson]

Could move the plug, wire, and coil and see if it follows. It sounds like you are on the right track, at least it doesn't sound like a mechanical problem

 

[Dantheman-2003]

Good suggestions. I will try and swap 1 and 5 and packs to see if it follows. Otherwise I will move on the the fuel, compression etc. I've heard others say could be o2 sensor, but I thought it had its own code. Po305 is specific to C5 misfire. Thanks for the inputs!

 

[OR VietVet]

If you do a compression test, you should have all plugs out. If you want a short cut, remove #3 and #7, each side of #5 and do the compression test and compare. Check for a vacuum leak too. I still say, swap the #5 plug with #3 plug, easier to get to, and see if misfire follows.

 

[Dantheman-2003]

UPDATE: I hope someone can find some humor here. I’m standing there over the engine bay contemplating my next move. Compression test first? Or fuel pressure test? Maybe start swapping wires and coils?

I’m ready to pull the trigger, coveralls on and all. So I look down and at a glance see my air filter indicator laying there at the bottom of the engine bay! WTF? Anyhow… all fixed now. Smooth operator all codes cleared. Back on the road.

Hope everyone has a great weekend go 9ers!

Dan

 

[OR VietVet]

That is weird. Even that air has to go thru the MAF.

 

[donjetman]

That is weird. Even that air has to go thru the MAF.

Right

 

[Dantheman-2003]

UPADTE 2: Yes this is weird. So I put that stupid thing back on, CEL turns off with no codes. Drive it for a minutes and rough rough idle and throttle. CEL back on. I’m leaning towards vacuum leak or MAF sensor bad. My question is, that little air module in there pretty good. How did it just pop out?

Thoughts?

 

[OR VietVet]

UPADTE 2: Yes this is weird. So I put that stupid thing back on, CEL turns off with no codes. Drive it for a minutes and rough rough idle and throttle. CEL back on. I’m leaning towards vacuum leak or MAF sensor bad. My question is, that little air module in there pretty good. How did it just pop out?

Thoughts?

I have leaned in to look at things and hit that and pushed it out. That should have not caused a misfire. That is why I said what I said.

Maf effects all cylinders. You have a one cylinder misfire. At least that is what you have said. Vacuum leak or plug, IMO. The plug is easy to check, switch like I said.

 

[Dantheman-2003]

I have leaned in to look at things and hit that and pushed it out. That should have not caused a misfire. That is why I said what I said.

Maf effects all cylinders. You have a one cylinder misfire. At least that is what you have said. Vacuum leak or plug, IMO. The plug is easy to check, switch like I said.

@OR VietVet - good points and good call on the MAF. I’m back to the drawing board on this sob. But it’s odd that CEL went off and code disappeared when I put it back in. Nonetheless, tomorrow is another day.

 

[OR VietVet]

The CEL can pick whenever it wants to go out. A thorough test drive is always needed, if the light goes out by itself or is turned out, then reattach a scanner and check/double check for codes.

 

[j91z28d1]

I think it's interesting that it ran smooth for a bit when cold, warmed up and started to miss again.

have we seen the fuel trims and stuff in this one. maybe that alcohol % and running rich enough to miss?

 

[Dantheman-2003]

At this point I’m thinking maybe fuel or air issue. I’d be surprised if it was something internal in the cylinder. But we will find out. Gonna enjoy my weekend for now. On Monday I’ll take it into the steelership and have them diagnose it. $$$

Go from there. Will report back.

 

[OR VietVet]

Did you at least swap the spark plug yet? Check for vacuum leak? If is running rich or lean, that would effect more than one cylinder, IMO. But a vacuum leak or spark plug can narrow down to one cylinder.

If you take it to another shop, make sure they explain why whatever they find, is bad and documented. Good shops will even document their diagnostic steps and results of tests.

 

[mikez71]

maybe inspect coil wiring harness/connector?

 

[mikez71]

Did you at least swap the spark plug yet? Check for vacuum leak? If is running rich or lean, that would effect more than one cylinder, IMO. But a vacuum leak or spark plug can narrow down to one cylinder.

Vacuum leak, hmm.
I wonder if it's leaking at the intake manifold gasket.

 

[Fless]

We have no idea what the fuel trims are doing, if it's going into closed loop, what the MAF is reporting, etc. Without some live data and knowledgeable eyes on that, all we have is conjecture.

Once the dealer diags it, please let us know what they find.