P0304-Misfire Help!

[SubZ3r0]

Hey all.

It seems that I have this ongoing issue of a misfire on cylinder 4 on my 08 LT. These are the things I have fixed/verified thus far.

1.) Verified the spark plug is getting a spark from the coil pack.
2.) Replaced spark plug.
3.) Replaced Spark plug wire.
4.) Tested injector resistance = 12.7 ohm
5.) Still replaced injector for cylinder 4.
6.) Stethoscope reveals a slight squeak under the valve cover near cylinder 4. Not sure if its a bad guide rod or just because of the actual misfire.
7.) MAF sensor looks clean and free of debris.
8.) All 4 oxygen censors look fine according to voltage read outs.


If anyone has experience with a similar misfire issue on the 5.3 v8 flex, Let me know what the fix was. Im sure its a case by case basis with these kind of things.

 

[rockenthebowtie]

Did you check compression or for an intake vacuum leak on that cylinder ?

 

[SubZ3r0]

No. I lack the tools to do so.

 

[Metaldrgn]

Is it all the time or just occasionally? If it's intermittent it's more likely an electrical issue. You could try and just swap coil packs/injectors with another cylinder and see if the problem moves and that way it won't cost you anything extra. Check the pins too. One could be recessed. It could also be the PCM or connector to it or wiring in between. If it's all the time it still could be an electrical problem.

If it's all the time I would recommend starting with a leak-down check (different than a compression check) before the compression check because if it is low you can isolate whether it's top end or bottom end. It doesn't cost very much for the tool. Harbor freight sells one if you have one close to you, but look for directions online. The one it comes with is actually wrong. You could also check to see if you had cam issues by taking off the valve cover and reconnecting everything out of the way and starting it to verify all the rocker arms are moving like they should.

 

[rockenthebowtie]

Yea the leak down tester is less expenssive but now you have to go out and buy a $200 compressor air line hose and air line fittings and adaptors im not sure how someone see's that as cheaper and easier lol.. get a compression gauge there only like $30 less work and time consuming.. if your under 100k on your mileage and it is low compression you can call GM to get your local GM dealer to work with you on the cost

 

[SubZ3r0]

Its not intermittent. You really only notice it at a low idle. ODB scanner shows the constant misfires. usually 40-46 per minute. the only thing i havent done ignition wise is change around coils. the plug is new so ill switch coils tonight and see if the misfire changes cylinders.

 

[samotahoe]

I have a buddy with a Silverado, and same issue but his is on #6. apparently there was a recall on some 5.3L for the oil pump, and caused a loss of pressure. now that is replaced he cant get rid of the misfire. ill let you know how it goes cuz I don't have access to his truck all the time to do testing but I would like to see how this turns out also. there is another test you can do to see if its internal damage, if you have a compression tester then you might also want to try doing a running compression test after doing the compression test. this will tell you if you have bent or burned valve.

But firstly I would definitely start with the less time consuming and cheaper way by swapping around the coils. make sure you mark your coil so you will remember which one it is lol. see some guys just move it and forget which one it is and have to wait till the light comes back on to figure it out lol.

 

[SubZ3r0]

Thanks for the info samotahoe. I swapped the coils around today. I switched 1 and 4. Cleared the ECU before hand. And when i started it the misfire counter was still going up on cylinder 4. It seems i have ruled out ignition.

ALSO...check this tool out that i use. I get this along with the torque pro app for your phone.. it does everything the $200+ scanner tools do!!! Live readouts of everything your ECU supports.

http://www.amazon.com/gp/product/B005NLQAHS/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1

So as i said.. once i started up i watched my phone and cylinder 4 started rising. about 8-10 misfires per minute. I guess ill have to checkout compression next!

Keep me updated samotahoe on that silverado. Mine is an 08. what year was his?

 

[rockenthebowtie]

Again once you find out its low compression call GM if you have less than 100k on it they will work with you on the cost on getting it repaired

 

[SubZ3r0]

Again once you find out its low compression call GM if you have less than 100k on it they will work with you on the cost on getting it repaired

Thanks.. Ill just bring it over to a Chevey dealership. Ill do the compression test or I might just let them do it. By the time i get home its already 8-9

 

[Metaldrgn]

Yea the leak down tester is less expenssive but now you have to go out and buy a $200 compressor air line hose and air line fittings and adaptors im not sure how someone see's that as cheaper and easier lol.. get a compression gauge there only like $30 less work and time consuming.. if your under 100k on your mileage and it is low compression you can call GM to get your local GM dealer to work with you on the cost

It isn't $200 for an air compressor... Cheap small ones are less than $100 I believe and you can probably get one from craigslist for $50 with everything you need. Not like you wouldn't use it anyway.

 

[Metaldrgn]

This site seems to have some good info. http://www.fullsizechevy.com/forum/...ntenance/488286-misfire-service-bulletin.html

To me most likely would seem to be a collapsed AFM (Active Fuel Management) lifter

There's also a video on youtube by ericthecarguy where he swaps out the engine and tears the old one down on a tahoe like yours for similar problems I think. I didn't watch it all, but may give you some insight if the lifter isn't what's bad.

 

[SubZ3r0]

This site seems to have some good info. http://www.fullsizechevy.com/forum/...ntenance/488286-misfire-service-bulletin.html

To me most likely would seem to be a collapsed AFM (Active Fuel Management) lifter

There's also a video on youtube by ericthecarguy where he swaps out the engine and tears the old one down on a tahoe like yours for similar problems I think. I didn't watch it all, but may give you some insight if the lifter isn't what's bad.

That page has some great info. Kinda confirms what I have been thinking. I do hear a squeek or chirp under the valve cover on the 2/4/6/8 side. Ill just do a compression check *I went out and bought the kit for $50* so that will save on some of the labor and then ill bring it to the dealership for the fix.

 

[Metaldrgn]

If you get low compression you may still want to do a leak down check since that checks sealing integrity and doesn't rely on the lifters working properly. Sticking valves can cause issues too but you can isolate that.