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Misfire!!!!! What should I test next???

Fabian505

Full Member
Joined
Oct 14, 2021
Messages
11
Ok guys, i assure you that before posting this, i have done lots of research and im hitting dead ends. Here goes;

New to me 07 avalanche with a misfire on cylinder 6 (please keep reading before saying AFM issues, maybe it is, but Im not there yet). So the misfire only happens once a day if i drive consistently or after being parked for over 2 hours. Typical scenario- back up from my driveway with no issues, drive for about 2 miles, and flashing check engine light with light engine shake, it blinks for about 2 or 3 minutes then back to normal, sometimes the check engine stays on, sometimes it turns off, i do get the stabilitrack warning light as well. After that, no issues, and never a lifter tick even when its misfireing, engine has about 20k miles acording to previous owner, truck has 195k miles.

This is what I have done so far:
1. Moved coil from cyl 6 to cyl 3
2. Move injector from cyl 6 to cyl 2
3. Replaced all spark plugs and wires (only because they were Autolite and figured AC Delco would be better)
4. Checked for vacum leaks with carb cleaner and nothing found.
5. Ordered a Range to keep it in 8 cyl mode. I will receive that today, so I have not installed it yet. I will update when i test it.

Misfire cyl 6 still coming up, no other codes.

Like I said, no lifter tick and the problem goes away after just driving.

What would you check next? Any sensors? Wiring maybe? Drive it off a cliff? You tell me.

Thanks in advance!
 
Only code is P0306- cylinder 6 misfire.

Yeah, i guess im going to start doing the lifter no compresion testing. I keep thinking its not the motor just because it doesnt have lifter tick and runs strong, but might as well start ruling out or confirming bad lifter. I really hope its not the lifter.
 
I do not have an AFM engine, but I do read about other's experiences.

Am I correct in understanding that placing the transmission in tow mode turns off the AFM?

And how about shifting the transmission out of overdrive.

Perhaps you can do some testing with those until your Range device arrives.

Preventing the AFM from activating should imitate what the Range device is going to do.

Just a thought.
 
I do not have an AFM engine, but I do read about other's experiences.

Am I correct in understanding that placing the transmission in tow mode turns off the AFM?

And how about shifting the transmission out of overdrive.

Perhaps you can do some testing with those until your Range device arrives.

Preventing the AFM from activating should imitate what the Range device is going to do.

Just a thought.
Yup!!! That is a good easy and free test! Trying that as soon a i get out of work. I did not think of that. 👍
 
Ever Google code? plenty of repair sites explain how to go thru trouble shooting procedures as no sense guessing and just replacing parts willy nilly.






plenty more.
 
I do not have an AFM engine, but I do read about other's experiences.

Am I correct in understanding that placing the transmission in tow mode turns off the AFM?
AFM can still engage in tow/haul mode from what I understand. To prevent AFM, shift to 'M' and select a max gear of '3' for the 4-speed transmission (2007-2008 ), and '5' for the 6-speed (2009+).
 
Mine has a crazy mis at an idle/low gear and I'm praying it's just the plugs and wires. In the mail now!
Hopefully is just that. From what I understand, you should be good unless you can hear the lifter ticking. Its a very distinctive sound. Good luck! Let me know what happend after you replace the wires and plugs.
 
Ever Google code? plenty of repair sites explain how to go thru trouble shooting procedures as no sense guessing and just replacing parts willy nilly.






plenty more.
Thanks. Trust me I have done my research. The odd thing is that it only mises for a few minutes and then its back to normal. Its a lot harder to diagnose. I posted my question hopeing someone has seen it before and could point me in the right direcction. So far i have rulled out, coil, wire, plug, injector and vacum leak. Im not that good at chasing electrical but i would rather it be electrical than a lifter.
 
Hopefully is just that. From what I understand, you should be good unless you can hear the lifter ticking. Its a very distinctive sound. Good luck! Let me know what happend after you replace the wires and plugs.
I will keep you posted I should have time to throw em in this weekend!
 
FIXED!!! Kind off...

Ok. So I wanted to give anyone interested or with the same problem how I "fixed" my avalanche. So if you read the original post, i did try to isolate the cause of the misfire by moving components to diferent cylinders or by replacing common parts like plugs and wires. That did not change the original issue of cylinder 6 misfire or code P0306.

The fix was preventing it from going to 4 cylinders by using an OBD2 programer, in my case a Range from amazon. This is if you DO NOT have lifter tick. So the lifters in my case were not the problem. What is happening is that when the cylinders come back on, for some rason the injector starts spraying fuel, but the ignition system is not responding at the same time, for some rason it is delayed on my truck on cylinder 6. That problem or the actual root of the problem is NOT FIXED. But I was able to work around it by preventing the engine to go down to 4 cylinders. Even if you do not have misfires, many people recomend disabeling the AFM to maximize the life of your engine. Unfortunately the first years of AFM (07 to 09 on all GM V8s) are known to fail and require lifter and maybe cam to be replaced due to faulty AFM system. Im lucky it was an easy fix for me, maybe this will help someone else.

As of now, I do not have any codes or stabilitrack warning lights after installing the AFM desable. Knowing this, i might take my truck to an actual tuner to permanetly disable it, or send my PCM to a tuner. Best option would be to buy the components and rebuild my motor, but at this time, i do not think is necessary. For now Im just keeping the Range pluged in.
 
good news!

FYI the AFM will drain your battery so you must drive once a week or keep on trickle charger.
 
ygmn is correct, the older threads about the Range Technologies AFM delete device have mentioned that it can drain the battery if left plugged in in a parked AV. Best to unplug it if going to park for several days and not drive the AV. It should not be an issue if driven daily.
 
ygmn is correct, the older threads about the Range Technologies AFM delete device have mentioned that it can drain the battery if left plugged in in a parked AV. Best to unplug it if going to park for several days and not drive the AV. It should not be an issue if driven daily.
Yup. Thanks for the info. Its going to be my wifes daily driver so i will just leave it plugged in for now.
 
Hopefully is just that. From what I understand, you should be good unless you can hear the lifter ticking. Its a very distinctive sound. Good luck! Let me know what happend after you replace the wires and plugs.
Just got to replacing the plugs/wires this weekend and good news! it fixed the misfire but I also replaced my front impact sensor that was acting up and the airbag light is still on UGHHHHH. But on other good news I found out there are tps sensors in the tires so thats cool.
 
Just got to replacing the plugs/wires this weekend and good news! it fixed the misfire but I also replaced my front impact sensor that was acting up and the airbag light is still on UGHHHHH. But on other good news I found out there are tps sensors in the tires so thats cool.
Nice! Sometimes the fix is just good maintance. I know people say that spark plugs should last 100k miles, but theres nothing wrong with replacing them early. The air bag light is not good news, but not the end of the world. Could be another easy fix.
 
It's great that you have sidestepped your issue by disabling the AFM with the Range device. I see you have an '07 Avy which is the same as me. Just to let you know, you could have "disabled" the AFM by driving your truck with the transmission gear selector in "3" instead of "D" as BainMan suggested earlier in this thread. This disables the AFM but also keeps you from going into 4th gear, which can be an issue on the highway. This is what I did for the first few years I owned my '07 since I did mostly around town driving. I considered getting a Range AFM disabling device, but I didn't like the fact it had to always be plugged into the OBD port in order for it to work and I had read about the battery drain issues. I ended up buying a DiabloSport Predator P2 tuner for only $100 or so more than what the Range would have cost. It was the single best investment I have made in my truck in the 6 years I have owned it. You plug it into the OBD port, turn on the ignition, follow the instructions to disable the AFM and, while you are at it, flash the 87 octane performance tune to your truck. Then you turn your truck off, unplug the tuner and you are good to go. Nothing plugged into the OBD port and no battery drain - just a completely transformed driving experience and no AFM. The tuner also allows you to read and clear diagnostic codes and adjust a number of other settings.

If you are considering going to a shop or sending your PCM out to have the AFM disabled, that is going to cost you way more than buying a tuner and doing it yourself. Plus you won't get the advantages of a tune, unless you have the shop do one which will be even more $$.

Just sharing my experience FWIW. There are quite a few threads on this forum about tuners - you should check them out.
 
What SRS error codes are you seeing?
 
It's great that you have sidestepped your issue by disabling the AFM with the Range device. I see you have an '07 Avy which is the same as me. Just to let you know, you could have "disabled" the AFM by driving your truck with the transmission gear selector in "3" instead of "D" as BainMan suggested earlier in this thread. This disables the AFM but also keeps you from going into 4th gear, which can be an issue on the highway. This is what I did for the first few years I owned my '07 since I did mostly around town driving. I considered getting a Range AFM disabling device, but I didn't like the fact it had to always be plugged into the OBD port in order for it to work and I had read about the battery drain issues. I ended up buying a DiabloSport Predator P2 tuner for only $100 or so more than what the Range would have cost. It was the single best investment I have made in my truck in the 6 years I have owned it. You plug it into the OBD port, turn on the ignition, follow the instructions to disable the AFM and, while you are at it, flash the 87 octane performance tune to your truck. Then you turn your truck off, unplug the tuner and you are good to go. Nothing plugged into the OBD port and no battery drain - just a completely transformed driving experience and no AFM. The tuner also allows you to read and clear diagnostic codes and adjust a number of other settings.

If you are considering going to a shop or sending your PCM out to have the AFM disabled, that is going to cost you way more than buying a tuner and doing it yourself. Plus you won't get the advantages of a tune, unless you have the shop do one which will be even more $$.

Just sharing my experience FWIW. There are quite a few threads on this forum about tuners - you should check them out.
Thanks for the info. You are right, i would of got a lot more by just spending 100 more dollars. Defenetly on my list of uogrades for the Avy. Its probably going to have to wait until after the holydays unless I see a crazy deal for black friday.
 
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