Random Missfire

rjgogo · January 28, 2012

I have a random missfire going on it happens only in specific situations. Vehicle is a 2003 Suburban 2500 with the 6.0.

The light only flashes when I am crusing on the highway and the tach is at about 2600 or 2700 RPM. If I am accelerating through that range it does not occur, and infact it never occurs when I am accelerating from zero all the way up, only when I am cruising and it does not happen all the time. It will flash for a minute and then won't flash again for a while and then it will happen again.

The light does not stay lit after the event but the dreaded P0300 code is set. No idea what cylinder. I have replaced the plugs and used a couple of tanks of fuel with Techtron injector cleaner, and changed the fuel filter but none of that has helped. I have also checked the fuel pressure and it is within specs.

I have misted water on all the coil packs and wires and no arcing. I have 5 also cracked open the valve on a propane cylinder and kind of dusted it all over there and no change in engine speed. (I did this outside BTW and only a little at a time) I can see no evidence of any cracked hoses but I could have missed something.

My code reader is pretty old and has already paid for itself about 100 time over so I am not opposed to getting something new that would assist in the diagnostics but would prefer it be usable with a mac or iphone ap

Any suggestions and where to go next besides the dealer? Is there something to tell me which cylinders are missfiring and how often.

Just when through this on my old BMW and it threw a code that told me cylinder 5 so it was about a 20 minute fix. This is random and I am out of diagnostic ideas.

Gurrzt · January 29, 2012

If your "SES" light (refering to 01 Z28) is "flashing", and you are throwing a "P0300" code, your vehicle is running in "limp" mode. This means instead of using the normal sequence of firing the cylinders, you are firing one bank, than the other (left, then right).

The problem is MOST likely that your ECM has lost the relationship of the cam sensor to the crank sensor, if this is the case, you will need to do a "relearn" proceedure on a GM Tech II or above machine, or one of the aftermarket companies that make the machine. Most professional automobile repair facilities have this machine, as it is now a standard to work on these vehicles.

The worst cenario (sp) is that the "cam sensor" is defective, and needs to be replaced. It is a $20 plastic sensor, and only $200 worth of labor to get to it (on my 01 Z28 (Sara) it was under the intake). If that is the case you need to do the cam crank relearn proceedure after the install.

GOOD LUCK!!!!!

rjgogo · January 29, 2012

If your "SES" light (refering to 01 Z28) is "flashing", and you are throwing a "P0300" code, your vehicle is running in "limp" mode. This means instead of using the normal sequence of firing the cylinders, you are firing one bank, than the other (left, then right).

The problem is MOST likely that your ECM has lost the relationship of the cam sensor to the crank sensor, if this is the case, you will need to do a "relearn" proceedure on a GM Tech II or above machine, or one of the aftermarket companies that make the machine. Most professional automobile repair facilities have this machine, as it is now a standard to work on these vehicles.

The worst cenario (sp) is that the "cam sensor" is defective, and needs to be replaced. It is a $20 plastic sensor, and only $200 worth of labor to get to it (on my 01 Z28 (Sara) it was under the intake). If that is the case you need to do the cam crank relearn proceedure after the install.

GOOD LUCK!!!!!

Thanks, the engine runs as sooth as can be and it is intermittent. If the cam sensor needed to be relearned wouldnt this happen all the time?

Gurrzt · January 29, 2012

Thanks, the engine runs as sooth as can be and it is intermittent. If the cam sensor needed to be relearned wouldnt this happen all the time?

No it should not, if it does you either have a bad sensor or a bad reluctor ring.

zedbear · February 5, 2012

I get this code and the flashing when coasting down long hills on the interstate with the throttle closed or mostly closed and ONLY when towing something. Runs fine. I was thinking perhaps a slight intake gasket leak but perhaps I will check into the cam sensor. I used to have an 89 Lesabre that would do the same thing (not towing) when coasting down hills. Wierd.