8.1 Vortec phantom misfire

Brandon Roy · September 5, 2020

Hello, new user with a strange issue here. I've got a 01 Suburban 2500 thats throwing misfire codes but doesn't seem to actually be missing at all. Codes are P0300 and p0307 that show up like clockwork after clearing the computer. Check engine light is NOT illuminated, motor firs right up and runs smooth and strong, doesn't feel like it's misfiring at any point. The number 7 spark plug came out looking beautiful with no fouling, plug wires all look and tested good, swapped the number 5 and 7 coils around only to have the misfire stay on cylinder 7. The truck has been into my local dealer and has them totally stumped as well, (no fuel issues, no egr problems, no vacuum leaks, all cylinders have strong and consistent compression, no weird data from any of the various sensors) And yet the codes are still showing up.

Is it possible the computer is faulty, and would it be worth my while to have a re-flash performed on it? Any help would be greatly appreciated.

EDIT: Previous owners replaced the engine 10 years ago after they ran the original one out of oil, not sure if this is somehow part of the problem.

Edited September 5, 2020 by Brandon Roy

davester · September 5, 2020

Maybe

-look at the knock sensors electrical output with an oscilloscope

-look in the cylinders with scope via the spark plug, and with it up the intake and exhaust to get a look at the valve seats

Brandon Roy · September 5, 2020

28 minutes ago, davester said:

Maybe

-look at the knock sensors electrical output with an oscilloscope

The techs at the dealership have done this, with no abnormal results

28 minutes ago, davester said:

-look in the cylinders with scope via the spark plug, and with it up the intake and exhaust to get a look at the valve seats

The new motor only has about 32000 miles on it, so i doubt the valve seats would be that badly worn already. However stranger things have happened.

Doug_Scott · September 5, 2020

From your description it sounds like the truck is not notifying you of these error codes, you are going looking for them. If it is not setting the CEL, maybe ignore it until it does. The ECM is the last place to look, and if you cannot say for certain(as in the truck is dead and has visible damage to the ECM) then keep looking at traditional causes. Its been a while, aren't some codes stored as historical and will only clear themselves after so many starts?

Don't think that just because something is new it can't be defective. Parts have warranty just for that reason. With that being said, have you checked the valve springs?

Brandon Roy · September 5, 2020

2 hours ago, Doug_Scott said:

From your description it sounds like the truck is not notifying you of these error codes, you are going looking for them. If it is not setting the CEL, maybe ignore it until it does. The ECM is the last place to look, and if you cannot say for certain(as in the truck is dead and has visible damage to the ECM) then keep looking at traditional causes. Its been a while, aren't some codes stored as historical and will only clear themselves after so many starts?

Don't think that just because something is new it can't be defective. Parts have warranty just for that reason. With that being said, have you checked the valve springs?

I had originally went looking for codes because it feels like the TCC is randomly locking/unlocking at highway/freeway speeds. I believe misfires can cause the computer to command the torque converter to unlock when detected. I've been clearing the codes out of the computer with my scanner but they always seem to return and stick around after driving for a short while.

I hadn't thought of the valve springs, I will mention that to tech today.