Another 19 GT P0300 - Misfires

[pmor4243]

Hoping I can get some help here on problem that popped up out of nowhere. Got a CEL a couple of days back for a P0300 - Random Misfires and am having a hard time figuring out how to fix this. The car has been 100% fine for the last 2 years and no parts/tunes have been changed or added.

Mode 6 data shows misfires on every cylinder without any once cylinder being obviously worse than the rest. Cleared the code but it will come back or at least show pending fairly quick. Datalogging what is going on, I'm seeing all of the misfiring happening at very light throttle when cruising, outside of that really no misfires. But with light throttle cruising, several hundred misfires will take place within just a minute or two.

So far I have:

• I drained my fuel and filled up with fresh fuel from a different station
• Changed out all of the plugs with brand new NGK 6510s (all gapped)
• Compression test (all cylinders were good)
• Smoke tested my intake manifold (saw no signs of leaks)
• Reloaded my tune just in case
• Ran a crank relearn

None of this has made any difference.

I did just put on a new serpentine belt the day before all this started happening due to it starting to squeal. Prior to the new belt, I degreased and sprayed down the front of the engine to get things cleaned up and any gunk off my pulleys. I did not soak the entire motor, but I'm not sure if water could have made its way somewhere that would be causing this to now happen.

At this point, I'm not really sure what else to check. I guess maybe the crank sensor or MAF could be replaced. Anyone have any ideas or have any experience with misfires like this only at very light throttle?

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[SheepDog]

What do your fuel trims look like?
Have you verified that you have good consistant fuel pressure?
A broken Intake manifold runner shaft (IMRC) can cause this. You would need to remove the intake manifold and see if the shaft is broken. Usually you would also have a code for IMRC stuck open. P2004, P2005

 

[pmor4243]

What do your fuel trims look like?
Have you verified that you have good consistant fuel pressure?
A broken Intake manifold runner shaft (IMRC) can cause this. You would need to remove the intake manifold and see if the shaft is broken. Usually you would also have a code for IMRC stuck open. P2004, P2005

I’ll grab a log looking at those things specifically. As of right now, I’m only getting the P0300 code.

 

[pmor4243]

Here is a screenshot from a log of a 7 minute drive just going down the road by my house. Total of 463 misfires logged in that period and all in the conditions seen in the screenshot below.

Nothing really jumps out to me as abnormal. If I hadn't had a CEL, I actually wouldn't have a clue anything was even wrong. So far, I have not felt or heard a misfire, the car idles fine, and during the times when the misfire count is actively increasing, I can feel nothing abnormal in how the car is behaving.

• Around 5-8% throttle
• Fuel trims are all within 2%
• Fuel Pressure looks normal

Mode 6 data from this drive. I think I only have 2 short drives after the tune reflash so no codes yet, but it is right at the threshold.

Based on what others have posted with a generic P0300, it seems like some have had issues/success with the O2 sensors, Crank Sensor, and IMRCs (still applicable on the 18+??) as mentioned before. I just don't want to keep throwing parts at the car if there is an intelligent way to go about diagnosing this.

 

[pmor4243]

Pulled the intake manifold and didn’t see anything broken or stuck with the IMRCs…

Crank sensor is the next cheapest in line then o2s so I guess I’ll go in that order unless anyone has some insight on something else to look into first.

 

[Call_Me_Bruce]

Fuel pressure?

 

[pmor4243]

Fuel pressure?

According to the logs, fuel pressure looks fine.

 

[NGOT8R]

Plug gap?

 

[SheepDog]

It's pretty unlikely to get almost the exact same amount of misfires on all 8 cylinders, and only at low RPM, low load situations. What did your tuner say about this?

Maybe a bad knock sensor

When you removed the intake manifold, did you verify that the vacuum source to the IMRC pods is intact and not cracked or broken?

 

[pmor4243]

Plug gap?

Installed brand new plugs gapped at .028. That was actually the first thing I did, but no luck there.

It's pretty unlikely to get almost the exact same amount of misfires on all 8 cylinders, and only at low RPM, low load situations. What did your tuner say about this?

Maybe a bad knock sensor

When you removed the intake manifold, did you verify that the vacuum source to the IMRC pods is intact and not cracked or broken?

I agree, that's why I'm so confused at this point. Its globally bad yet everything seems to look fine. They are not all exactly the same but Id say they are all within 50 or so misfire events which isn't much when they are all in the hundreds.

The intake manifold and IMRCs all looked really good. I did not see anything visually wrong with them or the vacuum lines, but I will admit, I didnt do any kind of pressure test on them to verify there was not a crack somewhere I couldnt easily see.

I have yet to reach out to Wengerd as this has not been a problem for the past couple of years running his tune, so I'm very doubtful it is tune related. Not saying it isn't, but I likely would have seen this a long time ago if that was the case.

 

[pmor4243]

Took off and inspected the intake manifold last night. Did 2 short drives this morning. I have not had a code come back up yet, however, the car is still registering a lot of misfire events, still all low low and low throttle, so it will likely present itself soon.

Out of curiosity and for my sanity, if anyone could do a short drive (like 5-10 minutes) and log their misfire count, I would love to see what values the rest of you are seeing. I tested this on my Raptor and only logged 4 misfire events in a 15 minute drive.

 

[SheepDog]

Took off and inspected the intake manifold last night. Did 2 short drives this morning. I have not had a code come back up yet, however, the car is still registering a lot of misfire events, still all low low and low throttle, so it will likely present itself soon.

Out of curiosity and for my sanity, if anyone could do a short drive (like 5-10 minutes) and log their misfire count, I would love to see what values the rest of you are seeing. I tested this on my Raptor and only logged 4 misfire events in a 15 minute drive.

Send your log to Wengerd. He is always willing to help his customers.

What are your knock sensors showing during these events? Did you get a bad tank of Gas? Are you on E85?

 

[pmor4243]

Send your log to Wengerd. He is always willing to help his customers.

What are your knock sensors showing during these events? Did you get a bad tank of Gas? Are you on E85?

Ya I may need to reach out. Knock sensors are not showing anything. I thought I had bad gas so I drained the tank and filled up with new gas from a different station and threw some boostane in there to be sure. Technically that was my first step before plugs, but that did not seem to be the issue.

 

[Call_Me_Bruce]

Have you replaced any OEM sensors with aftermarket parts or OEM parts purchased off of ebay at a really good price?

 

[pmor4243]

Have you replaced any OEM sensors with aftermarket parts or OEM parts purchased off of ebay at a really good price?

Nope, I have yet to replace any sensors other than a MAF that I messed up on initial install, but that was replaced with a factory sensor from the dealership. Any other hard part has always been replaced with OEM.

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