Misfires, all cylinders

Hey, there. I have and '11Colorado, I5, 4wd, with trim issues and occasional misfires.

Let me start by saying that if you were to just drive this vehicle, you would likely not notice anything out of the ordinary. But, when I bought it a year ago, I went out of my way to decide that I was going to learn more about working on my own vehicles. I'm fairly mechanically and diagnostically inclined, I just never got very far into the automotive side of things. So, when I got this one, I also quickly got a scan adapter and began trying to learn how all this works.

So, I notices that the LTFT stayed a little high. They went from about 9.4 - up to 13, at all speeds under acceleration. Found a small vacuum leak in the resonator (patched with liquid plastic), changed the MAF, MAP, and both O2 sensors, changed the plugs, and cleaned the throttle body twice. Not all at the same time (and yea I realize that I prob could have skipped some of that). Found oil in the plugs, so found and repaired the valve cover gasket (paid a pro, it's a PITA on these).

So, the issue seems to get a little better at times, but it always returns to stay. Sometimes the STFT spikes all the way to 29 for about a second. This seems to correlate with a misfire or two on random cylinders. But, I looked back through some old logs and realized that there were a couple of times that it didn't. What actually seemed to be going on was that the "Fuel flow rate" was dropping very low, or to zero. And, I'm guessing that that is what is actually causing the misfire. Today while digging around, I noticed that I punch it when in park, I can recreate the misfires at about 1500-2000 rpm's. If I gradually reach those rpm's, I didn't notice it. I've also checked the pressure at the rail before, and it was within spec. But today, the PID read up to 74 with KOIO, when 50-60 is spec (but I don't know how reliable that is).

I know I've written a lot, but the quick version:

* High LTFT when gas is applied, all loads. Idle trim fluctuates.
* Random misfire on any cylinder. Sometimes 2, but never all at once.
* Noticed that "Fuel flow" drops or is at zero.
* Can create misfires when gas is applied sharply
* There is no serviceable fuel filter on these vehicles.

I'm afraid it's the fuel pump, which is inside the tank. But, any thoughts? I can provide some previous logs or take new ones if it would help. Thanks!

Have you put a fuel pressure gauge on it? I think these have a schrader valve on the fuel rail. See if it drops to 0psi

Previously yes, but it's been a while back. I can try to do it again tomorrow. The key on level was within spec.

Are you saying to see if it drops while running and pushing the accelerator?

Yes key on engine off isn’t really a stress test on the pump. See if it drops out when it starts running rough accelerating as you describe

Hey, so this is the best that I could do with my crappy gauge. Everyone is out of town for Christmas, so I didn't have anyone to help out. Does this even tell you a whole lot, or do I need to go borrow a better one from a parts store? Snap starts at about 6 sec.



Thanks!

Think you need to tape the gauge to the windshield and do a wide open throttle run where the cops won’t get ya. I think it dropped below 40 psi in the video. What’s spec supposed to be?

I just went through about 5 pages of Google, and my analog Haynes manual. I don't see it. All I can find is the ignition off spec, which is between 50-60 psi (I think that was fine).

Haynes also mentions not wanting to see the pressure drop to quickly after turning it off, Ima go check both again...

Ok, so key on was def fine. After shutting off the engine, the psi actually increased by about 12 psi.

I can't watch the gauge at the same time, so I took another video for my self doing a slow rev to about 2500. Other than the shaking, it seems to read fairly steady between 40-45. PID gauge reads staying at about 43.5 give or take 1/2 point.

So, I think we've established that fuel pressure drops significantly under a quick load (not the same thing as 'heavy', right?), and climbs over 10 psi after ignition is off.

Does that tell us anything?

Cause and effect? or effect or cause ? And ignition misfire will look to the upstream O2 like a lean mixture and cause the engine computer to add fuel. An ignition secondary misfire is most like to occur during acceleration when the cylinder pressure at the end of compression is higher.

Hey, so I understand what you re getting at in general terms, that correlation does not imply causation.

But it would seem unlikely that a lack of fuel "flow" would be the result of a misfire, if the O2 called for more fuel.

And to your second point, if it was a cylinder problem, wouldn't it be more readily reproduced in only a particular cylinder, instead of various?

It would seem that the cause would need to be something that potentially effected all cylinders.

But, I'll completely admit that my understanding of the systems is still new. I may be misunderstanding you, or simply not have the knowledge to understand more specifically what you are getting at.

Any help is appreciated!

Are you ever actually noticing a misfire while driving? Or are you basing that solely on the misfire counters? Likewise, do you ever notice a lack of power, or are you only concerned about the trim numbers?

I gotta be honest, after reading the thread, I don't think there's anything wrong. :silly:

Sometimes the STFT spikes all the way to 29 for about a second. This seems to correlate with a misfire or two on random cylinders.

Was this while you were letting off the throttle? Because that's pretty typical for newer GM's when the PCM will shift into Open Loop and/or fuel cut decel with a closed throttle. Here's a shot from a '12 Captiva with very similar behavior:

* Random misfire on any cylinder. Sometimes 2, but never all at once.

How high do these counts go up to? Does the History PID on any cylinder ever show counts? Again, I sometimes see this on known good vehicles. The misfire monitor isn't perfect. :lol: If the miss counter ticks up to 2 then disappears, and the History never shows anything, I usually don't worry about it.

What scanner are you using, specifically? I was surprised to hear about a 'fuel flow rate' PID, as I've never heard of that on GM products. The Fuel Pump Control Module scan data is your best bet for ID'ing a pump problem. It'll have actual pressure, desired pressure, as well as pump trims. Keep in mind that desired pressure will change based on driving conditions, and isn't static.

Hey, there. Thanks for taking the time to reply. I work in a busy downtown area, so this is one of my busier weeks. Happy New Year! :whistle: :ohmy: :pinch:

What you had said had definitely crossed my mind, that there's not really any major issue, but I couldn't find any evident in research to support that. But, that's part of why I came here. I'll defer when needed to those with more practical experience and knowledge than I have!

You can see above that it def occurs when I snap/punch the throttle. (Do you make anything of what looks like a 10 psi drop on that shitty gauge?) I have noticed it occasionally when letting off/coasting as well, just not as often.

I'm using Torque. I have a hard-wired computer set-up as well (forget which program, not one of the major ones, but def usable), but I use Torque as the one to watch in real-time and to log trips. I have a screen set up with all 5 cylinder's misfire counters and their history, and it will show 2 for a few seconds, move it to history, and then erase it after maybe 20 seconds. Occasionally I'll get only a count of 1 misfire, but 90% of the time it is a 2-count.

I have a little bit of a trim issue, regardless, but nothing that I would be immediately worried about if not for this additional weirdness, at least in my mind. The LTFT can stay in the 9.5 - 12.5 range on the interstate. Your pick shows a spike up to 15, and my #'s are showing a spike up to what looks like it's maximum, but you think it might be just be open-loop anomaly? I'd be cool with that!

But no, I would not likely have noticed much if I weren't looking at the #'s. I'm definitely not looking for a problem just to find one, but I'm using this truck to learn about things. I've had it for almost a year and noticed the slightly high fuel trims a while back, and have been as much exploring, in order to understand better, more than I've actually been "concerned". The crazy spike and misfire thing just brought me down a little bit of a rabbit-hole to the point that I've thought something like, "Ok, I actually need to figure out what's going on and take care of that."

classic fuel delivery problem... most likely a fuel pump going out. snap throttle says it all. any time you have a loss of psi in fuel pressure when you raise RPM is a fuel delivery problem. the misfires are a result of the lean condition. pcm is adding fuel that's why the high trims. check your power and ground at the pump. if you have an amp meter you could check pump amps and see the motor failing. check your fuel pressure regulator for fuel leakage in the vacuum line. if no fuel leaking, its the fuel pump. do all the test before replacing any part. check fuel lines, filter and the power and ground of the pump.

ScannerDanner has many videos related to this issue.

Yea, as you can see from the last line in the original post, that's kind of what I was initially leaning towards...

Unfortunately I have a medical thing that's gonna put me out of commission on much lifting for a little while (like moving a truck bed, or cranking on a stuck bolt), but I may be able to at least check the voltages next week. Even though I don't expect to find an electrical issue, it'll be a good learning experience.