You might clean the throttle body and check what the fuel pressure is at idle and when you rev up the motor to say 3k rpm.

 

Hmm okay, I actually did clean the throttle body today. I can definitely also check the fuel pressure at 3k, but there typically are no sputters under load, especially anything higher than 1500rpm. It runs smooth on the road and the freeway.

I’ll check it anyway and see what it comes out to be.

 

I'm assuming you don't have any engine lights or codes? Do you have access to a scan tool? If so you can take a look at fuel trims at idle and while increasing RPM. While it could be a throttle body or TPS issue, failed (or failing) oxygen sensors or issues with the catalytic converter, as well as fuel pressure like Davester mentioned, could be causing your issue as well. I've fired the parts howitzer at my truck before as well, I'd hate to see you keep swapping parts without identifying the true cause

 

Correct. No codes. Yes, I do have a scanner with fuel trims feature, I’ll check those too.

Indeed, I don’t like throwing money at problems without clear diagnosis either. The GM TPS is $95 and the GM TB that comes with the TPS is $219, but given that the GM one is failing (presumably), Dorman also makes a TB/TPS combo unit for $109, that I would try, so not all that terrible if it fixes the issue.

I’ll check the fuel trims and fuel pressure before replacing the TB/TPS.

Do you know off hand what kind of trims and/or pressure I should be looking for?

 

I'm not a total expert on fuel trims, but I have a general understanding. Ensure the engine is warm before checking fuel trim, as the system will be in "open loop" and the readings wont be of much value. The scan tool should say something like OL and CL with the fuel trim bank reading telling you it is in open or closed loop. My understanding is they should generally be pretty close to zero (within a few numbers) at idle. High negatives means you have a rich condition and high positive is lean (I also think the max either way is -25 and +25). I would see what they look like at idle and also compare banks one and two against each other, it may point you to a specific set of cylinders causing the issue if it is fuel related. You can also look at the O2 sensor data between the port and starboard banks which should be consistent.

 

I lent my fancy scanner out, so of course I don’t have it when I need it. I’ll check fuel trim once I get it back, and fuel pressure in the meantime.

If those are okay, I think I’ll have to reconsider the Dorman TB/TPS unit to see if that solves it.

 

I lent

when was the last time you put in new spark plugs?

 

Ok, finally got my bluetooth scanner back and had a chance to get some live data.

Long term fuel trim seemed very steady, so I didn’t really include it as comparison with other sensors.

All these charts show engine at idle over a 60 sec period (except for the last two that are noted with orange writing, which are 30 sec timelines).

The dips in Short Term Bank 1 are when the more feelable stumbles took place.

 

Here are the readings from the O2 sensors.

It appears that something is happening with the O2 sensor at Bank 1 Sensor 2.

Every time this sensor dipped low, the engine stumbled, and the more feelable stumbles are when both Sensor 2 (on both banks) dipped low.

This is 60 secs of engine at idle.

 

I figured I would compare the Sensor 2 (O2) sensors in comparison to Short Term Fuel Trims.

This is at idle. Whenever Oxygen Bank 1 Sensor 2 dipped is when a stumble was felt.

 

Since the Bank 1 Sensor 2 oxygen sensor seemed like an accurate reflection of when the engine stumbled, I wanted to compare it against some other modules.

Even at idle the MAF is fluctuating, and so is the intake manifold pressure.

 

I also checked engine RPM vs intake manifold pressure in comparison to the stumbles felt via the O2 sensor graph.

This is also at idle.

 

Interestingly enough… The EVAP pressure looks like it is building up and then releasing?

Is it normal for it to fluctuate this much, even at idle?

 

Also, interestingly enough… RPM and intake manifold pressure are a mirror image, even during a stumble (highlighted in orange notation).

When the engine stumbled (RPM dipped), intake manifold pressure spiked, along with fuel rate.

This was also at idle, the spikes on the right side of the graph are when I tapped on the throttle.

 

Absolute throttle position also spiked during a stumble while at idle.

The spikes on the right side of the graph are when I tapped the throttle pedal.

 

Does anything stand out as a potential culprit from this data?

 

Wow, this is a lot of info. It does look like that Bank 1 oxygen sensor is acting very erratically. It also correlates to the short term trims being off. If you look at the y-axes for your two fuel trims, Bank 2 is pretty centered around zero, but the left y axis which I believe is for Bank 1 is consistently off of zero and overall centered around 1-2. The system should be shooting for zero on both which is the optimal air to fuel ratio, so that seems odd. Since the oxygen sensors communicate with the PCM to tell it how much fuel to send, I'm wondering if there's an issue with the bank 1 oxygen sensors. I'm not trying to send you on a wild goose chase, because it could also be the oxygen sensors reacting to something else affecting that fuel ratio...

 

Indeed that was my first thought as well… However without clear indication, it might be difficult to diagnose without throwing some money at it and start replacing potential culprits to rule things out.

One other tidbit here… I did just replace a broken exhaust manifold bolt that was causing a small exhaust leak on the driver’s side (Bank 1), which I was hoping would cure this stumble, to no avail.

I’m temped to replace the post-cat O2 sensor and see if that changes anything, but perhaps I’ll run some more live data first.

 

Post cat sensor won't change it. That is just used to check how well the cat's are working, it's not used for controlling how the engine is run.

 

Ahh okay, got it.

Btw I ran some more live data to compare both sides of pre-cat and post-cat O2 sensors.

Pre-cat sensors on top, and post-cat sensors at the bottom…

Is it normal that the each side of sensors show such varying results from each side?

 

Well, today I noticed that the stumbling is not only during idle, but even at low RPM even when moving.

And when going up a big hill, it seemed to be happening more, and slightly underpowered… and I also heard a few clunks from what sound like the transmission.

How can this be related to the transmission??

 

So… Installed fresh O2 sensors (all four), new MAF, TB and TPS… Stumble still there. Driving me nuts.

And I did find another broken exhaust manifold bolt on the passenger (near the firewall, of course), so I’m hoping it’s somehow related. I’ll extract it next week.

Otherwise, maybe dirty injectors?? WTH else could it be??

 

Wish I could help you more with the symptoms, and I hate for you to keep swapping parts out because sometimes you'll practically build a new truck and still have the issue. It's almost acting like its periodically losing fuel. Have you measured fuel pressure while it's running to see if maybe the fuel pump is going?

 

Indeed it feels like I’m rebuilding the ignition system to start, but glad I haven’t replaced the fuel pump or coil packs… yet

I was just about to measure fuel pressure, but then the check engine light finally did come on with misfire on cylinder 2 (at least now I know I’m not crazy!), so I’ve been chasing this down.

I swapped coil packs, misfire again today on #2. Now I switched the spark plug wire to #4, and the spark plug to #6. We’ll see if the misfire moves.

I also did a compression test on #2 and #4, which measured at 170ish and 175ish, respectively.

The saga continues…

 

Solved!

Just wanted to follow up this thread with a final update.

Turns out it was a faulty spark plug wire… brand new AC Delco!

I’ve always sworn by AC Delco wires, but seems like the quality control may have slipped up.

I’m wondering if it’s been that way for a while, given how widespread this issue seems to be (based on so many others on the internet who have simply chosen to live with a slightly rough/stumbly idle because even the dealer had told them there is nothing they can do about it)… was it the AC Delco wires all along..?

Anyway, I went with the red Edelbrock Max Fire wires this time and must say I am pleasantly surprised. The quality is impressive, and they come with new heat shields too.

My idle is now absolutely, perfectly stable… it literally doesn’t bounce or rumble at all.

Time will tell how they hold up, I’ll update this thread if anything does change.

 

I’ll post some EKG readings as well as comparison to the stumble idle.

 

Thanks for posting the update.

 

Really glad you figured it out 👍🏻

 

Yowie

thank you my 2015 was doing the same thing. Thought it was trans at first because of the sputter at low rpm and idle. Found your thread changer my wires and bam. Fixed.
Thanks again