Summoning the geniuses of the forum: Problem accelerating from a stop / hill, engine stumble [updated]

[davidavidd]

Hi TYF friends, I'm having a hard time finding a diagnosis for this problem on my Tahoe (2007, stock Engine 5.3, RWD, 218.000mi, 275/55/R20) so I humbly ask for your collective help, sorry for the long post:

When accelerating from a stop the engine acts as if it freezes for 1 second, as if I were trying to pull out a tree stump, like backfiring (sorry I can't describe it well in English).
If I let go of the accelerator it stops happening, but if I accelerate a little more it repeats one additional time and then continues accelerating as if nothing happened. On two occasions when I tried to accelerate the engine completely dies, I was left without power steering and brakes, not pleasant at all, fortunately in the area where I live in Colombia there are no highways, only urban roads.

When I started the engine again I got a horrible idle, vibrating like it was running on Flintstones gasoline. I turned it off again and when I turned it back on 10 seconds later it was normal with smooth idle at 600rpm.

But here's what's got me thinking:
If I accelerate very gently it does not happen and I can reach normal speed. And if I fully apply the throttle (WOT) in parking/neutral nothing happens and it reaches 4000 rpm, there is no hesitation/vibration like a new engine, you can see it in the attached videos.

Videos:

1. https://youtu.be/LH3FueoAwIo (00:27, I was pressing the brake and trying to accelerate, you can hear the engine and see the RPMs bouncing)
2. https://www.youtube.com/shorts/W_O7a98SEXg (street driving, the problem is visible and audible between 00:03 and 00:06, RPMs bouncing but after accelerating I was able to continue normally to home, when parking the engine stall, I get into my house and 10 minutes later I was able to drive another half hour for food, it didn't turn off or fail again).

Background story:
Last week the CEL came on - first P0496 and then the problem started P0336.

I thought it was going to be an easy fix, I was about to change the EVAP system a few days ago so I did, new vapor canister and solenoids. It rained carbon pellets when I blew out the line that goes to the engine and the one that comes out of the gas tank (I didn't drop the gas tank just used compressed air).

The P0496 code disappeared and I continued with the P0336: new CKP, relearn with the scanner, the P0336 did not appear again. I thought the problem would be fixed, stupid me, apparently the P0336 was just a symptom not the cause.

What I have checked or concluded so far:

- I have already used up the entire tank of gas and visited another station to rule out bad gas.
- It happens with the engine cold or hot, in open or closed loop.
- The truck always starts normally.
- There are no new error codes, no misfires, even when the vibration occurs.
- I opened the differential, changed the oil, nothing out of the ordinary.
- The transmission was repaired a year ago, it is not the torque converter clutch because the failure also occurs in the first gear. Temp OK, oil looks good and run fine.
- There are no apparent vacuum leaks (I have not tested with a smoke machine).
- The oil and coolant look and smell normal, there is no white or black smoke, oilky milk shake or strange odors.
- The MAP, MAF and throttle body sensors act normally.
- It does not appear to be an electrical problem, the battery is fine and all accessories work.
- I do not have enough experience to interpret the oxygen sensors, in the video 1 you can see that when the failure occurs the sensors go flat.
- All cylinders are working, with the scanner I activated and deactivated them one by one with the engine balance test function.
- It does not seem to be the AFM system involved, because when it happens is in V8, no ticking noise, oil pressure OK.

I'm getting a fuel pressure gauge, that test is pending and I think that's where I should have started.

Do you have any theories to continue hunting the failure?

 

[Bigburb3500]

Have you tried a new MAF sensor? That is an expensive single part but from my previous Suburban I had, the truck would hesitate and shut off regularly because it could not correctly mix the fuel/air ratio. It did not have a specific code and thru random codes. I believe one symptom was the “dancing” RPM needle.

 

[petethepug]

If you haven’t replaced the f/p after that milage, it’s due 100k miles ago. Rock Auto & Parts Geek have the best pricing on OEM f/p that carry a lifetime guarantee. Local stores get about $2h more for OEM f/p.

Replace it, swab the tank of pellets and blow the lines again. If the problem persists, the battery cables were due 100k miles ago too. They rot from the inside out and do a lovely job of freaking the truck out and making false DTC.

Once both of those are done anything else you find can be addressed systematically without doubt.

btw… nice job on attacking the charcoal canister. That’s a win.

 

[Fless]

Compare and graph the throttle position sensors with a good scanner. Dropouts can cause stumbling.

 

[davidavidd]

Have you tried a new MAF sensor? That is an expensive single part but from my previous Suburban I had, the truck would hesitate and shut off regularly because it could not correctly mix the fuel/air ratio. It did not have a specific code and thru random codes. I believe one symptom was the “dancing” RPM needle.

Nope, but what I did try was to test with the sensor unplugged and the hesitation occurs anyway. So for now that sensor I think may be out of the equation.

 

[davidavidd]

If you haven’t replaced the f/p after that milage, it’s due 100k miles ago. Rock Auto & Parts Geek have the best pricing on OEM f/p that carry a lifetime guarantee. Local stores get about $2h more for OEM f/p.

Replace it, swab the tank of pellets and blow the lines again. If the problem persists, the battery cables were due 100k miles ago too. They rot from the inside out and do a lovely job of freaking the truck out and making false DTC.

Once both of those are done anything else you find can be addressed systematically without doubt.

btw… nice job on attacking the charcoal canister. That’s a win.

I will keep that in mind, my dad knows a lot about cars too and gave me the same advice.

 

[davidavidd]

Compare and graph the throttle position sensors with a good scanner. Dropouts can cause stumbling.

I'm going to do that today, first of all I'm going to clean it.

 

[j91z28d1]

a fuel pressure gauge hooked up and run up to the windshield so you can see it while driving would help know if the fuel pump can keep up with the fast throttle movement.


I haven't watched the videos, but you said the o2 voltage went flat line, as in 0 voltage. that sounds like you're having a lean bogg. which also leads to the fuel pump can't hold pressure. checking it with a gauge would be helpful. even thou yes, it's passed it's replacement time. I have 160k on my stock pump, it ready for a change too but all these new parts are so low quality these days, I'm scared to touch it. seems like everything I do to this this truck takes doing it over 2 to 3 times to get it right over something.

 

[davidavidd]

a fuel pressure gauge hooked up and run up to the windshield so you can see it while driving would help know if the fuel pump can keep up with the fast throttle movement.


I haven't watched the videos, but you said the o2 voltage went flat line, as in 0 voltage. that sounds like you're having a lean bogg. which also leads to the fuel pump can't hold pressure. checking it with a gauge would be helpful. even thou yes, it's passed it's replacement time. I have 160k on my stock pump, it ready for a change too but all these new parts are so low quality these days, I'm scared to touch it. seems like everything I do to this this truck takes doing it over 2 to 3 times to get it right over something.

Thanks for your reply, you're right, sometimes getting good spare parts is difficult. This is the second pump on the truck, the first one died a few miles ago due to gasoline contaminated with dirt.

 

[R467X]

Definitely sounds like potentially a fuel pump pressure problem/ fuel pump starting to go. Like others said, get a gauge. Also consider running a quality fuel system cleaner, I always run Red-line every 6 months or so.

I also think someone here mentioned electrical. Would be a good idea to check all of your ground connections and clean them up.

Also look at your negative battery cable ground wire. It goes between the body / frame on the front passenger side. With your mileage, id venture to assume your body mounts are shot like mine were. Long story short, my ground wire looked like a rat chewed through it due to body rubbing it against the frame. Solved a lot of problems when I replaced it.

 

[Bigburb3500]

Nope, but what I did try was to test with the sensor unplugged and the hesitation occurs anyway. So for now that sensor I think may be out of the equation.

Depending how long it has been on going the computer could have “learned” to compensate. Don’t these GM computers “learn” for a long while?

The battery and cables are a great place to look as well as mentioned earlier by @davidavidd. I used to have Audi’s and if the battery voltage dropped too low the whole car freaked. Good luck with the diagnosis! I am following along and interested in the final repair.

 

[davidavidd]

Depending how long it has been on going the computer could have “learned” to compensate. Don’t these GM computers “learn” for a long while?

The battery and cables are a great place to look as well as mentioned earlier by @davidavidd. I used to have Audi’s and if the battery voltage dropped too low the whole car freaked. Good luck with the diagnosis! I am following along and interested in the final repair.

Lol, the ECU "learn" against my pocket, gas consumption has been skyrocketing. Posting update with pictures right now.

 

[Bigburb3500]

Lol, the ECU "learn" against my pocket, gas consumption has been skyrocketing. Posting update with pictures right now.

Unfortunately, the old trifecta: Power, Reliability, Efficiency. You can only have 2.

 

[davidavidd]

Ok, I started the cleaning process, this is what I found so far:

- Small coolant leak in a hose, easy to fix.
- EVAP solenoid hose very brittle (I'm going to change it)
- Partially open throttle body, with a considerable layer of sticky oil and carbon, the gasket was crushed and in very poor condition
- The intake manifold is covered in a paste of grime, carbon and oil (expected). The gasket was apparently allowing air in at several points.
- I have seen several charcoal pellets, even a small buildup near the vlom. 100% sure it is charcoal, not mice p*#p lol.

- The valves and ports look relatively clean, not carbonized, BUT I have removed these manifolds before and have never smelled gasoline as much as this time. I would venture to say that there are small puddles of gasoline on about four valves. This makes me suspect that some injectors are in direct flow without containing the pressure (maybe contaminated with carbon from the EVAP system?)

- Oil pressure sensor filter in good condition, not clogged.

- The injectors look very dirty, 1 bad o-ring.

Tomorrow I will finish washing the parts, I will also take the injectors to be cleaned and tested.

I'm waiting for the fitting so I can use my pressure gauge on the fuel rail.

The battery cables look good, I always apply dielectric grease to the terminals, they do not look brittle, corroded or rotten.

Thank you all for your responses, they have been very helpful.

Some pictures:

 

[j91z28d1]

having the injectors clean and tested is a good idea, maintance and rule them out as what's casing the issue.

not thst it would effect much of this, but a catch can is a total worth while investent on these trucks. I don't know how much it would cost down your way, but Amazon has ones most use for about 30$. mine catches a good about of oil between changes.

 

[davidavidd]

having the injectors clean and tested is a good idea, maintance and rule them out as what's casing the issue.

not thst it would effect much of this, but a catch can is a total worth while investent on these trucks. I don't know how much it would cost down your way, but Amazon has ones most use for about 30$. mine catches a good about of oil between changes.

That's the point, I already have one catch can installed , I don't know if it's not working properly, maybe I should relocate it.

 

[j91z28d1]

That's the point, I already have one catch can installed , I don't know if it's not working properly, maybe I should relocate it.

oh that's interesting. definitely something to look it into. might double check the hose routing. also what type, it seems like ones a that have metal mesh inside them work better than ones that are just a plain can.

 

[davidavidd]

Quick update: The rain delayed me a bit but I finished the cleaning process, some injectors were dirty but nothing out of the ordinary, all working, new gaskets for everything; unfortunately the problem continues.

Tomorrow I'm going to test the fuel pressure parked and driving.

 

[Carisvan]

How old are your spark plugs and wires? I did not see them being mentioned in the thread. At that mileage, you should have replaced the plugs at least twice and the wires at least once. I just did mine on a 2017 Tahoe at 91,000 miles, and the improvement is remarkable. My truck was running a bit sluggish and hesitating during acceleration. The plugs I pulled out were filthy.

 

[clubdriven]

I have had a similar issue on other vehicles and it was one of the ignition coils. It was intermittent until the coil melted due to arcing.