Engine 'Stumbles' When When Accelerating

[PNW_IT_Guy]

My 2010 Yukon SLT 5.3 has a weird issue that I haven't been able to find anything online that's exactly the same. When I'm approaching an on-ramp and I hit the gas, sometimes I can feel the transmission downshift, then the engine revs up and starts to accelerate, but then it just... pauses for a moment. It's just a moment then it gets right back into and goes. Maybe I'm just not searching for the right word. It's not that it generally 'runs rough' but smooths out when accelerating, it's not that it's always rough when accelerating, it doesn't die completely and need to be restarted.

It'***** it, downshift, pause, go. It does seem to have a slightly rough idle, but generally seems to run reasonably well.

Also, no lights on the dash at all. Transmission was drained and filled and oil changed just this past fall. It's a used truck that I bought with 180k a bit more than a year ago. Hopefully I've answered all the basic questions. Thoughts?

 

[Marky Dissod]

1st thought:

It'***** it, downshift, pause, go.

What did you say? This forum operates under the assumption that words hurt.

2nd thought: Before you fire the parts cannon, consider firing the parts derringer, crank & cam position sensors.
Clean the MAF sensor.

3rd thought: Can you get someone (preferably a tuner) to sit next to you with their laptop hooked up to your OBD2 port,
reading the realtime telemetry as you drive, especially as this peculiar behavior exhibits itself?
Before you drive off, check the throttle position sensor for linear response.

 

[PNW_IT_Guy]

geez, sorry I guess I missed a space at the worst time. *hit it, not what they thought i said.

Crank and cam sensors aren't too spendy, so I guess I could try that. But shouldn't I get a check engine light if they are bad? I think I have cleaned the MAF sensor but it's been a year so I could give it another go.

I wish I knew a tuner. I do have the Torque app so I'll check to see if it shows throttle position. Pretty sure it does.

Thanks for the input!

 

[Marky Dissod]

You get a Check Engine light if they are bad ENOUGH. If they're not good, yet not bad enough,
you can get problems like this.

 

[intheburbs]

180k? Have you done anything with the fuel system? Before just throwing money/parts at it, I'd run some Techron or BG44 through it. Maybe more than once.

 

[Doubeleive]

My 2010 Yukon SLT 5.3 has a weird issue that I haven't been able to find anything online that's exactly the same. When I'm approaching an on-ramp and I hit the gas, sometimes I can feel the transmission downshift, then the engine revs up and starts to accelerate, but then it just... pauses for a moment. It's just a moment then it gets right back into and goes. Maybe I'm just not searching for the right word. It's not that it generally 'runs rough' but smooths out when accelerating, it's not that it's always rough when accelerating, it doesn't die completely and need to be restarted.

It'***** it, downshift, pause, go. It does seem to have a slightly rough idle, but generally seems to run reasonably well.

Also, no lights on the dash at all. Transmission was drained and filled and oil changed just this past fall. It's a used truck that I bought with 180k a bit more than a year ago. Hopefully I've answered all the basic questions. Thoughts?

I would say clean the throttle body, maf, new air filter. then either some injector cleaner back to back for a few tanks of gas or injector cleaning service.
see how that pans out....
also get a tune, it makes a difference and can get a buggy shift like that addressed.
if i was going to throw parts at it I would do the maf and throttle body @180k they are both questionable in my eyes and directly correlate to shifting & stumbling.
a drive recording with a scanner is always a good idea.
and the question that should be asked is what maintenance has been performed already?
bought it used @180k
changed the oil
changed the transmission fluid
and???? anything else? plugs/wires?, 02 sensors?

 

[PNW_IT_Guy]

I have been thinking about doing the air filter. It doesn't look too bad but it's also pretty cheap. I'll look into just replacing the MAF. I'd love a tune but don't really know what all goes into doing that... seems like it would be expensive.

Sorry for not giving more details on my servicing:
-Several fuel system cleanings in past year, Even ran a tank of E85 through it. (Gas mileage is down around 13mpg so I've been trying everything I can think of to improve that)
-2 or 3 oil changes in the past year with Mobil1 Full Synthetic
-Oil flush on last oil change
-Transmission drain/fill in December using Mobile1 Full Syn ATF
-MAF cleaning a year ago
-Throttle body cleaning a year ago
-Removed fuel pump ground, cleaned surface of rust and replaced. (Significant surface rust over entire undercarriage)

I have plans to do a big 3 upgrade and clean all grounding connections in the engine bay.

 

[j91z28d1]

sounds like you're going to need a scanner of. some sort, pretty decent one go see what it's doing during that cut out.


as for a tune, black bear here is common. it's not much more expensive than a oem maf would cost.

 

[Doubeleive]

I have been thinking about doing the air filter. It doesn't look too bad but it's also pretty cheap. I'll look into just replacing the MAF. I'd love a tune but don't really know what all goes into doing that... seems like it would be expensive.

Sorry for not giving more details on my servicing:
-Several fuel system cleanings in past year, Even ran a tank of E85 through it. (Gas mileage is down around 13mpg so I've been trying everything I can think of to improve that)
-2 or 3 oil changes in the past year with Mobil1 Full Synthetic
-Oil flush on last oil change
-Transmission drain/fill in December using Mobile1 Full Syn ATF
-MAF cleaning a year ago
-Throttle body cleaning a year ago
-Removed fuel pump ground, cleaned surface of rust and replaced. (Significant surface rust over entire undercarriage)

I have plans to do a big 3 upgrade and clean all grounding connections in the engine bay.

so plugs/wires/02 sensors are unknown age/mileage?

 

[PNW_IT_Guy]

Shoot, I knew I was forgetting something. Plugs and wires replaced in the past year as well. o2 sensors are unknown. It's been a busy year and I took care of as much as I could after buying her.

 

[Doubeleive]

Shoot, I knew I was forgetting something. Plugs and wires replaced in the past year as well. o2 sensors are unknown. It's been a busy year and I took care of as much as I could after buying her.

this information just helps for anyone helping to know what your baseline maintenance is thus far.
my personal experiences with stumbling have been #1 throttle body #2 maf
it could be many things though....you just have rule things in/out as you go along
if the air/fuel information is off from what it is expecting then it's going to hiccup, the maf can cause all kinds of issue's if the reading wrong or if there is un-metered air getting in
hence a scanner that can read live data is very helpful

 

[PNW_IT_Guy]

I can't believe I missed the air filter. I got a new K&N swapped in and she's running much better. When it drops a gear it just goes now. She's still running a bit rough so I'll clean the MAF sensor this weekend. I also get a faint ticking occasionally, so I'm really hoping that's not lifters.

 

[donjetman]

@PNW_IT_Guy
Ticking may be caused by lifter preload being too tight or loose. Preload is a function of push rod length on stock LS engines.

I'll try to explain how to set LS engine lifter preload on a stock engine.

First you need to measure a pushrod. I have a friend with dial calipers that measure up to 8".

I removed a rocker arm, then the pushrod, that was on the base circle of the cam. You can tell it's on the base circle because it feels a little loose when you grab the rocker arm tip and move it side to side. If it's on a ramp of the lobe it will be tight and not wiggle. When on a base circle it will wiggle.

Measure it. Mine was 7.388" long. I assumed the other 15 were too.

Now reinstall the pushrod. Reinstall the rocker arm. Tighten the hold down bolt until it is just finger tight (zero lash). Now, using a torque wrench, count the number of turns it takes to reach 22 ft lbs. If less than 3/4 turn pushrod is too short. If more than 1 1/4 turns pushrod is too long.

Check the preload of all 16 this way. Write down the # of turns for each. Some rockers will be tight because they are on a ramp of the camshaft. After checking all the loose ones using this method bar the engine crankshaft over around 270* to 360* degrees.

Now, many rocker arms that had been tight will be loose because they are now on the base circle. Now loosen those hold down bolts and retighten it until finger tight (zero lash) then check and record how many turns it takes to reach 22 ft lbs. Again, If less than 3/4 turn? pushrod is too short. If more than 1 1/4 turns? pushrod is too long.

Keep doing this procedure until you've recorded the # of turns it takes for all 16 valves.

With one of my LS engines only 2 of the 16 lifters had preload that fell in the 3/4 to 1 1/4 turn range, the other 14 took more than 1 1/4 turns and needed a shorter pushrod, either 7.350" or 7.375" to get them in the correct preload range.

LS pushrods can be purchased in many lengths from many sources for <$10 each.