Hesitation on Rapid Acceleration

Jkb242

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May 19, 2019
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Just noticed today and the first time this has happened. This follows the Throttle body cleaning which was quite dirty and removal and cleaning of MAF sensor. I’m sure both of these were cleaned properly and carefully inspected before Installation.
There is a noticeable initial hesitation upon full throttle from load speed to full throttle and a slight hesitation when snapping the throttle at idle. Idle is smooth, no missing or misfires, or codes during these conditions
Gradual acceleration seems normal no missing. There is a noticeable difference in the delay to full throttle.
MAF reads between 4 and 5 at idle. Fuel trims look normal.
Any ideas on this new drop of under load?
The CAT on bank 2 is apparently bad unfortunately as I thought the sensor had taken card of the issue and because the O2 readings look normal after replacement and a 50 mile test ride.
Im also hearing a tale tell rattle on the bank 2 side as well but not sure it’s actually coming from the CAT.
Thanks for any suggestions or ideas.
 

Mooseman

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Dec 4, 2011
25,262
Ottawa, ON
Is this on a 5.3? If so, you may need to get a relearn done. Contrary to the 4.2, the 5.3 and other LS engines need a relearn when the throttle is cleaned.

Video
 
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mrrsm

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The hesitations under load speed and when making the Throttle Snaps correlate with the problem of the "rattling" sound emanating from the Left Bank CAT. You can easily investigate this by reaching under the vehicle after the Engine is Ice Cold and lightly tapping under the bottom of the Left Side CAT with a Dead Blow Hammer. If you can hear any rebound sounds that are similar to 'Dropping a Handful of Change into a Ceramic Bowl'... this will point to what is causing the performance problem.

What may have occurred is that once the in-dwelling Ceramic Rare Earth Metal(s) Coated Honeycomb Plank inside the CAT has shattered... the pieces may have fallen towards the bottom of the metal container and canted sideways enough to obstruct the even flow of the exiting exhaust. Such back pressure being present when the engine is loaded can make for the Cylinders on that Bank struggle to easily vent their Exhaust Streams and this could induce the hesitation problem here.

If this notion proves to be correct and If you live in a state that has stringent EPA Annual Vehicle Inspections, seek out the OEM (much more expensive) Dual CAT "Y" Pipes that meet the Federal CARB Standards and shine the less expensive ones on, lest you wind up having to plow this expensive field twice in order to have the SUV pass inspections.
 
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Jkb242

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Member
May 19, 2019
239
CLT
Is this on a 5.3? If so, you may need to get a relearn done. Contrary to the 4.2, the 5.3 and other LS engines need a relearn when the throttle is cleaned.
Just noticed today and the first time this has happened. This follows the Throttle body cleaning which was quite dirty and removal and cleaning of MAF sensor. I’m sure both of these were cleaned properly and carefully inspected before Installation.
There is a noticeable initial hesitation upon full throttle from load speed to full throttle and a slight hesitation when snapping the throttle at idle. Idle is smooth, no missing or misfires, or codes during these conditions
Gradual acceleration seems normal no missing. There is a noticeable difference in the delay to full throttle.
MAF reads between 4 and 5 at idle. Fuel trims look normal.
Any ideas on this new drop of under load?
The CAT on bank 2 is apparently bad unfortunately as I thought the sensor had taken card of the issue and because the O2 readings look normal after replacement and a 50 mile test ride.
Im also hearing a tale tell rattle on the bank 2 side as well but not sure it’s actually coming from the CAT.
Thanks for any suggestions or ideas.
Thanks for the replies thus far.
Just noticed today and the first time this has happened. This follows the Throttle body cleaning which was quite dirty and removal and cleaning of MAF sensor. I’m sure both of these were cleaned properly and carefully inspected before Installation.
There is a noticeable initial hesitation upon full throttle from load speed to full throttle and a slight hesitation when snapping the throttle at idle. Idle is smooth, no missing or misfires, or codes during these conditions
Gradual acceleration seems normal no missing. There is a noticeable difference in the delay to full throttle.
MAF reads between 4 and 5 at idle. Fuel trims look normal.
Any ideas on this new drop of under load?
The CAT on bank 2 is apparently bad unfortunately as I thought the sensor had taken card of the issue and because the O2 readings look normal after replacement and a 50 mile test ride.
Im also hearing a tale tell rattle on the bank 2 side as well but not sure it’s actually coming from the CAT.
Thanks for any suggestions or ideas.
Feedback 3/3/2020:
This is a 5.3L engine
1. I watched attached the video by posted in Mooseman’s reply, not experience any erratic or abnormal idle mentioned in video. If a relearn procedure is required I do not have a compatible scanner to do this and wondering if this is necessary given that I have no issues at idle highlighted in video.
2. Jacked up passenger side of the Denali (Bank 2) to get access to CAT. Using hard rubber hammer striking CAT as hard as possible while lying under vehicle, did not hear pronounced rattle from CAT as expected. Noticed only a slight rattle from clamp upstream from CAT but clamp was not loose. Therefore was unable to detect an obvious failure of particle separation in CAT, no doubt it’s defective but is it related to the hesitation is the question.
I have not detected a reduced power mode, no codes except the CAT code and have not experienced “Limp Mode”.

It seems obvious the CAT will need replacing but with the Covid 19, I’m trying to avoid “kicking that snake” in avoiding contact, so it will be a while before I can get it in for this service. Meanwhile I need to use it.
Any additional comments regarding further checks I can make and report back.
 
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Mooseman

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Dec 4, 2011
25,262
Ottawa, ON
Is overall power low? Like it won't accelerate? That would be a sign of a plugged cat(s). Removing the upstream O2 sensors would confirm this.
 

Jkb242

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May 19, 2019
239
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No there is no indication of low power, it’s only the slight hesitation heard at the instant the throttle is snapped.
I’m trying to attached a sound clip to the post but it appears that only photos are supported.
 

Reprise

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I'm coming in a little late here, and just finished watching the video Moose linked to.

Normally, I'd say anything that guy told you to do / not do was money in the bank. He used to be on this site a lot, and he seriously knows what the hell he's doing.

(BTW, Moose... the back half of the video, after he does the relearn, was hilarious. Thx for linking it)

However... I'm going to report that while I disconnected the battery when I cleaned my TB in both the 5.3L Envoy and the 6.0L Sierra (both are similar gen trucks)... I know I didn't do an idle relearn in either, w/ my Tech2. In the case of the Envoy especially, I didn't even own my Tech2, then.

Anywho... maybe it was b/c I had the battery disco'd... but when I started them up, I had no high idle or throttle problems. I let them warm up a couple of minutes, took them on a 30-40min drive (which is basically a relearn proc of its own), and both of them ran like a top afterward.
Neither was throwing codes, etc., before cleaning -- I just saw it needed cleaning (while doing other things) and did it while I was there. But I did disco the battery on both.

The other thing I knew not to do was move the butterfly with everything connected, as it will f*ck with the TPS (channeling my inner May03LT here...lol) If you cleaned yours without at least disconnecting it from the harness on the pass. side -- that might be why yours is 'off' now, too.
 

Jkb242

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May 19, 2019
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UPDATE 4/4/2020

The throttle body was disconnected and completely removed from the vehicle. I can't imagine anyone attempting to clean it installed but maybe there is something to that as well, a short cut maybe but I never considered it. No issues with idle when it was reinstalled and started.

The attached is a link to a youtube post that contains no video, just the audio of what I hear when I perform a snap throttle in park from idle. Listen carefully at the beginning of each of the three events of snapping the throttle. You will hear a very brief rush sounding like air intake then the rev of the engine. It sounds like the Quardrajet of the older 350cc form the early 60's when I was in high school and rebuilt many of these for friends and of course the one on the family car when needed. I think I could do it today even--some things really stick, many don't!

Anyway this is what I am referring to as the hesitation I do not recall hearing before. When I have punched this 5.3L in the past the acceleration was instantaneous, no delay, not the slightest but now that I have noticed it, when I am driving it, I hear this throttle open a split second before she's ripping the RPM's. Please have a listen and see if this sounds normal. Please let me know if there is any issues with the link as well. Thanks.

 

Jkb242

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May 19, 2019
239
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fuel trims normal? what's that? what kind of vacuum readings are you seeing?
FT's look normal. I can still see occassionally a -7 on short term at idle but typically on bank 2 but it isn't constant as I recall and isn't those ridicuously high - number of the original problem before the MAS was removed and cleaned with MAS cleaner only spraying pickup resistors at a distance of ~6".
 

Mooseman

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Dec 4, 2011
25,262
Ottawa, ON
Maybe it'll relearn itself and will smooth itself out after a while? Are you sure it wasn't there before? Maybe because you just cleaned it you're just more sensitive to it. My old 4.2 used to actually stumble off idle. Always did it and I never was able to resolve it so I became used to it after a while. Only after I did something to it (changed a part or cleaned the throttle) I was reminded it did it. The 07 doesn't do it though and neither does the Saab.
 

Jkb242

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May 19, 2019
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Were you able to hear the hesitation I referred to clearly enough?
 

Mooseman

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Dec 4, 2011
25,262
Ottawa, ON
Yes but it's not as pronounced as the one I had in the '02 4.2. I have nothing to explain it.
 

mrrsm

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If you have an OBD2 Scanner that can collect and log Live Data, see if you can reproduce these Hesitation conditions while collecting information on these parameters:

(1) The Engine RPM.
(2) Note The Vehicle Speed during each moment of these Hesitations.
(3) The TPA (Throttle Position Angle) Percentile.
(4) The Two Hi-Lo Voltage Measurement variances sensed by the ACC Pedal Module during these conditions with sudden "Pedal to The Metal" actions.

If the Plastic-Nylon Gear Set inside the Removable Side Cover of the Throttle Body Gear Box displays either Missing Teeth or any Cracks in the Gear Wheels that only spread apart during the 'Snap Moments' from pressing hard on the ACC Pedal... this might be the next place to look for the Culprit.
 

Jkb242

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May 19, 2019
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I do have a scanner that collects the data MRRSM mentions. I’ll need to check if the TPS field is degrees or percentage. Would appreciate more clarification in capturing the voltage measurement extremes of the pedal position and how this differes from the TPS OBDll parameter.
Smooth or linear pedal advance produces no hesitation. Im also pretty certain that is the case as well with even a rapid pedal advance. Only a forced Instantaneous full pedal advance (throttle snap) results in that brief hesitation.
Thanks!!
 

mrrsm

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Please Consult with our GMTN Expert on PID Designs for OBD2 Scanners and Blue Tooth Device: @TJBaker57 ...as he is very capable of guiding you towards how they need to be set up to observe and break down these Sensor-Module Data Elements and make good sense of things.

The "Winning Combination" for problems of Controlling Air Flow in a Modern Motor that simply responds to how well that AIR is controlled versus one that uses Fuel Delivery via Carburetor Fuel Spray. These are the Main Players required to make it all work:

(1) MAF (Mass Air Flow) Sensor WITH IAT (Inlet Air Temperature).

This primary metric works by WEIGHING THE INCOMING AIR STREAM in Grams. The technique works better than any other Method because it is NOT Affected by Altitude causing Lower than Sea Level Air Pressure... say while driving up to Pike's Peak vs. riding along the New Jersey Shore of the Atlantic Ocean. The MAF sensing mechanism informs the PCM constantly with data about "What the In-Coming Air Weighs at Varying Temperatures" to provide the PCM with that portion of what it adds to the Fuel Delivery Algorithm.

(2) The Oxygen Sensor(s) Four for V8s...2 for the I-6 Engines.

The Heated Oxygen Sensors role is to use a shielded Cubic Zirconia Sensor poking its "Thumbs" into the Exhaust Streams and simply measures the presence of Excess Oxygen. This device is considered "Narrow Band" because it only uses the "Perfect Stoichio-Metric" reading of 14.7:1 for the Air to Fuel Ratio. Oxygen Sensors Do NOT measure the presence of Un-Burned Gasoline within the Exhaust Stream. It only provides the information on the "Left Over" Oxygen discarded from improper combustion F/A ratios happening well ahead of it. The O2 Sensors provide this data to the PCM to be added into the PCM Fuel Delivery Algorithm. The Aft Oxygen Sensor data stream alerts the PCM as to efficiency of how well the Catalytic Converter is chemically converting the by-products of incomplete Engine Combustion into less Noxious or Polluting Vapors along with Water as by-products of combustion.

(3) The Throttle Position Sensor (TPS.

The Throttle Position Sensor on modern EFI Engines controls and meters the In-Coming Air Stream using a 12 Volts DC Motor and Nylon Gear Reduction Mechanism set up to provide very fine Electro-Mechanical control of the position of the Butterfly Plate that either Opens or Closes to deliver More or Less Air on Demand and informs the PCM by means of its PWM (Pulse Width Modulation) to control the Drive Motor to Plate movements with Very Fine Movements. Along with controlling this activity via PWM., the PCM is capable of Reading the Angle of the Butterfly Plate moment by moment and thereby adds that information into its Fuel Delivery calculus having figured out what the Weight and Temperature of the In-Coming Air is up to that point and adds that information to its Fuel Delivery Algorithm from (Fuel-Air Tables) stored in Memory.

(4) The Accelerator Pedal Module (Accelerator)

The "Gas Pedal" in modern EFI Fuel Air Engines is anything but... It is more akin to having Two Potentiometers that measure in Milli-volts (mV) on Two Separate Channels communication with the PCM and the Throttle Body. This means that as the "Gas Pedal" gets Pushed Down... One signal in Milli-voltage (mV) Decreases while the other Increases. Likewise, when the Gas Pedal is released, these two PWM Monitored (mV) signals get sent to the PCM... and provide Opposite mV Values for referencing. In this manner, the PCM can use this combination of the Position of the Gas Pedal AND the Angle of the Throttle Body Butterfly Plate to determine the INTENT of the Driver to want More Power from the Motor... and that leads to the next Major Player in this Drama...

(5) Electronic Fuel Injectors (EFI)

The GM Multec II Design of the EFIs provides the ability for the PCM to vary its Pulse Width Modulation (PWM) and change whether the (6) EFI Pintle (Pintel) Valves are either Open or Closed ...and even more importanly...for How Long in order to achieve more Time based subtlety in How Much Fuel is being Atomized into a Mist per Cylinder with greater or lesser intervals to either increase or decrease the Amount of Fuel that is available at varied RPM and Engine Power Band requirements. The Fuel Rail provides adjustable pressure via the responsive Fuel Pump to ensure that the Fuel Pressure does not drop below an acceptable level via the PCM providing (PWM) responding signals to it as well to raise or lower its RPM.

(6) The Vehicle Speed Sensor (VSS)

To a Greater or Lesser degree... The 4L60E Hydro-matic Transmission requires regular PCM input in order to know at what particular RPM it needs to Shift Gears into (or PREVENT any Gear Shifting) to provide the proper PRNDL123 Gearing depending upon the need for Power being fed by the Engine into the Torque Converter. The PCM uses Pulse Width Modulation to signal the Solenoids inside the Transmission to move either up or down in the internal Gear Ratios and achieve a balance of Power Needs vs. Power Delivery (...and not Destroy the complex Gear Box inside by accidentally Down Shifting at Highway Speeds)

I'm probably missing quite few other Engine Air/Fuel Regulatory Modules... but THESE are the main Players to be considered in any instance of the engine suddenly misbehaving or going into either Reduced Power or Limp Home Mode as a precaution necessary to prevent the Accidental Runaway of an Engine OUT OF CONTROL.. Should any of these F/A Delivery Mechanisms FAIL.
 
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Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
Try unplugging the MAF. It will run on pre-programmed MAP calibrations instead. Driving it may become dicey as the tranny will be shifting hard with it like this, which is what happened to me when I once forgot to plug it.
 
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Jkb242

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May 19, 2019
239
CLT
[QUOTE="


Nice summary indeed and a good review for me I appreciate this information. I began educating myself a few years ago in this area after retiring from my career in RF Engineering so that I could effectively understand and troubleshoot these areas in taking care of my own vehicles. I knew I would need a good code reader and purchased a Foxwell ($125.00) scanner that also included Antilock brake and seatbelt module testing feature. I recently switched to a WiFi based module that would work with an iOS OBDll app for use with my ipad. It does not offer bidirectional support but allows good code capability and offers live data and capture feature.
I don’t have any Android devices otherwise I would have opted for the Torque module.
Thanks again for your reply. I will search for any sticky notes from TJBAKER57 for additional info and I will give this issue a bit more time if there is some “learning time” needed and maybe following the Bank 2 CAT replacement. By the way, when revving the engine while parked, I see the CATS are doing their job producing H₂O!
 
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