Hesitation when accelerating from a dead stop

[TobyP]

Hello, I am having an issue that I cannot trace down the source of. I am getting a slight hesitation (about 1-1.5 seconds) when pressing the throttle coming off of a full stop. The idle is smooth, acceleration is fine once the hesitation corrects itself, and it never has acceleration problems once out of a stop. Plenty of power when cruising at any speed, it's only the slight hiccup when starting up from a full stop. This happens whether cold or at operating temperature. Short stops (quick stop sign), it generally doesn't do it, it's more at a stop light when it sits for a bit that it hesitates. I have put about 3k miles since the engine swap, and this has only presented itself in the past few hundred miles. I daily drive about 60 miles round trip, 5 days per week. Vehicle has full power once past the initial little hiccup, cruises fine, downshifts and revs fine, never any lack of power.
Some history:
2003 TB LTZ, 4.2 LL8
Purchased the vehicle with 86k miles with a blown engine. Swapped in an 75k engine. Used original throttle body (OEM, thoroughly cleaned) from blown engine, original PCM, all new sensors (crank, ECT, both O2's, IAT, MAP, etc... All are ACDelco/ Delphi, nothing off brand), did the crank relearn. This model only has a MAP, no MAF.
New fuel pump (ACDelco) & wix fuel filter, fuel injectors (genuine Delphi), fuel pressure regulator (Delphi).
New plugs (ACDelco 41-103), new coils (Denso)
I switched the throttle body with another used OEM, did the pcm relearn (pulled PCM fuses for 2 hours), same result. Both throttle bodies have been THOROUGHLY cleaned. It starts right up every time, no issues there. There are no codes: current or history. It has been sitting since I 2008, all fluids are new, engine was taken apart and resealed /new gaskets, minus the head gasket, otherwise all are new. Fuel system was flushed (had NASTY gas in it from sitting), have run BG 44K and seafoam thru separate tanks to remove anything residual.
I am at a loss what to try next. Idles perfectly fine, though pressing the peddle when in park has the hesitation as well. The only issue is directly from a stop or sitting and idling, never anywhere else at any speed, kicks down and has plenty of power everywhere else. If I his the throttle aggressively from a stop, it happens slightly less, but under normal acceleration it is quite noticable. It has never died, it catches itself before that, but it's still something that needs to be addressed.
Thanks in advance for any assistance!

 

[TJBaker57]

Do you have one of those inexpensive Bluetooth cloned ELM327 OBD2 adapters? If I were experiencing such issues I would record a handful of related parameters during the event. Things like throttle position in relation to accelerator pedal, MAP values, fuel trims, fuel trim cell, fuel trim index, injector firing rates. That sort of thing.

 

[TobyP]

Do you have one of those inexpensive Bluetooth cloned ELM327 OBD2 adapters? If I were experiencing such issues I would record a handful of related parameters during the event. Things like throttle position in relation to accelerator pedal, MAP values, fuel trims, fuel trim cell, fuel trim index, injector firing rates. That sort of thing.

View attachment 106499

I have an Autel that does the live data, I will try those. I have checked fuel trims (short and long) over about a 40 minute drive. It ranged from about -9 to 5 ST, and -3 to 2. There weren't any crazy fluctuations, and nothing out of the ordinary when it would do it.
I also have a Tech II if I need more in depth data.

 

[TJBaker57]

I also have a Tech II but honestly it sucks for data acquisition. I have an Autel AP200 but I have never even opened the package!!

 

[budwich]

Do you have one of those inexpensive Bluetooth cloned ELM327 OBD2 adapters? If I were experiencing such issues I would record a handful of related parameters during the event. Things like throttle position in relation to accelerator pedal, MAP values, fuel trims, fuel trim cell, fuel trim index, injector firing rates. That sort of thing.

View attachment 106499

Interesting graphs. Can you "expound" a bit about the "desired" and "indicated" TP being "exact duplicates". I haven't ever graphed them on torque. I would have maybe thought some difference would appear... not significant as that would "code" but think that the "desired" reading would come from some "reading" of pedal versus the "indicated" as what was set in the TB.... but maybe I misunderstand what these PIDs are. Just curious.

 

[TJBaker57]

Interesting graphs. Can you "expound" a bit about the "desired" and "indicated" TP being "exact duplicates". I haven't ever graphed them on torque. I would have maybe thought some difference would appear... not significant as that would "code" but think that the "desired" reading would come from some "reading" of pedal versus the "indicated" as what was set in the TB.... but maybe I misunderstand what these PIDs are. Just curious.

Don't know what I can add but to say these are both PIDs that the PCM has available. The Tech 2 displays them as TP Indicated and TP Desired angles under the selection "Engine Data 1". As you say I would expect a variation beyond some unknown threshold should set a code.




As you can see by the date of the graph it was a few years ago. I have a boatload of these files saved in Car Scanner. The app lets me select what parameters I wish to see and I can output to separate graphs or a single and can adjust the scales, X and Y.

So I tossed those two parameters together for a better look and here we can see there is very slight variance occasionally.



I would speculate that the PCM takes the pedal position along with other parameters and arrives at a desired throttle opening. Having that much the PCM would send what it believes is the required signal(s) to drive the throttle to the desired position but there is likely some error to be monitored??

 

[Mooseman]

All I can add is that I had that on my '02 and was never able to solve it. It didn't do it if stop and go but if stopped for a while.

 

[budwich]

Don't know what I can add but to say these are both PIDs that the PCM has available. The Tech 2 displays them as TP Indicated and TP Desired angles under the selection "Engine Data 1". As you say I would expect a variation beyond some unknown threshold should set a code.

View attachment 106514


As you can see by the date of the graph it was a few years ago. I have a boatload of these files saved in Car Scanner. The app lets me select what parameters I wish to see and I can output to separate graphs or a single and can adjust the scales, X and Y.

So I tossed those two parameters together for a better look and here we can see there is very slight variance occasionally.

View attachment 106516

I would speculate that the PCM takes the pedal position along with other parameters and arrives at a desired throttle opening. Having that much the PCM would send what it believes is the required signal(s) to drive the throttle to the desired position but there is likely some error to be monitored??

EXCELLENT!!!! I think this helps in "visualization". The points of interest appear to be at places where the throttle "drops off"... meaning less throttle. At those points, the difference between desired and indicated appear to be the "greatest difference" assuming the logging is accurate. Note the one "dip" which I think corresponds to idle... actually, pedal release? The desired (pedal?) is lower than the indicated (tb?)... by less than 1% which is good (ie. not enough to set a code).... but it indicates a somewhat "significant" difference versus "normal" running. Further, of note is the actual value... in the "just under 20%" area. This jives with what I see with at least two tb's that I have used on my 2008. However, in a number of other posts on this site, I have seen a number shown less than 10% which seems rather small (ie. the tb is almost fully closed). To me this may cause some "throttling" issues at times... a generic statement maybe should say "mixture issues" coming out of that area. It doesn't mean its a tb / pedal issue, it means there is something driving the system to cause it to set the "physical controls" to this point at times (ie. idle). NOTE: also the other "drop" (to the left), the two curves are "reversed" from the other dip.

On my truck, I have significant lag at acceleration times which I attribute to possibly a plugged cat but maybe my pedal sensor has issues at points in its "sweep" but not all the time.... hot versus cold versus use versus sitting. OR maybe a timing issue... too much / not enough advance. I haven't found things that I could correlate just yet.

I think the OP should consider looking at similar data for his vehicle to see if anything "jumps out".... go from the result. Of course, FT's at idle are important also along with some of the other data.

 

[movietvet]

More Information for GM GENUINE 15150990

I had a similar problem on my 05 Z71 and I did the TPS but still was there and ended up be a communication problem between the TPS and this part, like @TJBaker57 mentioned.

 

[TobyP]

Thank you everyone for the advice. I am going to pull out the scanner and get some graphs of what it is doing this weekend.

 

[TobyP]

Hi everyone, sorry it has been a hectic few weeks & bad weather, so I hadn't gotten around to much.
I was able to get fuel pressure, it's sitting around 45 psi at idle with the needle bouncing quite a bit, when I rev the engine the needle stays at a consistent 45psi. A new fuel pump is on order already & new fuel filter just in case.
I have noticed when we go from R to D or D to R, it hesitates a lot more, nearly dying. The trans fluid level is perfect and clean. Would a faulty TCC solenoid cause hesitation if it is stuck on?
I am still not getting any codes from the engine or trans.
I did check the TPS desired and actual, they both match and have no variation throughout the sweep.
I check fuel trim again, the ST bounces all over the place at idle from -8 to +3. The LT is sitting at a consistent 1. Even when revving the ST seems to stay in the negatives, is that normal? I would think demanding more fuel would command a positive FT?

 

[mrrsm]

Another GMTN Legacy Thread... describing similar symptoms... and a few more suggestions from "The Roadie" to investigate:

Lack of acceleration / hesitation

Recently I picked up a 2003 Trailblazer EXT LT 140,000 miles and a brand new GM transmission. Right from the first day I noticed that when I would tap the gas after the truck was at a warm idol (Aprox 600RPM's ) the engine will almost stall for half a second before reving up. I've also noticed...

gmtnation.com

 

[budwich]

Hi everyone, sorry it has been a hectic few weeks & bad weather, so I hadn't gotten around to much.
I was able to get fuel pressure, it's sitting around 45 psi at idle with the needle bouncing quite a bit, when I rev the engine the needle stays at a consistent 45psi. A new fuel pump is on order already & new fuel filter just in case.
I have noticed when we go from R to D or D to R, it hesitates a lot more, nearly dying. The trans fluid level is perfect and clean. Would a faulty TCC solenoid cause hesitation if it is stuck on?
I am still not getting any codes from the engine or trans.
I did check the TPS desired and actual, they both match and have no variation throughout the sweep.
I check fuel trim again, the ST bounces all over the place at idle from

-8 to +3. The LT is sitting at a consistent 1. Even when revving the ST seems to stay in the negatives, is that normal? I would think demanding more fuel would command a positive FT?

I have "returnless" fuel system but I always thought that the injector systems expect fuel pressures in the upper 50 / low 60 range.

What was the actual throttle position at idle? Just wondering.

 

[TobyP]

I have "returnless" fuel system but I always that the injector systems expect fuel pressures in the upper 50 / low 60 range.

What was the actual throttle position at idle? Just wondering.

My system has a return, with a new pressure regulator.
Idle was at 7 I believe, then it went steadily higher the farther the pedal was pressed.

 

[budwich]

Not sure about the trailblazer system, but I believe the return system is vacuum driven (based on quick look of picture). I still think that the pressure should be higher but ultimately, the difference might not be "fuel related" (ie. fuel pump / regulator) as the vacuum may not be correct.

At idle, a throttle opening of 7% ? is pretty "thin". I don't know what it would be "normally" but again, based on "old experience" (carb type and readings of my TB), I am thinking it could be hard for the system to readily adapt from almost fully closed to going the other way.

 

[TobyP]

Okay, some updates. Still as confused as when I started.

Replaced (All ACDelco parts now!)
MAP Sensor - was Delphi, seemed to lower the rpm's on cold starts.
Vapor Canister, Purge & Vent Solenoids. I blew out all the lines with compressed air while it was apart, no fuel or debris in any of the lines.
Upper O2 Sensor - was Delphi
Throttle pedal - just to rule it out.
Fuel pump - pressure is sitting around 52 now. Also cleaned all the grounding points.

The only thing that has not been replaced is the coolant temp sensor, it got held up in shipping. I don't know that it would cause this, but it is original so it will be replaced this coming weekend. It does not seem to be malfunctioning, my temp display is very consistent.

All of this with zero change.
I monitored the throttle position commanded and actual for a longer time. It gets down to 4% at low idle. Engine still idles fine, in the 620-650 range. When I barely tap the throttle, the commanded/actual jump to 20-30%, the rpm drops to the low 400's for a second, then revs like normal. This is very consistent if you wait 45-60 seconds, then try again. If you do not give the the engine time to idle down low again, it does not hesitate at all. Could the PCM just be letting the middle get down too low? It has never actually died, it catches itself in time. However, the constant "what if it does this time" thought it always there, and very unnerving.

I also ordered a refurbished & reprogrammed PCM, if it is a communication issue, this seems to be a likely culprit from research I have done.

I haven't been driving it, I don't want to get caught in a bad situation and have the thing actually die on me. Which is a pain as I bought this for a commuter vehicle to save miles on my others.

If the temp sensor & new PCM don't fix it, I fear I am out of ideas and will have to take it to a mechanic. I am very skeptical of mechanics, that's the last thing I want to do, who knows what they'll try to sell me on.

 

[Mooseman]

Not sure if you mentioned it but, did you check for vacuum leaks? I'd do a carb cleaner test around the intake manifold, the throttle body and any other vacuum points.

I doubt very much a PCM will fix this. They either work or they don't.

Taking it to a mechanic may not yield the results you're looking for unless they are at South Main Auto or Scanner Danner advanced level of a diagnostician.

Mine did it for years and never stalled on me. Just annoying.

 

[TobyP]

Not sure if you mentioned it but, did you check for vacuum leaks? I'd do a carb cleaner test around the intake manifold, the throttle body and any other vacuum points.

I doubt very much a PCM will fix this. They either work or they don't.

Taking it to a mechanic may not yield the results you're looking for unless they are at South Main Auto or Scanner Danner advanced level of a diagnostician.

Mine did it for years and never stalled on me. Just annoying.

I have, partially. I have sprayed the bottom of the intake, I meant to do the check upper when I had the resonator off and forgot, I'll do that this week or weekend. I would hope not as it has new gaskets, but then again I did have an exhaust manifold leak with a new gaskets, so it's possible.

I am not holding my breath the PCM fixes it, but I figured at least I'll have the latest updates now. I doubt that the current PCM has ever been flashed. I don't know exactly how long it had been sitting, it was last registered in 2008.

I'm not normally one to just throw parts at a problem, but considering it's age and the relative cheapness for new electronic components, I don't mind since it has been sitting for so long. With it not throwing any codes at all I am really at a loss where to go next. I know I should probably drive it more so it can possibly throw a code, I just don't want to be the guy stalled on traffic during rush hour.

Heck yeah! I'd take it to those guys in a heartbeat, that's a bit of a trip though for me unfortunately, I'm in New Mexico. On the up side, I never have to deal with rust 😂

 

[TJBaker57]

doubt that the current PCM has ever been flashed

Just for giggles have you checked what your current PCM calibration is?

I'm in New Mexico

I'll be in The Bootheel for about 6 weeks this springtime!

 

[TobyP]

Just for giggles have you checked what your current PCM calibration is?




I'll be in The Bootheel for about 6 weeks this springtime!

How would one go about finding that out??

What are you doing down there in the land of nothing?? I grew up in Alamogordo, so we have explored all over that part of the state when we were young, dumb, and full of energy.

 

[budwich]

So your "work" seems to have impacted your fuel pressure a bit but I think it is still a bit low. The "touch of the pedal" system response seems "interesting" going from almost totally closed to 20+. Of course, at idle, the pcm is likely doing "full control" of the tb while the touch is requiring some pedal "adjustment".
Your earlier observation about your "jumping around fuel pressure" might indicate some form of "vacuum change" or pump issue... maybe. Is that still happening? maybe, could also be a "measurement technique difference" (ie. how gage is connected).

Something changed to get 10% more pressure but still 15% plus from "normal" (upper 50's low 60's).

Might want to try tracking down a vacuum gage and "T-ing" on the vacuum line going to the regulator to see what's happening there.

 

[TobyP]

So your "work" seems to have impacted your fuel pressure a bit but I think it is still a bit low. The "touch of the pedal" system response seems "interesting" going from almost totally closed to 20+. Of course, at idle, the pcm is likely doing "full control" of the tb while the touch is requiring some pedal "adjustment".
Your earlier observation about your "jumping around fuel pressure" might indicate some form of "vacuum change" or pump issue... maybe. Is that still happening? maybe, could also be a "measurement technique difference" (ie. how gage is connected).

Something changed to get 10% more pressure but still 15% plus from "normal" (upper 50's low 60's).

Might want to try tracking down a vacuum gage and "T-ing" on the vacuum line going to the regulator to see what's happening there.

I did replace the fuel pump. From what I found in the repair manual, normal is anywhere from 50-60. The pressure gauge it a T style, I connected it right at the fuel rail. The jumping around of the needle has stopped since replacing the pump.
I have not tested vacuum on the regulator, though with it being connected to the resonator box, not the manifold, I feel like it doesn't take very much vacuum to make it function. I am thinking it allows more fuel to pass at idle, and less fuel to pass when vacuum is applied? I used a mightyvac to put a vacuum on it, no change, still hesitates. I swapped the old regulator back in at one point as well and no change. I don't believe the regulator was the problem, the vacuum line smelled like gas, but it was not wet & the regulator was not leaking anywhere.. just replaced it since I'm sure the diaphragm inside was brittle.
I'm honestly totally stumped at this point, but it sounds like it's a common-ish issue everyone just seems to live with. I just can't, the mini heart attack I have every time it does it is not worth it.

 

[TJBaker57]

How would one go about finding that out??

A Tech 2 can read that out for you but I am unsure about just where in the menu structure it is. Surely someone here can chime in.

With an OBD2 adapter and a serial terminal app you read out the values as hexadecimal by using the data block read service, "3C". The values are in data blocks 0A through 0F. Not that I think anyone here is going to do this! Looks like this with one such calibration highlighted. The highlighted calibration is for speedometer. 8E B1 9C hexadecimal equals 9351580 decimal seen in the second image below for speedometer. There are like 6 calibrations for various services in the PCM. I believe you can still check online for what the calibrations available are.

 

[TJBaker57]

What are you doing down there in the land of nothing?

I assist long distance backpackers who are hiking the Continental Divide National Scenic Trail from the Mexican border up to the Canadian border. Been doing this since 2014.

 

[TobyP]

A Tech 2 can read that out for you but I am unsure about just where in the menu structure it is. Surely someone here can chime in.

With an OBD2 adapter and a serial terminal app you read out the values as hexadecimal by using the data block read service, "3C". The values are in data blocks 0A through 0F. Not that I think anyone here is going to do this! Looks like this with one such calibration highlighted. The highlighted calibration is for speedometer. 8E B1 9C hexadecimal equals 9351580 decimal seen in the second image below for speedometer. There are like 6 calibrations for various services in the PCM. I believe you can still check online for what the calibrations available are.


View attachment 106772

View attachment 106774

View attachment 106775

I do have a Tech II, maybe I will mess around with it this weekend and see if I am able to find it. I'm pretty sure it would have whatever it left the factory with. Is that printout what has been flashed? Or what is available for update? Where did you get it from? I would be interested to see what mine is.

 

[TobyP]

I assist long distance backpackers who are hiking the Continental Divide National Scenic Trail from the Mexican border up to the Canadian border. Been doing this since 2014.

Oh wow, that's awesome! That... Is a heck of a hike!

 

[TJBaker57]

Interesting to note that there is a reference to hesitation in one of the engine calibration listings. My 2002 does not have that updated calibration. I have 12575728.

I am a lightfoot so I likely wouldn't ever notice hesitation issues.

 

[TobyP]

Interesting to note that there is a reference to hesitation in one of the engine calibration listings. My 2002 does not have that updated calibration. I have 12575728.

I am a lightfoot so I likely wouldn't ever notice hesitation issues.

I... Do not have a light foot. I like power. I don't gun it off the line or anything, but I do like to hear the rev every once in a while. That's what the LQ9 is for though.
If I let off the brake and let it coast for a few feet / seconds before pressing the accelerator it rarely does it, it's generally going straight from brake to throttle.

 

[budwich]

"brake to throttle" causes vacuum "fluctuations" which will be seen at the MAP. It would take some "science" to see what's happening during those events especially at the "outside edge of operations" (ie. normal throttling / engine run).

 

[Mooseman]

I tried to get calibration info using both FF and Edge at https://www.acdelcotds.com/subscriptions under Resources>SPS Info however was getting errors. It does work with Chrome. There you're supposed to be able to put in your VIN and tell you what's available for all your modules. You do have to create an account to access this.

 

[TobyP]

I tried to get calibration info using both FF and Edge at https://www.acdelcotds.com/subscriptions under Resources>SPS Info however was getting errors. It does work with Chrome. There you're supposed to be able to put in your VIN and tell you what's available for all your modules. You do have to create an account to access this.

I'm not sure whether this explains anything or not, but here's the info.

 

[TJBaker57]

What is needed is your current calibration numbers. Those can be read out by your Tech 2.

 

[mrrsm]

For some more, possibly "Mundane Mechanical" Causes for Hesitation...

...and Check out this "Two Tone Trailblazer"... :>)

Chevy TrailBlazer slow acceleration causes and how to fix it

Driving your Chevy TrailBlazer with engine hesitation or slow acceleration is not only infuriating, it can also be dangerous when overtaking or when...

www.wheelsjoint.com

 

[Beacon]

Did you change the fuel filter before or after the BG42 sea foam treatment? Is possible the fuel filter needs changed again?

Do you think your cat's are bad? Can you take it for a ride up a long steep hill?
I'm suggesting these items because, if I remember correctly, you don't have any codes, and these items sometimes cause problems without throwing a code

 

[Beacon]

Does it only hesitate when you apply the throttle? What happens if you just let your foot of the brake, no throttle, and let it idle along, like in an empty parking lot?

And since I don't have one, can you command the throttle with your tech 2, or other scanner?

 

[TobyP]

Did you change the fuel filter before or after the BG42 sea foam treatment? Is possible the fuel filter needs changed again?

Do you think your cat's are bad? Can you take it for a ride up a long steep hill?
I'm suggesting these items because, if I remember correctly, you don't have any codes, and these items sometimes cause problems without throwing a code

This started before I changed the fuel filter, but when I changed the fuel pump last weekend, I went ahead and changed the fuel filter (Wix) as well.
The cat is new, it was missing when I bought it, it's not OEM (not dropping $2k on an OEM cat), it's a Walker off of Rock Auto. I had driven it 3500 miles on that cat with zero codes or issues. I suppose it could have failed, but I would think that would throw a code?
I took it on a road trip that was fairly mountainous & we took a trip up to the peak of a mountain recently, 30 miles up a 10-12% grade the entire way, again zero issues. This was before the throttle problem started though.

 

[TobyP]

Does it only hesitate when you apply the throttle? What happens if you just let your foot of the brake, no throttle, and let it idle along, like in an empty parking lot?

And since I don't have one, can you command the throttle with your tech 2, or other scanner?

Yes, only when I go from brake to throttle. If I just let off the brake for a few seconds and let it roll a few feet, then throttle, it's normal. It will sit and idle smooth the entire time, once it's passed it's initial "hiccup" it's fine, revs fine, never lacks for power. It's just the right off of a stop hiccup.
If I do a rolling stop, it's fine. If I hold the brake at a stop & lightly hold the throttle so the rpm's don't drop to their normal low (mid 600's from the rpm gauge), keeping it around 800, it's fine.
I don't believe so, at least I have seen that option on the Tech II or Autel. I can just read the position demanded and actual.

 

[Mooseman]

Wild hunch here... Try replacing the brake booster vacuum valve and gasket. I remember faintly about someone with driveability issues because of that valve. Maybe...

 

[TobyP]

Wild hunch here... Try replacing the brake booster vacuum valve and gasket. I remember faintly about someone with driveability issues because of that valve. Maybe...

That's a great idea, I didn't even think of that, my other GM's are hydroboost so I forget about that potential vacuum leak. I'll pull the line and cap off the port tomorrow, see if it makes a difference.

 

[TobyP]

Well, I have an update. I'm not sure if this was entirely the fix or not, but it seems to be doing better. I blocked off the brake booster as suggested and there was no change, the valve was functioning properly.
I was checking fluids & decided to check the oil catch can... It was bone dry... Nothing in there at all. This got me wondering why, as there should at least be some sort of film in there from the vapors. The engine side of the PCV was plugged completely, no air flow through it at all. I filled the engine side hose with carb cleaner and let it set, then filled it with barrymans a few times while it slowly leaked down. Once it leaked down a few times and had time to penetrate and start breaking down whatever sludge was in there, I blew compressed air through the system (removed the oil fill first to give the air somewhere to go). It was slow at first, hit it with a few bursts of air, refill the hose with barrymans, and repeat. It eventually got to where the air would flow freely. I put everything back together and let it warm up, took it for a spirited drive to clear out anything that might have accumulated. It's not perfect, there is a very minor hesitation every once in a while, but for now it seems to have fixed it, fingers crossed. I'm sure there is still some gunk/ sludge in the system somewhere, I'm going to drive it next week and give it a few more barrymans soak treatments next weekend. I will give an update next weekend once I get some miles on it to test it more next week.