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2003 Trailblazer LTZ
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11 Posts
Discussion Starter · #1 ·
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.
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, all new sensors (crank, ECT, both O2's, etc... All are ACDelco/ Delphi, nothing off brand), did the crank relearn.
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.
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!
 

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2002 Chevrolet TrailBlazer LTZ 4.2L
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524 Posts
What was the year, make and model of the donor vehicle. Did you swap the PCM from the donor? Did you check and change your air filter?
 

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2003 Trailblazer LTZ
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11 Posts
Discussion Starter · #3 ·
The donor vehicle was a 2003 TB LT. The only difference was it came with the EGR valve, my original engine just had the block off plate. I installed the block off plate on the replacement engine since I am using the stock PCM.
No, the PCM is from the original TB.
Yes, new wix air filter. Even without the intake plenum installed, it still does it. I removed it to watch the throttle body to see if there was erratic behavior, it appeared to be operating smoothly, no erratic movements.
The problem has just arose in the past ~400 miles. I have been daily driving it for 2800 miles (about 6 weeks) with zero issues (about 60 mile round trip), as well as a 450 mile road trip over Christmas.
It does this whether it is cold or warmed up.
If it is a quick stop and go, it doesn't do it as much (ie: quick stop sign), but at a stop light where I am sitting for a bit, it will always do it. If I let off the brake and let it naturally coast for a few feet, then hit the accelerator, it usually doesn't do it.
 

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'05 TB 4.2 EXT
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6,192 Posts
I KNOW you're talking a 5.3 here 'cause there ain't NO EGR on our 4.2s.

You're gonna have to take snapshots on a scanner --- even movies if they are enabled any more (I'm behind the 8-ball maybe on that).

But you need to see the problem as it happened, 'cause you'll never find it actually happening ---> unless by accident.

Here's where I'd go in not necessarily the best order ----> I warned you ...

1. MAP Sensor.
2. Knock Sensor.
3. MAF Sensor
4. B1S1 & B1S2 Oxygen Sensors --- is/am it/they new - is it Bosch? If so, Bad Robot.
5. Did you do the Crank Relearn Procedure?
6. Tell me you don't disconnect the battery to use Voo-Doo to fix it --- please!
7. Tell me <please> that you don't have the wrong plugs in the engine --- go 'wan --- what brand is in there?

I think it's a waste of time ---​
time you already spent anyway -​
to mess with the TPS or Throttle Pedal thingy -​
whatever that acronym is .. <sigh>​
 

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2003 Trailblazer LTZ
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11 Posts
Discussion Starter · #5 ·
Thank for the reply. Sorry, I am accustomed to working on LS engines, this is my first gm without a 5.3 or 6.0. It is the smog/ air pump, not EGR.

1. New, Delphi
2. New, Delphi
3. No MAF, this one only has MAP & IAT (also new, also Delphi)
4. Both new, both ACDelco
5. Yes, crank & cam sensors both new, both Delphi, crank relearn completed.
6. Absolutely NOT! I have a techII scanner & Autel. I pulled the #10 & #28 fuses per another post that said whenever disconnecting the throttle body to pull these so it goes into relearn.
7. ACDelco 41-103 plugs.

Everything is OEM, I learned long ago GM and aftermarket parts do not mix. I have owned and worked on many LS engines, but have never had any sort of throttle issues. I am fairly well mechanically inclined, but this has me stumped, especially without throwing any codes.

I have a hard time believing it is the actual throttle body, the chances of 2 throttle bodies having the exact same symptoms are slim.

What live data should I be looking at? I have a TechII scanner, so I am able to access everything as far as I know. I
 

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2004 chevy trailblazer_lt
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3,439 Posts
Have you checked to see what your fuel pressure is and if your fuel pressure regulator is working properly and not leaking?

Have you ever changed the fuel filter?
 

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2003 Trailblazer LTZ
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11 Posts
Discussion Starter · #7 ·
Have you checked to see what your fuel pressure is and if your fuel pressure regulator is working properly and not leaking?

Have you ever changed the fuel filter?
I have not checked the fuel pressure yet, I was hoping I could find it on the scan tool, but it looks like I will have to crawl underneath and test it manually.
The regulator is new & not leaking. I did pull the vacuum line off & pulled a vacuum on it manually with a mityvac, it did hold vacuum.
The fuel filter is also new, a wix.
The fuel system was completely flushed, it had been sitting since 2008. I also ran BG 44k during the first tank to eliminate anything residual left in the lines.
The injectors are new Delphis. I suppose 1 could be bad. I hate to pull the entire intake to test them unless I absolutely have to, it's a bit of a pain to get everything off.
 

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2003 Trailblazer LTZ
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11 Posts
Discussion Starter · #8 ·
I KNOW you're talking a 5.3 here 'cause there ain't NO EGR on our 4.2s.

You're gonna have to take snapshots on a scanner --- even movies if they are enabled any more (I'm behind the 8-ball maybe on that).

But you need to see the problem as it happened, 'cause you'll never find it actually happening ---> unless by accident.

Here's where I'd go in not necessarily the best order ----> I warned you ...

1. MAP Sensor.
2. Knock Sensor.
3. MAF Sensor
4. B1S1 & B1S2 Oxygen Sensors --- is/am it/they new - is it Bosch? If so, Bad Robot.
5. Did you do the Crank Relearn Procedure?
6. Tell me you don't disconnect the battery to use Voo-Doo to fix it --- please!
7. Tell me <please> that you don't have the wrong plugs in the engine --- go 'wan --- what brand is in there?

I think it's a waste of time ---​
time you already spent anyway -​
to mess with the TPS or Throttle Pedal thingy -​
whatever that acronym is .. <sigh>​
[/QUOTE]

Thank for the reply. Sorry, I am accustomed to working on LS engines, this is my first gm without a 5.3 or 6.0. It is the smog/ air pump, not EGR.

1. New, Delphi
2. New, Delphi
3. No MAF, this one only has MAP & IAT (also new, also Delphi)
4. Both new, both ACDelco
5. Yes, crank & cam sensors both new, both Delphi, crank relearn completed.
6. Absolutely NOT! I have a techII scanner & Autel. I pulled the #10 & #28 fuses per another post that said whenever disconnecting the throttle body to pull these so it goes into relearn.
7. ACDelco 41-103 plugs.

Everything is OEM, I learned long ago GM and aftermarket parts do not mix. I have owned and worked on many LS engines, but have never had any sort of throttle issues. I am fairly well mechanically inclined, but this has me stumped, especially without throwing any codes.

I have a hard time believing it is the actual throttle body, the chances of 2 throttle bodies having the exact same symptoms are slim.

What live data should I be looking at? I have a TechII scanner, so I am able to access everything as far as I know.

I have seen many forums where the throttle body & throttle pedal are notoriously problematic. I suppose it could be the throttle pedal, but I don't know why it would only be at acceleration and not problematic anywhere else in the power band it it was faulty.
 

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2004 chevy trailblazer_lt
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3,439 Posts
OK on hoping you could read the fuel pressure with the Tech II. I doubt that there is a fuel pressure senor in the fuel injection system.

Since no one else has asked, what DTCs are present (current, pending and historical)?
 

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2003 Trailblazer LTZ
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11 Posts
Discussion Starter · #10 ·
OK on hoping you could read the fuel pressure with the Tech II. I doubt that there is a fuel pressure senor in the fuel injection system.

Since no one else has asked, what DTCs are present (current, pending and historical)?
There are no codes at all. It has not thrown a single code since it has all gone back together. I have checked the PCM & BCM, both are completely clear. The battery has not been disconnected or died since it all went back together back in November. This is part of the reason I so stumped, no modules are reporting any problems.
 

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2004 chevy trailblazer_lt
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3,439 Posts
OK. The reason why I asked is because if you were looking at a throttle body or an accelerator pedal problem, then a pending DTC would have likely shown up when you read the OBDII system. So, in the meantime, let's keep this simple and look for horses and not zebras.

Find out what the fuel pressure is, and I would be monitoring the short and long term fuel trims to see if something shows up (a sudden swing to the positive or negative) when you start to accelerate and feel the hesitation.

Now for another question. When you fill up with gas, do you keep goosing the gas nozzle after the pump initially shuts off in order to get a full tank (or a better dollar amount on the gas pump), or do you stop gassing up and return the nozzle to the pump? Please answer honestly.
 
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· Registered
2003 Trailblazer LTZ
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11 Posts
Discussion Starter · #12 ·
OK. The reason why I asked is because if you were looking at a throttle body or an accelerator pedal problem, then a pending DTC would have likely shown up when you read the OBDII system. So, in the meantime, let's keep this simple and look for horses and not zebras.

Find out what the fuel pressure is, and I would be monitoring the short and long term fuel trims to see if something shows up (a sudden swing to the positive or negative) when you start to accelerate and feel the hesitation.

Now for another question. When you fill up with gas, do you keep goosing the gas nozzle after the pump initially shuts off in order to get a full tank (or a better dollar amount on the gas pump), or do you stop gassing up and return the nozzle to the pump? Please answer honestly.
I monitored fuel trim on my drive this morning, nothing out of the ordinary.
ST ranged between -9 to 5
LT ranges between -3 to 2.
There were no sudden drops or spikes during a stop and acceleration. It naturally went more negative when I slowed and stopped, more positive when accelerating, but nothing with huge swings.

I round it up to the next full dollar when fueling, but I don't "top it off", this does have the extra capacity fuel tank, thankfully with the mpg this thing gets. I average 14.5 with mostly highway driving. I knew these things God bad MPG, but my Yukon XL with a 6.0 gets 16mpg..
 

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2004 chevy trailblazer_lt
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3,439 Posts
OK on the fuel trims. I am a little suspicious of the STFT in that it sees a larger change of 14% points. The LTFT looks OK.

Have you ever monitored both the STFT and LTFT values over a time period, say 10 to 30 minutes? What I would be looking for are swings to the negative by 10+ percentage points. It could indicate a fuel saturated charcoal canister. What happens in that case is the EVAP valves open allowing air to flow through the charcoal canister and become a fuel carrier and is introduced to the engine. When this happens, the STFT will go highly negative (indicating a rich condition) for a period of time indicating that the PCM is cutting back on the amount of fuel being introduced into the combustion chamber.

I had this happen to me on a 1999 Buick Century. When I filled up I would try and round it off to the nearest tenth of a dollar or sometimes just keep going until the tank was full. Well, after several years of doing that, it finally caught up with me and I wound up changing out the charcoal canister. The old canister was significantly heavier than it's replacement (an ACDelco OEM part). I imagine it would have taken years for all the fuel to be purged from the charcoal, if ever.

I'm not saying this is the cause of what you are experiencing, but it might be worth watching your fuel trims over an extended drive time of 20 - 30 minutes.

Also, when you find out what your fuel pressure is please let us know.

Good Luck!
 

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2003 Trailblazer LTZ
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Discussion Starter · #14 ·
OK on the fuel trims. I am a little suspicious of the STFT in that it sees a larger change of 14% points. The LTFT looks OK.

Have you ever monitored both the STFT and LTFT values over a time period, say 10 to 30 minutes? What I would be looking for are swings to the negative by 10+ percentage points. It could indicate a fuel saturated charcoal canister. What happens in that case is the EVAP valves open allowing air to flow through the charcoal canister and become a fuel carrier and is introduced to the engine. When this happens, the STFT will go highly negative (indicating a rich condition) for a period of time indicating that the PCM is cutting back on the amount of fuel being introduced into the combustion chamber.

I had this happen to me on a 1999 Buick Century. When I filled up I would try and round it off to the nearest tenth of a dollar or sometimes just keep going until the tank was full. Well, after several years of doing that, it finally caught up with me and I wound up changing out the charcoal canister. The old canister was significantly heavier than it's replacement (an ACDelco OEM part). I imagine it would have taken years for all the fuel to be purged from the charcoal, if ever.

I'm not saying this is the cause of what you are experiencing, but it might be worth watching your fuel trims over an extended drive time of 20 - 30 minutes.

Also, when you find out what your fuel pressure is please let us know.

Good Luck!
I was wondering about the vapor canister, since it sat for 14 years with NASTY fuel in it. It has never had any issues starting, it always starts right up. There was no fuel in the evap lines, but I will check out the canister this weekend when I crawl under there to get the fuel pressure.

The fuel trim was over about a 40 minute drive, highway and freeway driving, with a few stoplights in between.

The first few tanks I filled up it actually spilled out of the filler neck, the bracket that holds it to the frame was not connected, so that it now fixed. Perhaps it filled the canister with fuel then & had delayed symptoms.

I will update this weekend when I have a chance to investigate further, thanks for the assistance/ideas!
 

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2003 Trailblazer LTZ
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Discussion Starter · #16 · (Edited)
Disconnect and then plug up the purge line at the plenum --- that's gonna bring in the truth --- quickly.
I did this last weekend when I removed the throttle body to clean it, it was completely dry. The back of the throttle body had a good coating of oil on the back side of the butterfly, considering it only has about 3k miles on it.
The bottom of the resonator housing had clear oil runs in it, no puddles but you could tell where oil was getting sucked into it. I'm definitely installing a catch can soon, for only 3k miles I am blown away with how much oil was there. I already installed a catch can on the intake portion, which catches a small amount, I guess both sides need it.

Also, yes, I have checked the catch can multiple times for a vacuum leak for this problem, it is sealed tight with no leaks.
 

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'05 TB 4.2 EXT
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Catch cans are amputating your head for a bad haircut.

Fix the problem .... raise the bridge instead of lowering the river!

Just Say NO to Catch Cans!

If you've got oil anywhere in the air intake system, you've got bigger fish to catch first.

 

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Discussion Starter · #18 ·
Catch cans are amputating your head for a bad haircut.

Fix the problem .... raise the bridge instead of lowering the river!

Just Say NO to Catch Cans!

If you've got oil anywhere in the air intake system, you've got bigger fish to catch first.

Interesting video, I will throw some barrymans down the tube this weekend. I don't know that it will help, but worth a shot. This majority of this engine was apart, thoroughly cleaned, and resealed / new gaskets throughout. The only gasket not replaced was the head gasket. All the passages were cleaned out with compressed air and brake clean to dissolve any hidden buildup that could be hiding. If it has somehow clogged the hose in 3k miles, then this is an absolutely awful PCV system.
I have run a catch can on all of my vehicles for many years. It's not about "lowering the river," it's about installing a damn upstream so the river doesn't destroy the bridge. Why let oil vapor into the combustion chamber in the first place when it can be easily stopped from getting in there.
 

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'05 TB 4.2 EXT
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because ------ oil vapors are NOT SUPPOSED TO SQUIRT OUTTA THE S-SHAPED HOSE INTO THE AIR FLOW IN THE FIRSTPLACE.

The problem is the head gasket --- which you have glibly admitted to having not been changed.

The only place the passageways are small enough to fill with [email protected] is there .... and B-12 will slowly, surely clean it out after a while .... or you can go back in and change the head gasket.

If one actually watches the video and listens to it --- the airflow is well documented and explained so that even a caveman can understand it.

I kept the video as far from a technical training film as I could without spoiling the actual operation of the system ... don't make me make another one with lines and arrows and paragraphs explaining the lines and arrows.

I cannot stoop that low.
 

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Discussion Starter · #20 ·
because ------ oil vapors are NOT SUPPOSED TO SQUIRT OUTTA THE S-SHAPED HOSE INTO THE AIR FLOW IN THE FIRSTPLACE.

The problem is the head gasket --- which you have glibly admitted to having not been changed.

The only place the passageways are small enough to fill with [email protected] is there .... and B-12 will slowly, surely clean it out after a while .... or you can go back in and change the head gasket.

If one actually watches the video and listens to it --- the airflow is well documented and explained so that even a caveman can understand it.

I kept the video as far from a technical training film as I could without spoiling the actual operation of the system ... don't make me make another one with lines and arrows and paragraphs explaining the lines and arrows.

I cannot stoop that low.
Yikes, I didn't mean to strike a nerve. By all means, stoop that low, that is what people asking for advice need is to be belittled. This is the first time I have asked for advice on any sort of forum, when my own research could not resolve the issue. Thank you for reminding my why I do not reach out for help.
 
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