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'99 S10 4.3 2WD RegCab
1999, S10, 4.3, 2WD, Auto, RC-SB, Base
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19 Posts
Discussion Starter · #1 · (Edited)
1999, S10, 4.3, Auto, 2WD, Reg Cab, Base, 322K Miles

Getting a weird acceleration issue when I begin accelerating from a full stop. Engine seems to take a half second before it decides to go, occasionally with a small stutter.

The harder I press the pedal the worse it seems to be. If I go SUPER light on the pedal, it doesn't occur.

While cruising around there's no issues, drives totally normal. It's only from a stop that it does this, and it is occurring at running temp. Not sure about at cold, I'll check tomorrow morning. Also is idling totally fine.

I searched the forums and read some posts on similar issues. Most seemed to suggest checking:
  • TPS
  • MAF
  • Fuel Pressure & Filter
  • Exhaust O2 Sensors
I have recently replaced my Cat and exhaust gaskets, so I'm thinking of starting by checking those O2 Sensors.

Im looking for help trouble shooting. Anyone else have any advice or dealt with this problem before? Hoping to find a solution soon because it's very annoying when I need to clear an intersection...
 

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2,608 Posts
Check the IAC, take it out and clean it. I suspect you have a vacuum leak somewhere.
Do you have a scan tool that you can watch the fuel trims?
Have you tried running it with the MAF disconnected?
 

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'99 S10 4.3 2WD RegCab
1999, S10, 4.3, 2WD, Auto, RC-SB, Base
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19 Posts
Discussion Starter · #3 · (Edited)
Check the IAC, take it out and clean it. I suspect you have a vacuum leak somewhere.
Good idea. I'll look into those tomorrow morning.

Do you have a scan tool that you can watch the fuel trims?
Have you tried running it with the MAF disconnected?
No scan tool. Haven't tried with the MAF disco yet either. I'll give her a try tomorrow.
 

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Mr Goodwrench's Evil Twin
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567 Posts
When is the last time it had a tune-up? Spark plugs, distributor cap & rotor, fuel filter, clean the MAF sensor (use ONLY MAF sensor cleaner for this). Gotta start with the simple maintenance items first
 

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'99 S10 4.3 2WD RegCab
1999, S10, 4.3, 2WD, Auto, RC-SB, Base
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19 Posts
Discussion Starter · #5 ·
When is the last time it had a tune-up?
Specifically a tune up? Never. 322K+ miles. I mostly work on and maintain her myself as issues arise. Most recently rebuilt the steering gearbox.
Spark plugs, distributor cap & rotor, fuel filter, clean the MAF sensor (use ONLY MAF sensor cleaner for this). Gotta start with the simple maintenance items first
Just changed the fuel filter and it didn't fix it. Yeah working my way through the simple stuff. Cleaning the MAF is next, then spark plugs I guess though those aren't that old.
 

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1,661 Posts
I am going to fault the spider injection, although your year has the upgraded model I still feel it is suspect with that mileage.
 
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'99 S10 4.3 2WD RegCab
1999, S10, 4.3, 2WD, Auto, RC-SB, Base
Joined
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19 Posts
Discussion Starter · #7 · (Edited)
I am going to fault the spider injection, although your year has the upgraded model I still feel it is suspect with that mileage.
I don't think the spider is the issue. She's not misfiring or anything. I did the lower intake gaskets about a month ago, and had her out. It looked like it was in pretty good shape all things considered. I'll add that to the bottom of my list of things to check though.
 

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Mr Goodwrench's Evil Twin
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567 Posts
Specifically a tune up? Never.
My question was more or less asking if all the normal scheduled maintenance items I mentioned have been replaced and/or checked since this problem arose.
I mostly work on and maintain her myself as issues arise.
If you typically wait until problems arise before giving attention to things like spark plugs, fuel filter etc, that's all the more reason to check the items that are supposed to be done as "scheduled maintenance". I notice you didn't mention having ever done the dizzy cap and rotor. If you haven't replaced them or checked them since the problem started... check them. The distributor cap is 2 screws away from being visually inspected. If there are crustaceans on the electrical contacts inside of it, it should be replaced along with the rotor.

The one thing I've learned more than anything else in my 15+ years working on cars day in and day out, it's that when a customer says their vehicle has a hiccup, stutter, hesitation, runs rough, stalls or any other drivability complaint and there are no trouble codes stored... I ALWAYS check ALL of the regular maintenance items first. Spark plugs/plug wires, fuel filter, cap and rotor (if equipped), clean the MAF and clean the throttle body. These few items alone are the source of around 75% of drivability complaints that have no trouble codes associated with them. And whenever there is doubt or uncertainty of age or mileage... replace it. Far more often than not, this methodology gets rid of the drivability issue. And if it doesn't, the vehicle will be up to date on scheduled maintenance so the maintenance stuff can then be ruled out.
then spark plugs I guess though those aren't that old.
For what it's worth, when a customer tells me that a component is new or near new, it moves higher on my suspect list. Not lower. New and near new parts fail all the time. It's new does not mean it's good.
She's not misfiring or anything.
If it has a hesitation or sputter, that would be because of a misfire. At 322k miles it very well could have a compression issue. Engine could quite literally just be worn out.

TL;DR If it is scheduled maintenance and you have not yet checked it or cleaned it... check it, replace it or clean it
 

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'99 S10 4.3 2WD RegCab
1999, S10, 4.3, 2WD, Auto, RC-SB, Base
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19 Posts
Discussion Starter · #9 ·
My question was more or less asking if all the normal scheduled maintenance items I mentioned have been replaced and/or checked since this problem arose.
Ah, I think I misunderstood you. Yes, I keep her well maintained. I perform fluid changes and scheduled maintenance regularly, and I don't put in cheap replacements. I usually spring for ACDelco or other quality brands. Most people don't believe me when I tell them how many miles she has.

Here's what I've checked/done so far:
  • Replaced Fuel Filter - Wix filter. She was due for a new one in about two thousand anyway.
  • Cleaned MAF - It was already very clean, but I cleaned it again anyway
  • Cleaned Throttle Body - A little dirty actually, Cleaned her pretty good as far as my fingers would reach. Still need to take it off and clean it thoroughly though.
  • Checked Spark Plugs - ACDelco Iridium, 50K on them. Everything seemed fine here.
Some other things I've noticed:
  • The hesitation is somewhat intermittent. It does it more often than not, and seems to vary slightly in duration (about 1/2 a second).
  • She does not hesitate when the engine is cold; as in immediately after starting her up. Once she's up to temp she starts doing it.
  • The engine does not feel like it has lost any power and is running perfectly smooth when idling, cruising, and accelerating; except when accelerating from a full stop.
Next I'll examine the distributor and properly clean the throttle body. I removed the distributor cap and rotor about a month ago when I replaced her lower intake gaskets. Everything looked fine then, but I know that doesn't guarantee it now (and yes it's aligned correctly). I have a suspicion that it's the TPS causing the issue though.
 

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4,752 Posts
When the distributor was out, how was the wear on the shaft and gear?
Did you do a crank/cam relearn after intake repairs?
 

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Mr Goodwrench's Evil Twin
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567 Posts
I have a suspicion that it's the TPS causing the issue though.
I haven't seen many TPS issues on 4.3Ls. That said I don't see a whole lot of them with 300k+ on the clock either. Have you checked it for vacuum leaks? Perhaps something is wonky with the intake job you just did on it. And honestly, given the mileage, I would do a compression test on it before chasing things like sensors and fuel injection. Otherwise, if it does have low compression you'd be chasing a ghost.

If compression is ok then I would suggest going to your local parts store and get a loaner scan tool. The live data stream and fuel trim values would be extremely useful information.
 
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'99 S10 4.3 2WD RegCab
1999, S10, 4.3, 2WD, Auto, RC-SB, Base
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19 Posts
Discussion Starter · #12 ·
When the distributor was out, how was the wear on the shaft and gear?
Minimal to none. It was replaced a while ago, don't remember exactly when. Maybe 150K ago. I was very surprised it looked as good as it did for the mileage.

Did you do a crank/cam relearn after intake repairs?
I did not.
 

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'99 S10 4.3 2WD RegCab
1999, S10, 4.3, 2WD, Auto, RC-SB, Base
Joined
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19 Posts
Discussion Starter · #13 · (Edited)
Have you checked it for vacuum leaks?
I have and i didn't find any. Idle is fine which further suggests that there's no vacuum problems.

Perhaps something is wonky with the intake job you just did on it.
I had a family member who is a retired mechanic help me with that job so I'm pretty confident we did it right. Could be something went wrong though.

I haven't seen many TPS issues on 4.3Ls. That said I don't see a whole lot of them with 300k+ on the clock either.
I suspect the TPS I because it occurs when I re-engage the throttle after being stopped, and only once the engine has been running for a bit and is warmed up. I know sometimes heat can cause electronics to squirrel out. It almost feel like when an accelerator pump is wearing on on a carburetor vehicle.

The conditions to cause the hesitation seem so specific. It really makes me think the issue isn't spark, fuel pressure, or vacuum as I would image it would give me problems in other states of driving like a rough idle, struggling to accelerate while at speed, a lack of power - and she does none of those. Ive has those kind of issues with other vehicles, and even though it's not a very scientific answer, it just doesnt "feel" like those problems do. I'm pretty good with cars but by no means am I a real mechanic so I could be 100% wrong here and might sound like an idiot.

And honestly, given the mileage
If I had a nickel for everytime I've heard or said that...

I would do a compression test on it before chasing things like sensors and fuel injection. Otherwise, if it does have low compression you'd be chasing a ghost.
How do I go about doing that?

If compression is ok then I would suggest going to your local parts store and get a loaner scan tool. The live data stream and fuel trim values would be extremely useful information.
I wa actually think about stopping by somewhere and seeing if they could check for pending codes. I'm wondering if it's not triggering the CEL because it's not continually hesitating/misfiring, it should still have something stored in there right? Maybe I get something to point me in the right direction.
 

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Mr Goodwrench's Evil Twin
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567 Posts
For a compression test you'll need a compression test gauge like this one. They cost about $30 from most auto parts stores. To perform the test you will need to make sure you have a good strong battery that is fully charged. Then follow these steps:

1: Warm engine to operating temperature
2: Stop engine and remove spark plugs from all 6 cylinders (be careful not to burn yourself on the exhaust)
3: Disable fuel system by removing fuel pump relay
4: Disable ignition system by unplugging the ignition coil or ignition control module
5: Prop the throttle plate in the throttle body wide open using a wrench or screwdriver
6: Install compression tester into a spark plug hole
7: Crank engine over 5-6 revolutions. Because the spark plugs are removed from all the cylinders, the engine will spin easier than normal until it reaches the cylinder with the compression tester installed. Cranking speed will audibly slow when it reaches that cylinder, so just crank the engine while counting those compression strokes 5-6 times then release the key.
8: Watch the gauge while the engine is cranking over (it helps to have a buddy do the cranking). The compression should rise evenly on each compression stroke before reaching its highest reading. If a cylinder is weak on the first compression stroke and then builds on the following strokes, this indicates worn or sticking rings. If it is weak on the first stroke and fails to gain much on the following strokes, this indicates valve troubles.
9. Write down the max reading on the compression gauge. It never hurts to be a little redundant here and run the test twice on each cylinder to make sure you get the same reading on the gauge
10: Repeat steps 6-9 for each cylinder, using the same number of compression strokes for each cylinder reading (meaning don't let it crank over 8 times on one cylinder then 3 times on the next cylinder)

Interpreting the results:
Compare your readings for all 6 cylinders. They should all be somewhat similar. GMs spec is that no one cylinder should be below 100psi and the cylinder with the lowest result should be no more than 30% less than the cylinder with the highest result. Some people say 20%, some say 10%. In my experience most engines will start to exhibit issues around the 15-20% mark.

If any one cylinder falls outside that range, repeat the test on that cylinder but add a cap full of oil to the cylinder via the spark plug hole prior to running the test again. If the compression in that cylinder improves after adding the oil to the cylinder, then it's got worn or stuck piston rings. If the compression doesn't improve after adding the oil, the issue is valve related.

If neighboring cylinders both have low readings, this indicated the head gasket has failed between those two cylinders.

For example, consider the following numbers:
Cyl 1: 142psi
cyl 2: 139 psi
cyl 3: 150 psi
cyl 4: 141 psi
cyl 5: 140 psi
cyl 6: 160 psi

The above readings would be considered good. However, if the above readings were all the same except the cyl 2 reading was 115, that would mean there is an issue in cylinder 2. If you add oil to cyl 2 and the reading doesn't change, it's valve trouble. If you add oil to cyl 2 and the reading increases, worn rings. And for example if cyl 2 was 115 and its neighbor cyl.4 was 110, then the head gasket is no longer sealing between those two cylinders.

Hopefully that all makes sense. It sounds complicated but it is actually quite easy, you'll get the hang of it pretty quickly. Post your results if you have any questions about what they indicate. The numbers don't matter much as long as they are fairly consistent across the board. Where the numbers begin to matter is if they aren't consistent across the board
 
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'99 S10 4.3 2WD RegCab
1999, S10, 4.3, 2WD, Auto, RC-SB, Base
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19 Posts
Discussion Starter · #15 · (Edited)
For a compression test you'll need a compression test gauge like this one. They cost about $30 from most auto parts stores. To perform the test you will need to make sure you have a good strong battery that is fully charged. Then follow these steps:

1: Warm engine to operating temperature
2: Stop engine and remove spark plugs from all 6 cylinders (be careful not to burn yourself on the exhaust)
3: Disable fuel system by removing fuel pump relay
4: Disable ignition system by unplugging the ignition coil or ignition control module
5: Prop the throttle plate in the throttle body wide open using a wrench or screwdriver
6: Install compression tester into a spark plug hole
7: Crank engine over 5-6 revolutions. Because the spark plugs are removed from all the cylinders, the engine will spin easier than normal until it reaches the cylinder with the compression tester installed. Cranking speed will audibly slow when it reaches that cylinder, so just crank the engine while counting those compression strokes 5-6 times then release the key.
8: Watch the gauge while the engine is cranking over (it helps to have a buddy do the cranking). The compression should rise evenly on each compression stroke before reaching its highest reading. If a cylinder is weak on the first compression stroke and then builds on the following strokes, this indicates worn or sticking rings. If it is weak on the first stroke and fails to gain much on the following strokes, this indicates valve troubles.
9. Write down the max reading on the compression gauge. It never hurts to be a little redundant here and run the test twice on each cylinder to make sure you get the same reading on the gauge
10: Repeat steps 6-9 for each cylinder, using the same number of compression strokes for each cylinder reading (meaning don't let it crank over 8 times on one cylinder then 3 times on the next cylinder)

Interpreting the results:
Compare your readings for all 6 cylinders. They should all be somewhat similar. GMs spec is that no one cylinder should be below 100psi and the cylinder with the lowest result should be no more than 30% less than the cylinder with the highest result. Some people say 20%, some say 10%. In my experience most engines will start to exhibit issues around the 15-20% mark.

If any one cylinder falls outside that range, repeat the test on that cylinder but add a cap full of oil to the cylinder via the spark plug hole prior to running the test again. If the compression in that cylinder improves after adding the oil to the cylinder, then it's got worn or stuck piston rings. If the compression doesn't improve after adding the oil, the issue is valve related.

If neighboring cylinders both have low readings, this indicated the head gasket has failed between those two cylinders.

For example, consider the following numbers:
Cyl 1: 142psi
cyl 2: 139 psi
cyl 3: 150 psi
cyl 4: 141 psi
cyl 5: 140 psi
cyl 6: 160 psi

The above readings would be considered good. However, if the above readings were all the same except the cyl 2 reading was 115, that would mean there is an issue in cylinder 2. If you add oil to cyl 2 and the reading doesn't change, it's valve trouble. If you add oil to cyl 2 and the reading increases, worn rings. And for example if cyl 2 was 115 and its neighbor cyl.4 was 110, then the head gasket is no longer sealing between those two cylinders.

Hopefully that all makes sense. It sounds complicated but it is actually quite easy, you'll get the hang of it pretty quickly. Post your results if you have any questions about what they indicate. The numbers don't matter much as long as they are fairly consistent across the board. Where the numbers begin to matter is if they aren't consistent across the board
Thanks for writing all that for me.
I'm busy with some stuff for the week, but this weekend but I'm gonna runs some tests - Fuel Pressure check, Smoke test, and compression test. I'll let yall know what I find.

The only update I have for now is my buddy let me borrow his OBD2 scanner and the live data showed she was running lean at op temp idle in both banks (-9 to -13%). Does fine if at cold idle and if I accelerate at any temp. Seems fuel related so I'm curious to see what the fuel pressure test reveals. I'll probably start with that one then do the compression test. Fingers crossed its not the spider, but I guess I can't be mad if she made it 320K....
 

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Mr Goodwrench's Evil Twin
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567 Posts
Fingers crossed its not the spider, but I guess I can't be mad if she made it 320K....
If you're still on the factory spider at 320k then that's gotta be a record. Furthest I've ever seen a spider make it was about 230k miles. That said, a leaky spider would result in negative fuel trims, not positive. If the fuel trims are better while on the throttle and trend positive at idle, that's indicative of a vacuum leak or faulty MAF.
 

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'99 S10 4.3 2WD RegCab
1999, S10, 4.3, 2WD, Auto, RC-SB, Base
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19 Posts
Discussion Starter · #17 · (Edited)
If you're still on the factory spider at 320k then that's gotta be a record. Furthest I've ever seen a spider make it was about 230k miles.
Factory spider. No joke, 322,863 on her.
349757


That said, a leaky spider would result in negative fuel trims, not positive. If the fuel trims are better while on the throttle and trend positive at idle, that's indicative of a vacuum leak or faulty MAF.
Typo. It was negative (I edited it), but since I caught her doing that once she hasn't done it since, and tends towards rich. The hesitation is still there.
 

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'99 S10 4.3 2WD RegCab
1999, S10, 4.3, 2WD, Auto, RC-SB, Base
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19 Posts
Discussion Starter · #18 · (Edited)
Here's what the scanner's live data reads right now, while she's parked and idling at op. temp. Everything looks pretty good, but LTFT a bit rich:
349758

349759

349760

349761

349762
 

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'99 S10 4.3 2WD RegCab
1999, S10, 4.3, 2WD, Auto, RC-SB, Base
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19 Posts
Discussion Starter · #19 ·
And this one is while cruising @ 50MPH:
349764
 
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