2003 Tahoe 5.3 Flex a dog when accelerating

[76Nick]

Hey guys... So I have been slowly trying to work all the kinks out of my 03 Tahoe Z71 with the flex fuel 5.3. for the first time since I've owned it it has no check engine lights. Just replaced the cats and put a cat back with a Flowmaster super 44 and all 4 02 sensors, has a new AC Delco throttle body and this thing seems like a dog when I'm accelerating especially on the highway. Once I get it up to speed it's fine. The transmission was rebuilt 2 years ago and it shifts fine. It has 373 gears.

I noticed the problem most when I'm entering an on-ramp and need to get it up to speed quickly or when I have to go pass someone. I haven't towed with it yet but I just bought a trailer and I'm worried what that's going to be like. Also if I am pulling out of somewhere onto a busy street quickly I notice it. It feels like I'm starting off in second gear even though I'm not. Almost feels like it got worse since I replaced the cats. It used to have a catalyst efficiency code but I found out that's because the previous owner had punched out the cats so that is probably the case since now it has cats.

I will say that I have 35x12.50 tires on it which obviously exacerbates the problem put it still should accelerate faster than it does I would think. I have not changed the plugs or wires since I've gotten it so that's next on the list. I will also clean the MAF again as I did it last year when I was dealing with some RPM fluctuation issues. I also have to check for brake drag but I feel like if that was the case I would be smelling brake pad right? I was also thinking about a tuner but if they're underlying issues I'd like to fix those first.

If anyone else has any ideas or knows what this could be because they dealt with it it would be very appreciated. Thank you!

Oh, and I have a scan tool that can read data streams so I can check whatever to make sure I'm in the range I'm supposed to be...thanks!

 

[mikez71]

Are you saying it began right after the new exhaust was installed?

Check for any knock retard.
I had loose motor mount bolts that caused 10+ degrees of knock retard which severely limited acceleration.
You could have part of the exhaust touching something.

And since you mentioned the MAF, I would keep an eye on those readings to compare with others.

 

[76Nick]

Are you saying it began right after the new exhaust was installed?

Check for any knock retard.
I had loose motor mount bolts that caused 10+ degrees of knock retard which severely limited acceleration.
You could have part of the exhaust touching something.

And since you mentioned the MAF, I would keep an eye on those readings to compare with others.

Interesting.... I feel like it has probably definitely gotten worse since the exhaust but I didn't know if that was because it went from hollowed cats to actual cats. Then again I don't see how that alone would make it so doggish. I know my exhaust touches the frame in the back but with that cause that in the rear of the vehicle? Were you getting any codes with your issue? How did you find out that's what it was?

 

[mikez71]

No codes, just check your scanner for any knock retard.

If your exhaust is touching your frame, the vibrations can be picked up by the knock sensors, then the computer retards your timing.

Motor mounts seem to be one common cause of vibrations.
I had replaced mine not long before, apparently forgetting to tighten one side mount to block bolts.
But before I discovered that, I was banging my head, checking exhaust backpressure, checking datalogs.
Never heard any noise, but the sensors picked up vibrations.

 

[mikez71]

I know my exhaust touches the frame in the back

Definately try and undo that part, or maybe shove a rag in there to see if it helps. Or hammer the exhaust pipe clear of the frame.

 

[76Nick]

Definately try and undo that part, or maybe shove a rag in there to see if it helps. Or hammer the exhaust pipe clear of whatever it's touching.

Thank you I will definitely do that. I will also have to look on my scanner and see what I can see. I won't be back until tomorrow night but I'll dive into it as soon as I get home.

 

[Marky Dissod]

GM knock sensors work by detecting resonances Vibrations.
You can prove this by whacking your engine block, or in some cases even the transmission or main driveshaft,
with a metallic hammer - voila, perfectly repeatable false knock event.
@mikez71 said it best:

If your exhaust is touching your frame, the vibrations can be picked up by the knock sensors, then the computer retards your timing.

 

[76Nick]

GM knock sensors work by detecting resonances Vibrations.
You can prove this by whacking your engine block, or in some cases even the transmission or main driveshaft,
with a metallic hammer - voila, perfectly repeatable false knock event.
@mikez71 said it best:

Stupid question...and I know it might not be the same on all scanners but if I want to check if the knock sensors are retarding my timing or try to replicate this in real time as you said, what would the function or data stream more than likely be called in the scanner? Knock sensors, timing, etc? I've used it for other things but not this so I don't recall seeing anything. Probably because I was not looking for it...

 

[mikez71]

In my VCMSuite scanner software from HPTuners, it is listed under:

Engine > Spark&Ignition > Retard > Knock Retard.

Parameter [2321] from what I can tell..
Not sure if they all have the same hierarchy or what.

 

[76Nick]

In my VCMSuite scanner software from HPTuners, it is listed under:

Engine > Spark&Ignition > Retard > Knock Retard.

Parameter [2321] from what I can tell..
Not sure if they all have the same hierarchy or what.

Thanks Mike, I fool around with mine when I get home and see how it's set up.

 

[Burban22]

Throttle position sensor reading correctly? 100% when full throttle?

Intake tube completely on the throttle body? Not pinched at the bottom? Mass airflow sensor clean and reading correctly? You could try driving it with MAF disconnected to see if there is a difference.

Fuel pressure in spec? Have you changed fuel filter?

I notice a big difference going from stock 31.7” tires to 33” but it still gets on it and pulls hard from idle to 5.5k.

Here is a video 0 to 60 with 33 inch tires. I’m at 7000 feet elevation so it should be a second quicker near sea level.

 

[76Nick]

In my VCMSuite scanner software from HPTuners, it is listed under:

Engine > Spark&Ignition > Retard > Knock Retard.

Parameter [2321] from what I can tell..
Not sure if they all have the same hierarchy or what.

So I finally had a chance to mess with it and found the knock retard in my scan tool. It fluctuates between 0 and 6ish degrees when I really get on the gas. I have a Super 44 muffler so it can get loud sometimes but the change in retard didn't seem to coincide with when I was getting on the gas and the muffler was loudest all the time. I guess I should try again and try to seed if it happens more when I go over a bump. I will also try to put something in between the pipe and the crossmember to cushion it and see if that eliminates it.

Is 6 degrees enough to give a very noticeable change in acceleration?

 

[mikez71]

I had upto 14.8 degrees of retard! It felt like REP mode it was so extremely bad..
2-3 degrees maybe normal..? So I don't really know if 6 degrees is bad enough. Should be noticeable?

Are you able to see a graph or just an instantaneous reading?
I had one knock retard event last 25 seconds! It started at 14.9 degrees and tapered down. First ramp shown below..

I'm guessing real knock would tend to be eliminated quickly with the timing reduction, but my knock often tapered down slowly.
Keep us updated!

 

[Marky Dissod]

In theory, if it does not have enough timing in the first place, all the way up to the ideal spark advance with a 'perfect tune', 0° of retard.
If the knock is REAL, INSIDE the combustion chamber, and it takes X° of retard to stop the knock, take 1/2X° out of the spark table,
in that area and immediately surrounding it.
Normal amount of retard is 0.0°. Anything past 2° of real retard requires attention in that area.

If the knock is FALSE, it's most likely coming from OUTSIDE the combustion chamber -
our knock sensors are acoustic / vibration detectors. Prove this by whacking the block with a ball peen hammer while watching the knock retard.
If you watch the timing it subtracts when you whack it, it should be obvious that any correction made to the timing tables is irrelevant -
either something needs to be adjusted, or tightened, or dampened in some way, to cure the false knock.

It fluctuates between 0 and 6ish degrees when I really get on the gas.
I have a Super 44 muffler so it can get loud sometimes, but the change in retard didn't seem to coincide with when I was getting on the gas,
and the muffler was loudest all the time.
I guess I should try again and try to see if it happens more when I go over a bump.
I will also try to put something in between the pipe and the crossmember to cushion it and see if that eliminates it.

Is 6 degrees enough to give a very noticeable change in acceleration?

It can be. Depends where / when the retard occurs.
It's a good idea to get something in between the pipe and the crossmember, like a makeshift bumpstop, or even some spray-on cushioning might be enough.

 

[76Nick]

I can graph it Mike so I will take it for another ride later and record the graph. Also mine slowly tapers off back to zero as well. The retard and timing also seems to be a little bit delayed from when I get on the throttle if that makes any sense.

Throttle position sensor reading correctly? 100% when full throttle?

Intake tube completely on the throttle body? Not pinched at the bottom? Mass airflow sensor clean and reading correctly? You could try driving it with MAF disconnected to see if there is a difference.

Fuel pressure in spec? Have you changed fuel filter?

I notice a big difference going from stock 31.7” tires to 33” but it still gets on it and pulls hard from idle to 5.5k.

Here is a video 0 to 60 with 33 inch tires. I’m at 7000 feet elevation so it should be a second quicker near sea level.

I have a No name aftermarket intake, Amazon special. It's tight at the TB however where the filter clamps onto the tube is questionable. Definitely seems like the filter isn't meant to go on the tube that it came with and is too big for it so I did what I could to make it work. Also I have 35 inch tires as I stated so I'm sure this adds to it. I am however in Connecticut... Inland but still pretty close to sea level.

 

[555hp]

Throttle position sensor reading correctly? 100% when full throttle?

Intake tube completely on the throttle body? Not pinched at the bottom? Mass airflow sensor clean and reading correctly? You could try driving it with MAF disconnected to see if there is a difference.

Fuel pressure in spec? Have you changed fuel filter?

I notice a big difference going from stock 31.7” tires to 33” but it still gets on it and pulls hard from idle to 5.5k.

Here is a video 0 to 60 with 33 inch tires. I’m at 7000 feet elevation so it should be a second quicker near sea level.

Fellow 2003 5.3 Flex Burb owner. Interested in doing upgrades similar to yours -- tune, exhaust, 33 inch tires and was wondering what, if anything you would do different.

Already did 2in up country lift, planning on 285 70/17 on factory Z71 rinse and efan conversion. Thinking about HPtuner vs BB tune, though concerned about the learning curve (and/or making a mistake in tuning that will **** the engine).

Any advice appreciated!

 

[Marky Dissod]

Thinking about HPtuner vs BB tune, though concerned about the learning curve
(and/or making a mistake in tuning that will ... ruin(?) ... the engine).

PCMperformance Programming

$250, very highly reputable GM Gen3 V8 tuning for the past 2 decades

If you're gonna use HPTuners, old@$$ powertrains don't get ANY additional functionality from buying HPTuners past version 2.24.
It's not hard to avoid making mistakes that will disembowel your engine -
just make your changes GRADUALLY in reference to previous.
That said, a decent mail-order tune will save you months and be a solid long-term investment,
even if you do want to improve on your mail-order tune.

 

[555hp]

PCMperformance Programming

$250, very highly reputable GM Gen3 V8 tuning for the past 2 decades

If you're gonna use HPTuners, old@$$ powertrains don't get ANY additional functionality from buying HPTuners past version 2.24.
It's not hard to avoid making mistakes that will disembowel your engine -
just make your changes GRADUALLY in reference to previous.
That said, a decent mail-order tune will save you months and be a solid long-term investment,
even if you do want to improve on your mail-order tune.

Thanks for the info!!

 

[Yukongreed]

I recently put a new hose on my secondary vac air system (original was corroded and missing in places not passing smog) and my truck has changed in throttle response pretty well, WOT it moves out and doesn’t do that wrong gear feel anymore.

Can any mechanics second that making sure the secondary vac air system is working properly would do this? That’s the only thing I’ve done and it’s a noticeable difference in acceleration and smoother engine operation overall too.

Check your hoses lmk!

 

[Mudsport96]

I have a No name aftermarket intake, Amazon special. It's tight at the TB however where the filter clamps onto it.

Monitor your air intake temps. I'll bet they are actually worse than a stock intake box, since you are now pulling under hood air.

Also I have 35 inch tires as I stated so I'm sure this adds to it. I am however in Connecticut... Inland but still pretty close to sea level.

And this is most likely the majority of the problem right here. A 35 is an 11% increase in diameter. That turns your 3.73 into a 3.31 just by diameter alone. Then add in width increase to raise your rolling resistance. Oh and weight increase too. For instance a uniroyal Laredo 265/75r16 is 41 pounds, in contrast a 35x12.5r17 TSL ssr swamper is 82.1 pounds...literally double the mass.
What 35s are you running? Because rubber compounds differ and can influence how a vehicle acts.

Personal experience, I dynoed my old 96 blazer. On stock tires it made 188hp 250lb/ft. With a tune, mandrel bent 3 inch exhaust from the y-pipe through a dynomax race bullet all the way out the back.
Then I dynoed it with the 32×9.50s They were 10% taller but almost the same width as stock. And a good bit heavier. For instance a 235/75r15 uniroyal Laredo is 31 pounds the 32x9.50 tsl i was running was 52 pounds. And that is unsprung weight.
The dyno after was ridiculous. 168hp and 224tq. That is a loss of 20 horsepower and 25 lb-ft of torque. From tire change alone. VERY noticeable.
Keep in mind this was without torque management and on 93 octane with no knock retard.
What i would do in your place. Is see if a a friend has a set of stock tires you can use. Then take the truck out for a spin. I would bet a dollar to a doughnut it accelerates astonishingly better. Also ditch the hot air intake. That is another power killer. Like mentioned in another thread, the same stock box and filter are used for the 8.1 engine. It is not restricting a 5.3.