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.