Engine Struggling to Accelerate at WoT

[CrashDriver]

Hi, this is my first post on this forum; I have a 1994 K2500 with a TBI 350. Before I go down a bunch of useless rabbit trails of part purchases I figured I'd see if anyone has a clue about my issue on here.
So fairly recently I started to notice the truck was running... I don't know if I would quite describe it as 'rough' at around 50-60 mph, but it seemed to slightly struggle with throttle and kind of 'chug' a little.
It seemed to get worse over time, and the transmission was feeling like it was shifting weird. And it started to sound like it was backfiring out the intake when I was really going hard on it as well.
Then the other day just to test I went from a stop to WoT all the way to 60 mph - it was really rough. It didn't want downshift and it lugged hard and when I got up to 3,500-4 it actually started to knock I think. It didn't sound to me like a 'ping' really, it was more like I could hear each stroke clearly.
On a hunch I replaced the ISS and VSS sensors on the transmission (4l80e), and that helped greatly and it shifts normally now.
I can throttle it hard on the road and it's pretty good, the engine runs completely normal, except when I go from a complete stop to WoT. It shifts right but it still struggles, either backfiring out the intake or knocking or what, it's hard for me to tell since this is my first time dealing with engine knocks.
I'm wondering if this could be related to the engine not getting the right fuel mixture, or spark? my uneducated guesses are leading me to think I may need a new MAP sensor or TPS - or even plugs and wires. What do you guys think?

 

[movietvet]

Welcome to the forum from Oregon.

Let's start with info: Age of plugs, wires, cap, rotor, fuel filter.....Have you done a fuel pressure test? You can get a long enough hose to reach out back of hood and tape to w/s to read. Any CEL? Check for clogged cats?

 

[CrashDriver]

Welcome to the forum from Oregon.

Let's start with info: Age of plugs, wires, cap, rotor, fuel filter.....Have you done a fuel pressure test? You can get a long enough hose to reach out back of hood and tape to w/s to read. Any CEL? Check for clogged cats?

The plugs are fairly new, the wires have been on the truck at least two years (that's how long I've owned it, I think they were replaced right before I bought it), fuel filter is... I wanna say close to a year old, I replaced an old one. I have not checked fuel pressure yet, although the pump is somewhat new (for better or worse). There are no CEL codes, I will double check tomorrow. I don't have an OBD-I reader unfortunately. Also... the cat may or may not have been removed before I bought the truck.
So I suppose I'll check the fuel pressure to start with.

 

[movietvet]

Miles on the truck since you bought 2 years ago? Cat removed?....Know for sure. Remove plugs and show pics of what they look like. Make sure to identify the hole they came out of.

 

[JDGMC]

A couple things you can do in the meantime. Look at your spark plugs (all of them) and take a photo and post it. Look at your ink spray pattern at idle and at a higher rpm in park. Note pattern, drips and differences between banks.

 

[CrashDriver]

Miles on the truck since you bought 2 years ago? Cat removed?....Know for sure. Remove plugs and show pics of what they look like. Make sure to identify the hole they came out of.

I believe I bought it at 142kish, it's at 150 now, and yes, there is no catalytic converter whatsoever. I will put pictures up as soon as I can

 

[CrashDriver]

So I pulled the plugs out of the truck today, they all looked pretty much the same, like this picture.

You must be registered for see images attach

I guess it's kind of glazed looking, so I got some cooler running plugs and replaced all 8 (making sure they were gapped right as well).

There's no difference in how the truck drives whatsoever unfortunately, it still pops and it feels kind of weak.
Odd thing I noticed, sometimes it doesn't pop - but still lacks power. I think it's something to do with how high the rpms are when I floor it, but it's hard to pinpoint.

Also I don't have a t-junction to test the fuel pressure with so that's going to be a bit before I get to that.

 

[JDGMC]

It doesn’t look like a fuel issue to me. Double-check every vacuum connection under the hood. You can check the MAP, TPS, and ECT with a vacuum gauge/multimeter. The EGR should be checked also.

 

[JDGMC]

So I pulled the plugs out of the truck today, they all looked pretty much the same, like this picture.

You must be registered for see images attach

I guess it's kind of glazed looking, so I got some cooler running plugs and replaced all 8 (making sure they were gapped right as well).

There's no difference in how the truck drives whatsoever unfortunately, it still pops and it feels kind of weak.
Odd thing I noticed, sometimes it doesn't pop - but still lacks power. I think it's something to do with how high the rpms are when I floor it, but it's hard to pinpoint.

Also I don't have a t-junction to test the fuel pressure with so that's going to be a bit before I get to that.

You can get the manuals at the link below if you don’t have them already. You will need a hand vacuum pump w/gauge to check the EGR and MAP.

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

 

[Schurkey]

1994 K2500 with a TBI 350.

the engine runs completely normal, except when I go from a complete stop to WoT... ...It shifts right but it still struggles, either backfiring out the intake or knocking or what, it's hard for me to tell since this is my first time dealing with engine knocks.
I'm wondering if this could be related to the engine not getting the right fuel mixture, or spark? my uneducated guesses are leading me to think I may need a new MAP sensor or TPS - or even plugs and wires. What do you guys think?

Don't guess.

Check fuel pressure at prime, and under load--when it's backfiring and acting-up.

Assure that all the usual "tune-up" parts and procedures are functioning correctly--cap, rotor, plug wires, clean or replace the spark plugs. Assure the PCV and EGR systems are working properly. Fuel and air filters are not restricting flow. O2 sensor is reasonably fresh. Charcoal canister, and heated-air intake function properly.

Connect a scan tool, verify EVERY sensor and computer output. Check the fuel trims, especially when the engine is misbehaving.