“Lag”?

[Schurkey]

it just wants to bog out on me again under load.

That doesn't sound like a transmission problem.

Decided to test a theory and see what it does at revs. Full throttle the truck acts like it has like a launch control.

I don't know what that means.

Itll reach to about 3500-3800rpm and just hold there and sputter. Half throttle and I can full rev.

That's not a trans problem.

What is your fuel pressure under load, and when the truck "sputters"?

Connect a scan tool, look at the data stream before it "sputters"; and while it's "sputtering". Also check the misfire history. Maybe you can narrow-down the problem to one or two cylinders.

On standby I have a new distributor cap and rotor along with new wires.

Probably wise. Don't forget to check the spark plugs for wear, deposits, cracked porcelains, etc.

On the way I have the jumped wire for trans flare.

Also probably wise, but none of this sounds like a trans flare, it sounds like the engine misfires.

Now Im starting to believe my issue may be throttle body related?

That's a statement, not a question.

cause normally when you floor a pedal of any car itll hit the limiter or bounce off it. Mine? Half way then it just bounces like launch control. Its just wild.

"Launch control" means "rev limiter"?

These trucks have a speed limiter, and they've got fuel maps that--I think--don't allow excess RPM; but that wouldn't sound like a typical "hot-rod" ignition-based rev limiter.

MAYBE this is fuel-pressure related, MAYBE this is a failure in the ignition system--faulty coil, cap, rotor, plug wires, plugs.

TEST the coil with a spark-tester calibrated for HEI. If your coil will reliably fire the spark tester, the new coil you might be tempted to buy is probably lower-quality than the one on your vehicle right now.

 

[tooturntdad]

That doesn't sound like a transmission problem.


I don't know what that means.


That's not a trans problem.

What is your fuel pressure under load, and when the truck "sputters"?

Connect a scan tool, look at the data stream before it "sputters"; and while it's "sputtering". Also check the misfire history. Maybe you can narrow-down the problem to one or two cylinders.


Probably wise. Don't forget to check the spark plugs for wear, deposits, cracked porcelains, etc.


Also probably wise, but none of this sounds like a trans flare, it sounds like the engine misfires.


That's a statement, not a question.


"Launch control" means "rev limiter"?

These trucks have a speed limiter, and they've got fuel maps that--I think--don't allow excess RPM; but that wouldn't sound like a typical "hot-rod" ignition-based rev limiter.

MAYBE this is fuel-pressure related, MAYBE this is a failure in the ignition system--faulty coil, cap, rotor, plug wires, plugs.

TEST the coil with a spark-tester calibrated for HEI. If your coil will reliably fire the spark tester, the new coil you might be tempted to buy is probably lower-quality than the one on your vehicle right now.

Sure know how to break down a paragraph.
That “launch control” reference is just what best describes the way it sounds and acts. I come from the rally enthusiast community. Well, motorsport world.

Instead of launching a car at start full rev and cause spinning during initial acceleration from a dig you can set the rpm to whatever and can have it “bounce” while having the throttle completely to the floor. Cool stuff, I never really got into it with any of my cars though.

But anyway, originally I thought this was a trans issue before digging deeper into it. Reason being is that when I would go on an on ramp for a highway and put it under load- Full throttle itll want to sputter act like its slipping under I let off the throttle some then itll go into gear and pick up speed. Hence why I thought it was a trans issue. As someone suggested transmission flare. Still adding that jumper wire just to see if maybe thatll fix that issue. Think it was part 12762310 or something like that.

Now as for the misfire goes. Its wild to me that itll only do it at full throttle. But half throttle the truck doesnt misfire. Even at idle. Dont have any CEL indicating that a misfire is occurring, unless these trucks dont give a code for that. I know the gmt800s do. The CEL was on at one point but the only code pulled and current and in history was the EGR valve. Also, I dont have any fuel smell or burning oil smell from the exhaust either. No smoke of any sort either.

I’ll do the wires and cap tonight to see what happens if it improves. Now I only got the cap and rotor. AutoZone didnt have the full unit with the shaft and gear (ive seen some that came with it on rock auto just didnt wanna wait on shipping) They had just the cap and rotor in stock so I figured Id snag them, try it and see what happens. Was hoping it was just plugs. I didnt see any oil on the old ones when I swapped them, still properly gapped at 060, look to be original as there signs of corrosion at the outside of them.

I personally feel a lot of this is just 25 years of parts that never really got replaced and decided to give out on me lol. It is afterall a one owner truck with 209k and Im the second owner of it and had put just 3k on it since ownership

Will update further tonight with a follow-up after work

 

[Donald Mitchell]

At over 200k miles you might be looking at a stretched timing chain, also as one of the others mentioned the fuel pump, filter and pressure meeds to be checked.

 

[Erik the Awful]

By your description it's a fuel delivery problem; most likely the fuel filter or fuel pump.

 

[RichLo]

Sounds like the TPS sending a bad signal at WOT

 

[tooturntdad]

Alright.
Replaced the wires and the cap and rotor. Same issue occurs.
Decided to give it a few revs. Pedal to the floor it bogs at 2800rpm and backfires a little. Half throttle itll rev up to 5000rpm.

No CEL. No misfire at idle. No current codes.
Going to tackle the fuel filter next.
Im also curious if clogged catalytic converters could potentially cause it. But I dont have an overheating issue indicating a clogged cat. Or atleast I would assume thats what would happen if exhaust cant pass through and create pressure.

Also wondering if this could be something with the throttle being out of adjustment?

 

[Schurkey]

Verify fuel pressure. You can--and maybe should--change the fuel filter also, but unless the existing filter is plugged, it won't affect fuel pressure.

I had a plugged catalyst on my Trailblazer. Went from highway speed to barely moving in the space of twenty--thirty miles. Eventually, the vehicle would hardly crawl along. Never had overheating issues even though the converter was horribly plugged.

Don't wait for misfire "codes". Look at the data stream, specifically the misfire history--a cylinder-by-cylinder count of misfires. Maybe all the cylinders are misfiring, maybe none are, maybe it's just one or two. If you can figure out which cylinders are misfiring, you're a step or three ahead on the diagnosis.

The throttle cannot get out-of-adjustment in any meaningful way related to misfire. However, you could have a faulty throttle position sensor. A faulty TPS or TPS wire harness would be easy to detect once the scan tool is connected. You wouldn't have two minutes in that diagnosis, once the tool is set up and providing data.

To repeat Post 11: Verify fuel pressure, connect a scan tool and review the data stream.

 

[tooturntdad]

Verify fuel pressure. You can--and maybe should--change the fuel filter also, but unless the existing filter is plugged, it won't affect fuel pressure.

I had a plugged catalyst on my Trailblazer. Went from highway speed to barely moving in the space of twenty--thirty miles. Eventually, the vehicle would hardly crawl along. Never had overheating issues even though the converter was horribly plugged.

Don't wait for misfire "codes". Look at the data stream, specifically the misfire history--a cylinder-by-cylinder count of misfires. Maybe all the cylinders are misfiring, maybe none are, maybe it's just one or two. If you can figure out which cylinders are misfiring, you're a step or three ahead on the diagnosis.

The throttle cannot get out-of-adjustment in any meaningful way related to misfire. However, you could have a faulty throttle position sensor. A faulty TPS or TPS wire harness would be easy to detect once the scan tool is connected. You wouldn't have two minutes in that diagnosis, once the tool is set up and providing data.

To repeat Post 11: Verify fuel pressure, connect a scan tool and review the data stream.

So Im running into fluctuating pressures at idle. Range between 51-55psi. Whats the standard for these trucks? I know the 5.3 gmt800s are at 60psi at idle. Im guessing the 5.7s are about the same?

 

[stutaeng]

Spec is 60-62 for these engines, but my own trucks have run around 55 psi without a problem.

What is fuel pressure with key on, engine off? Does it hold that pressure for about 5 minutes? What does the gauge do when you flip the throttle? Better yet, try to stick the guage into your windshield and go for a drive and monitor the guage when it bogs down.

I went back to what your describe the symptoms and I'm going to agree with others about engine bogging down.

 

[tooturntdad]

Spec is 60-62 for these engines, but my own trucks have run around 55 psi without a problem.

What is fuel pressure with key on, engine off? Does it hold that pressure for about 5 minutes? What does the gauge do when you flip the throttle? Better yet, try to stick the guage into your windshield and go for a drive and monitor the guage when it bogs down.

I went back to what your describe the symptoms and I'm going to agree with others about engine bogging down.

Ill get the readings of it when driving here in a bit. Pressure with the key off dropped significantly. Went down 46psi.