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hydroboost uses the power steering hydraulic fluid and is connected to the power steering pump.

a vacume booster has a big rubber hose that connects to the intake somewhere.

I have never worked on one of your trucks though, just the older generation. but a vacume leak is a vacume leak and a vacume brake booster will leak mainly when the brakes are pressed, because the diaphram has a small rupture in it that may seal when its relaxed. and if its leaking really bad you will hear it when you press the pedal but not when the leak is small. when the whole thing ruptures you will end up with it being loud all the time and manual brakes. I had this happen coming up to a stop sign. it was kind of scary.

what I'm talking about though is a very simple test reading just the live data of your scanner.
 
Discussion starter · #22 ·
hydroboost uses the power steering hydraulic fluid and is connected to the power steering pump.

a vacume booster has a big rubber hose that connects to the intake somewhere.

I have never worked on one of your trucks though, just the older generation. but a vacume leak is a vacume leak and a vacume brake booster will leak mainly when the brakes are pressed, because the diaphram has a small rupture in it that may seal when its relaxed. and if its leaking really bad you will hear it when you press the pedal but not when the leak is small. when the whole thing ruptures you will end up with it being loud all the time and manual brakes. I had this happen coming up to a stop sign. it was kind of scary.

what I'm talking about though is a very simple test reading just the live data of your scanner.
Definitely hydroboost. I recognized it as soon as I opened the hood. Someone told me years ago this Sub had been set up like a HD truck with the hydroboost and a hood from a 2500 series. Anyway, I did some of the evaluation steps from the manual: Simple checkup. I do know my power steering fluid seems to lose a little overe time, but I've had lots of vehicles do that. Still.....worth checking later.
 
Check the fuel pressure and regulator. Easy to do and just rule it out. My 2003 flex should be 50 psi and disconnect regulator will hop to 60 psi since a vacumn. Should have a stable reading.
 
I had a 96 or 99 chevy that did the same thing...would die or studder every time I started accelerating. Turns out, a baffle in my gas tank broke so everytime I hit the gas, the baffle would ram my fuel pump until it stopped working 100%. Two fuel pumps later finally found the issue. Don't know how the tanks are constructed in these gmt800's but worth investigating.
 
Discussion starter · #26 ·
My life has been validated. 😋

The stumbling and dying makes me think weak fuel pump. When mine was going, I would have to sit with my foot bouncing on the gas and the other holding the brake to prepare for jumping into traffic. Talk about nerve racking...
I'm looking more that direction. Let me give the latest. Every time I drive the Sub I watch and listen for glitches. Last time out I noticed it would stumble when starting out on an INCLINE, sometimes steep, sometimes slight. It doesn't seem to happen on a declining road. Then, twice as I pulled away with no radio or any distractions I heard an odd "groaning" or "chortling" sound. Very faint and seemed to be below and in front of my feet. I cannot hear the engine at all, so it was noticeable both times. The second time I turned left and headed towards the railroad tracks near my house....slight DECLINE. It stumbled. I looked at the tach. It went from about 1500 to 2000. "2000"! That means the engine is actually still running....or sending a signal that it is running when it is not? Then it caught and lurched forward and ran fine the rest of the way. I can't seem to "make" it happen, but that was typical. Good thing I wasn't crossing the tracks trying to beat a big UP locomotive. So....fuel? Ignition? Grounds? WTH??
 
Discussion starter · #27 ·
I had a 96 or 99 chevy that did the same thing...would die or studder every time I started accelerating. Turns out, a baffle in my gas tank broke so everytime I hit the gas, the baffle would ram my fuel pump until it stopped working 100%. Two fuel pumps later finally found the issue. Don't know how the tanks are constructed in these gmt800's but worth investigating.
Did you know that those big dual fuel pumps is what the Corvette ZR-1 used??
 
Sorry...I meant to say that I replaced the fuel pump two times b4 figuring out the baffles broke free from the mount. And btw, the hesitation DID happen to me while crossing railroad tracks while a train was coming. Used to live 1/4 from the tracks... crossed it twice a day. One day the lights were on but train was a little bit away (slow freight train) ...stopped...looked...hit the gas, power for half a second then died...then resumed. So yes, get it fixed, lol!
 
Discussion starter · #29 ·
Sorry...I meant to say that I replaced the fuel pump two times b4 figuring out the baffles broke free from the mount. And btw, the hesitation DID happen to me while crossing railroad tracks while a train was coming. Used to live 1/4 from the tracks... crossed it twice a day. One day the lights were on but train was a little bit away (slow freight train) ...stopped...looked...hit the gas, power for half a second then died...then resumed. So yes, get it fixed, lol!
Yeah....I'm going in that direction as soon as possible. I could order a new fuel pump from Rockauto. But keep in mind, your 8.1 is a lot different from my 2005 5.3L flexfuel.

Oh, when I mentioned the 2 fuel pumps most people don't know those '90-'96 ZR-1s had TWO GMC big block fuel pumps in the tank. One set low to the bottom of the tank, and one set higher. It was so you never starved for fuel at 7-8000 rpms.
 
Discussion starter · #30 ·
It acts like a fuel problem, guys who are familiar with the old systems would tell you to rebuild the carburetor.
Of course, you have fuel injectors and a throttle body. The throttle body just delivers air to the engine! So I don't think it's the throttle body.
However, it doesn't hurt to clean it. I would just remove the air intake housing and spray some TB cleaner into the throttle body without removing it.
Don't pry, or try to move the butterfly, you can turn on the key, don't start the engine, but have someone push the gas pedal to open the butterfly and give it a 2nd spray.
I usually wait a few minutes before I start the engine. It should run fine without codes. Try a test drive.
I'm not one of the guys who 1st look toward fuel pressure; but I think in this case, it's a good spot to look.
When the truck is running, can you smell gas, there could be a fuel leak, the lines will and can rust through?
An update and a little more information. I was considering the transmission because it seemed to be not wanting to shift up from 1st to 2nd, and the engine was not stalling as I had thought (rpms went same or up). THEN...as I was coming back from the stores yesterday (same slight incline), it "stalled", and I heard a "pop" like a backfire. Since the cabin is very isolated from the engine compartment it's hard to tell,but I know the sound of a backfire...even a small one.

That would certainly explain the stumbling, but how does an engine/ignition system like this lose timing? Could it be that ground between the front of the crank and the coils that the guy a few years ago said the dealership repaired to fix his problem?? I don't know the circuitry, but if the crank position sensor is not grounded properly I could see a problem potential. Hmmm....????
 
Discussion starter · #31 ·
FYI: even after the backfire......NO DTC codes. Also read the troubleshooting map from the manual and the Crank Position Sensor was never mentioned as a cause for stumbling or dying. Neither did the Cam sensor.
 
Discussion starter · #32 ·
update 4/15/2022:

Still stumbling. Have not had time to take it to the stealership. Trying to relate to the member who's prob;em was solved by the dealer shop doing something to a ground wire up front. Since the crank position sensor is at the rear of the block under the starter,that cannot be the problem(?). But as I watch and listen I occasionally hear that slight "chortling" sound beneath my feet. When she stumbles it seems more and more like it isn't shifting. I had that ECU/PCM computer replaced back around 4/2012 at a staggering $1400. It better not be bad again. :(
 
I replaced my gas lines a while back and noticed the FCS or fuel compensation sensor casing partially broke and need to replace. This may cause some fuel issues since in very cold weather it's a little rough but gets better quickly. It's exposed so figure a rock hit it. At the time just caulked it. Easy to remove and install. 2003 Suburban LS 4X4 205000.
 
Discussion starter · #34 ·
I replaced my gas lines a while back and noticed the FCS or fuel compensation sensor casing partially broke and need to replace. This may cause some fuel issues since in very cold weather it's a little rough but gets better quickly. It's exposed so figure a rock hit it. At the time just caulked it. Easy to remove and install. 2003 Suburban LS 4X4 205000.
I'll have to check that. Where is the FCS located?
 
FYI: The Sub runs better if I warm it up for a few minutes before driving it. Less stumbling I think. Probably a common thing....but the decreased stumbling is important.

KK
The FCS or maybe I have it wrong. It is a metal box under drivers that helps control or distribute the E85 if that is right. Two fuel lines. One in and one out.
 
Discussion starter · #37 ·
The FCS or maybe I have it wrong. It is a metal box under drivers that helps control or distribute the E85 if that is right. Two fuel lines. One in and one out.
I'll try to get under there and look at it. If this problem was fuel related, wouldn't the engine die and the tach go to "0" when it stumbles? It's getting worse, and feels like it doesn't want to shift up quite often. :(
 
Discussion starter · #38 ·
The FCS or maybe I have it wrong. It is a metal box under drivers that helps control or distribute the E85 if that is right. Two fuel lines. One in and one out.
After months of trying to figure this out I had come to the conclusion it was transmission-related. I took it to a local tranny shop that was recommended. They did a thorough analysis and de5trmined it was NOT rransmission-related. The shop owner told me two important things: 1- even though there were no other codes, there are ABS codes. Not sure what that means. 2- he felt that the drivetrain or suspension/brakes were "pulling" down when trying to accelerate. Like a malfunctioning TC control or AWD suspension control.

I'm taking it into a shop on Friday that is supposed to be GM and GM truck experts. We'll see what comes next.
 
Discussion starter · #40 ·
Keep us posted. Thanks.

DING DING DING!!! Finally a solution....or, at least a "fix".

I finally got a solution. Not a real repair, but it has stopped the stumbling and hesitation.

First, I took the Sub to a good transmission shop who did a thorough trans/drivetrain diagnostic. They affirmed that all the trans/train stuff was OK. The tech had a suggestion and gave me the number of David Tilley's Automotive. VOILA! Great guys and great investigators.

A long-time retired GM tech found that the TCS is causing the stumbling. It's a little more complicated that this, but for now as long as I push the TCS button on the dash to the left of the instrument panel....that will disconnect/switch off the TCS. There is a warning light on the right side of cluster that tells me when it is OFF. With that system OFF, the Sub no longer stumbles....for now.

I'll settle for this right now and see what happens. HOW MANY OTHER OWNERS THAT HAVE TRIED EVERYTHING TO FIX THEIR SUBS AND YUKONS ARE STILL SPENDING $$$$$ TO NOT HAVE A "FIX"? I HOPE OTHERS SEE THIS,.

YAY!!!!!!!

KEN
 
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