'98 5.7 Shuddering / surging

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TJ_11

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Looking to get a little more detailed information for troubleshooting this issue. Im pretty sure ive narrowed it down to a section of the truck but looking for confirmation.

Recently truck has been shuttering while accelerating or starving for fuel. At 2500 rpms it just stalls but revs a little and won't accelerate. While it "seemed" worse lower the tank but thats not the case. Tank was at half when I started noticing the problem. Noticed really bad when going up a bridge and the truck would not accelerate, but think it was coincidence. If I go full throttle the truck loses power and once even cut off on me. Put in neutral and fired it back up with no issues. No code has ever been thrown or read in the background.

Fast forward today and I started getting voltage readings on the TPS to narrow it down. Put a meter on the grey wire which is my understanding is the power wire and got 5 volts (Good reading). Went and checked the black wire (should be ground) Ohmed it out so that left the other wire that runs to the PCM. Controlled throttle by hand and it was a steady increase to 4.5 volts with no stoppage.

Truck has a new battery, new plugs, rotor, fuel pump and fuel filter in the last couple hundred miles. Spark plugs changed on 9-13. This has been in the last tank or two before that it was running good. Its only my weekend truck.

What should my next step be?

***updated for things checked and changed***

Thanks TJ
 
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Erik the Awful

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Fast forward today and I started getting voltage readings on the TPS to narrow it down. Put a meter on the grey wire which is my understanding is the power wire and got 5 volts (Good reading). Went and checked the black wire (should be ground) and only got .5 when I think it should be reading between 10 and 12.5 volts. This leads me to believe a bad wire or PCM is acting up.
I don't have a '98 wiring diagram handy, but it sounds like you don't either.

I wouldn't jump to blaming the PCM until you get a look at the data stream.
 

JeremyNH

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The TPS is a 5V sensor so you have 5V power (gray), ground (black w/white stripe), and signal (blue) all coming from the PCM. I'm far from a diagnostic mechanic but shuddering in drive I would suspect to be an issue with the TCC.
 

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1. What is the fuel pressure at idle, and when shuddering (NOT "shuttering". Cameras have a shutter, GMT400s don't.)

2. Stop trying to voltage-test sensors with a multimeter. You're wasting time, effort, and enthusiasm. Connect a scan tool, let the computer tell you about every sensor in the vehicle in three minutes. In five minutes, you can learn about the outputs, too--EGR, IAC, AIR, etc.

3. How old are the spark plugs? When was the last time you inspected the plug wires, cap, rotor, air filter, EGR, intake gaskets, camshaft/crankshaft sensor synchronization, etc? Those plastic distributors are known for worn bushings, and worn distributor gears.
 

TJ_11

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1. What is the fuel pressure at idle, and when shuddering (NOT "shuttering". Cameras have a shutter, GMT400s don't.)

2. Stop trying to voltage-test sensors with a multimeter. You're wasting time, effort, and enthusiasm. Connect a scan tool, let the computer tell you about every sensor in the vehicle in three minutes. In five minutes, you can learn about the outputs, too--EGR, IAC, AIR, etc.

3. How old are the spark plugs? When was the last time you inspected the plug wires, cap, rotor, air filter, EGR, intake gaskets, camshaft/crankshaft sensor synchronization, etc? Those plastic distributors are known for worn bushings, and worn distributor gears.

***Fuel Pressure check on 9-14*** and I corrected the shuddering. Thanks
Fuel pressure was 52 at idle, so this is sounding like something near the tank or voltage.

What type of scanner would you recommend without getting to involved. I know my regular mechanic uses a Maverick.

Spark plugs were original with truck having 90k. Start of July the Rotor, cap were changed. Plugs "looked" good so I didn't replace. As of today the 13th of September they are changed and problem is still there. Intake gasket was replaced in the last couple months when injectors and harness was replaced. Cant speak on the synchronization also.
 
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TJ_11

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The TPS is a 5V sensor so you have 5V power (gray), ground (black w/white stripe), and signal (blue) all coming from the PCM. I'm far from a diagnostic mechanic but shuddering in drive I would suspect to be an issue with the TCC.
I corrected the symptoms in the main post. Its more of a starving / surging and won't accelerate with a level of acceleration through the gas pedal.
 

TJ_11

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Fuel Pressure checked today and it was 52 and while throttling up it actually dropped to 50.

We will say that was the problem is around the fuel tank system.
 
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Schurkey

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***Fuel Pressure check on 9-14*** ...
...Fuel pressure was 52 at idle, so this is sounding like a bad fuel pump which has a warranty.
The fuel pump, being powered by electricity, cannot be diagnosed as faulty until you've ruled-out power supply (and ground) problems.
1. You should test supply voltage as close to the pump as practical, with the pump RUNNING. Ideally, you'd test right at the pump, but that's impossible since it's sealed inside the tank.
2. You should test for voltage on the ground wire as close to the pump as practical, with the pump RUNNING.

GM uses under-sized wire for the fuel pump circuit, so there's always some voltage loss. But that loss shouldn't be more than about two volts on the supply, and one volt on the ground. With the alternator charging at 14+ volts, the pump should have 12V supply, and another 1 volt loss on the ground, so the pump is actually powered with at least 11 volts. More power is better!

3. You should test amperage draw of the pump. Typically runs 6.5-ish amperes.

If the pump is getting proper voltage and the amperage draw is reasonable, but it still won't make pressure--try a fresh fuel filter. If it still won't make pressure spec, the pump is defective or the filter sock is plugged. Either way, the pump comes out of the tank.

What type of scanner would you recommend without getting to involved. I know my regular mechanic uses a Maverick.
I ruined my Snap-On MTG2500 a few weeks ago. Had software and cables/adapters to connect to 1980 1/2--2006 vehicles. Bought it used on eBay back around 2004-ish; and it was years old then (I bought updated software after I bought the actual scan tool.

Replaced it with a Snap-On Solus Pro, again used from eBay, with software from 1980 1/2--2007. This scan tool was probably never updated, it likely sold new in 2008. There's many Solus and Solus Pro scan tools; all of them are newer than my old MTG2500. There's a wide range of software, from early or mid 2000s, to about 2017 when support ended for the Solus Pro. Domestic + Asian is pretty standard, European software is relatively rare. Any of them need an OBD-II adapter plus "Personality Keys" to match the 1996-newer vehicle, or OBD-I adapters to match the vehicle. There's at least three GM adapters, three or four Ford adapters, a couple of Chrysler adapters, a Jeep adapter, multiple Toyota, Nissan, Mazda, etc adapters for OBD-I.

Other folks use OTC scan tools, consumer-grade stuff from Launch or Innova or a dozen other Made-In-China brand names; Chinese Tech 2 clones, or a laptop plus cheap software plus a connector cable. I have ZERO experience with any scan tool except my two Snappys in the last thirty years, so I can't help you with other choices.
Spark plugs were original with truck having 90k. Start of July the Rotor, cap were changed. Plugs "looked" good so I didn't replace. As of today the 13th of September they are changed and problem is still there. Intake gasket was replaced in the last couple months when injectors and harness was replaced. Cant speak on the synchronization also.
Verify the plug wires and coil wire.

Use the scan tool you choose to assure the cam and crank sensor signal ("Cam offset" on some tools) is within that + or - 2 degree spec.
 

TJ_11

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The fuel pump, being powered by electricity, cannot be diagnosed as faulty until you've ruled-out power supply (and ground) problems.
1. You should test supply voltage as close to the pump as practical, with the pump RUNNING. Ideally, you'd test right at the pump, but that's impossible since it's sealed inside the tank.
2. You should test for voltage on the ground wire as close to the pump as practical, with the pump RUNNING.

GM uses under-sized wire for the fuel pump circuit, so there's always some voltage loss. But that loss shouldn't be more than about two volts on the supply, and one volt on the ground. With the alternator charging at 14+ volts, the pump should have 12V supply, and another 1 volt loss on the ground, so the pump is actually powered with at least 11 volts. More power is better!

3. You should test amperage draw of the pump. Typically runs 6.5-ish amperes.

If the pump is getting proper voltage and the amperage draw is reasonable, but it still won't make pressure--try a fresh fuel filter. If it still won't make pressure spec, the pump is defective or the filter sock is plugged. Either way, the pump comes out of the tank.
Ill poke my head around and see where the best place to get voltage tested. It really had me scratching my head just because its an AC Delco fuel pump that was put in and it only has 900 miles on.

In regards if all power is good, fresh fuel filter was already tried. New one was replaced in May when the fuel pump was replaced and that was actually my first replacement the other day. When I drained the fuel filter though it was contaminated because it was a rust color that came out the "intake" side.
 
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