1998 Chevy Silverado K1500 choppy cold/warm startup

I have a 5.7 vortec truck. All original 189,000 miles. Down to stock radio. I have replaced...

GM cap and rotor
Full tune up and 41-993 plugs
New MPFI Delphi injectors and Delphi FPR 
Had a coolant leak on backside of intake, new metal backed gasket set and thermostat and new belts/hoses. 
New distributor, with OEM cam sensor put in metal distributor. 
New crank sensor, GM genuine 
Fuel pump (GM Genuine) and fuel filter
New coil, tried new ICM but went back with OEM one when nothing changed
New O2 sensors (NGK) 
No cats installed. And fuel mileage is still good and plugs are not wet. 

Temp sensor reads ambient at startup, then 195 at OT.

New Delphi MAF updated design. New MAP as well

When you start it up it almost sounds choppy then straightens out. Then when you go into store and come out you can turn it over and it will chop around then die. Then try again it will come around and stabilize......no codes other than a P1351 if I'm remembering correctly. High ICM circuit. Just not a smooth startup. I have a bi directional LAUNCH scanner. I'm not sharp with it. But I can read it. Short term fuel trims are close to 0, and long term sometimes reach -7.........I have done a pressured smoke test on PCV valve and no leaks. As well as brake booster. 

I'm at a loss. I watch scanner Danner all the time and I got a monthly subscription hoping some smart mechanics will help me out. Truck looks too great to be sounding like it does when it's starting up 
Lol 

 

Since you tried so many avenues, I gave your post to chat gtp. It's not always right but sometimes it helps you move forward.
*Thanks for the detailed breakdown — it’s clear you’ve covered a lot of ground and are meticulous. That choppy startup and warm restart stumble with a P1351 (ICM high voltage) is a pretty strong clue, especially since you’ve tried a new ICM and coil, and the issue persists.

Here’s a step-by-step breakdown to help pinpoint the issue:


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? Key Suspects


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1. Ignition Control Module (P1351)

P1351 on a 5.7 Vortec typically means high voltage detected in the ICM control circuit.

You've tried another ICM but reverted — try checking grounds and ICM wiring:

The ICM gets power on pink wire, signal to PCM, and ground through the distributor plate.

Run a voltage drop test on the ICM ground with the engine cranking.

Corrosion on the distributor base or poor harness integrity near the coil/ICM pigtail is very common.

Consider splicing in a new coil and ICM harness pigtail — they’re notorious for heat cracking.



2. Distributor Position / Cam Retard Offset

Even though you have a new distributor and cam sensor, Vortecs are very sensitive to cam retard offset.

It must be within ±2° of zero at 1000 RPM or higher.

Your Launch scanner should show "Cam Retard Offset" under live engine data.

If it's outside that range, the PCM can delay ignition timing during startup, causing the stumble.

If off, re-time the distributor (not like a small block — you turn the whole thing for cam offset, not timing advance).



3. Fuel Delivery Hot Soak / Pressure Bleed-off

If it struggles on hot restart, consider:

A leaking injector or FPR (you have new Delphi parts, so likely OK).

Check your scanner for fuel pressure on key-on, if you have a sensor or port.

Consider rigging a manual gauge to check hot soak bleed-down.



4. Intake Valve Carbon / Compression Drop at Cranking

At 189k, some intake valve sealing loss from carbon is common.

This can cause choppy or low vacuum during crank, especially warm.

Consider a compression test or leakdown, or try a round of top-end cleaner / water decarb and see if startup improves.



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? What to Scan / Check


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With LAUNCH Scanner (bi-directional):

Cam Retard Offset at 1000+ RPM

Cranking RPM (if low, suspect starter drag or timing issue)

Injector Pulse Width during crank

Fuel Pressure (if available)

Live voltage on coil control wire and ICM ground during crank


Manual Checks:

Battery voltage at crank (should stay >10V)

Inspect coil-to-ICM and ICM-to-harness wiring

Wiggle test the harness during idle and monitor RPM/voltage spikes



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? Quick Experiments


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1. Cold Start: Try starting cold with MAF unplugged. See if it smooths out — that’ll force the PCM into speed-density.


2. Hot Soak Start: Try cycling the key twice (prime fuel pump), then start. If this helps, it’s likely fuel bleed-down.


3. Force Cam Retard Check: Hold it at ~1100 RPM and verify cam retard number — needs to be within ±2°.




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? Summary of Next Steps

1. Verify Cam Retard Offset at 1000+ RPM


2. Check coil and ICM wiring harness, especially grounds and connectors


3. Check for fuel pressure drop after key-off, hot soak condition


4. Verify ICM and coil feed voltages during crank


5. Optional: decarb for possible intake valve seal improvement




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If you're comfortable with it, upload a screenshot of your live data during a choppy startup or when it dies — especially cam retard, RPM, fuel trims, IAT, MAP, and voltage. That can help narrow it down faster.

       

That is idling warmed up. Has a misfire every now and then of 2 counts on cyl 1, 4 and 3.....I cleaned up some grounds and added a new ground strap. No change but it's all I had time to do to it today. 

And I set the Cam offset as well when I did the distributor. Set to TDC using a old cap with a hole drilled in it. Then set Cam offset at 1k and tightened down old style clamp.

Your MAF readings seem high for a hot idle. It should be closer to 5.7 g/s. I would install the original one and see what your reading is.