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Random Misfire at Idle, Fine Under Load

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As the title says, I have a P0300 for random misfire but only at idle. The truck is an '01 Silverado 4.8 with some mods done to it. I've chased this problem for longer than I'd like to admit, so here I am. I am both the builder and the tuner and I can post the tune file if anyone wants to take a look through it.


  1. 92 mm Throttle Body

  2. Airaid MIT Intake Tube

  3. ARP Head Studs

  4. Chevrolet Performance Lifters

  5. Corvette Fuel Filter Regulator

  6. Heads milled 0.020"

  7. Ligenfelter 160* Thermostat

  8. TBSS Intake Manifold

  9. Texas Speed 212/218 .550" 112 LSA

Things I've checked:

  1. Coil packs ohm'd out of spec, replaced all with ACDelco D581

  2. Coil pack plugs all @ 12V

  3. CKP replaced and relearned

  4. IAC replaced and relearned

  5. Injector plugs all @ 12V

  6. Knock sensors replaced with ACDelco 213-3521 when I built the truck over a year ago, RTV'd around the grommets to keep water out

  7. Spark plugs are NGK TR5IX gapped at 0.040"

  8. Spark plug wires replaced with ACDelco 9748HH - I broke some of my old ones taking off a set of long tubes I tried out

  9. TPS replaced and relearned due to a rough 1-2 shift - fixed the problem

  10. Vacuum gauge hooked up reads steadily in "late timing" area

  11. Vacuum leaks - I had one with my stock intake that was fixed after the TBSS intake swap

I'm genuinely at a loss for this problem. The truck does not misfire under load above about 1,000 rpm, the only time it ever misfires is at idle. I've attached a lengthy log so anyone with any input can see exactly when the misfire counts start going up. Any help is greatly appreciated.

HP Tuners log

Hi. What is your AFR ratio at idle? On some engines the lean mixture can cause misfire at light loads. Also i would check out the dwell time for ignition coils at idle to ensure they get enough charge time to fire the spark plug...

My pro link gets here today so I'll be able to log the AFR now. If the dwell time is off, how would that be corrected?

I'm not particularly familiar with HP tuners software but all stand-alone ECU have the option of altering dwell time. You just slightly increase the dwell time and check if it helps to solve the problem.

As far as I know, there is not a way to adjust dwell time in the PCM. The fuel system is all new except for the injectors. I wouldn't mind replacing them if they're the problem.

A P01 ECU : Engine> Spark> Dwell for that suggestion.

Spark plugs are NGK TR5IX gapped at 0.040" isn't that the hottest range they offer? I would use a colder heat range.


Looks like you are idling at 650 on this cam when miss fires are highest. Texas Speed 212/218 .550" 112 LSA try adding 100+ RPM

Thank you for the reply the issue has been fixed. The misfire tables needed scaling for the cam. Idles great now!

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