Random Misfire at Idle, Fine Under Load

[The Black Sheep]

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.

Mods:

• 92 mm Throttle Body
• Airaid MIT Intake Tube
• ARP Head Studs
• Chevrolet Performance Lifters
• Corvette Fuel Filter Regulator
• Heads milled 0.020"
• Ligenfelter 160* Thermostat
• TBSS Intake Manifold
• Texas Speed 212/218 .550" 112 LSA

Things I've checked and replaced:

• Coil packs ohm'd out of spec, replaced all with ACDelco D581
• Coil pack plugs all @ 12V
• CKP replaced and relearned
• IAC replaced and relearned
• Injector plugs all @ 12V
• 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
• Spark plugs are NGK TR5IX gapped at 0.040"
• Spark plug wires replaced with ACDelco 9748HH - I broke some of my old ones taking off a set of long tubes I tried out
• TPS replaced and relearned due to a rough 1-2 shift - fixed the problem
• Vacuum gauge hooked up reads steadily in "late timing" area
• 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.

random misfire at idle.hpl

[anniversaryss]

Are these confirmed as real misfires or just "false misfires" because of the cam?

[The Black Sheep]

Confirmed. The the spark graph doesn't stay "steady" like other cammed and stock vehicles do. You can clearly hear the engine stumble on itself. To me, it seems like a trigger problem, like it gets the idle right for two or three lopes, then loses it again.

[5FDP]

Even with a higher commanded idle still the same issue?

Like 750 or 825 instead of 650?

[kevin87turbot]

The idle speed is a bit too low. I'd bump it up to 700-725 rpm or so. Have you desensitized misfire detection yet? If not, your misfire counts are actually very low for that cam with that low idle speed. Was your truck originally equipped with the D581 coils?

[The Black Sheep]

My truck originally had the Mitsubishi style coils. I'm not sure what you mean by desensitized misfire detection, Kevin. Can you explain or post a link to this process? From what I've read this cam shouldn't have a problem idling at 650 RPM as it's pretty mild. I'll try a 700 and 750 RPM idle and post another log. My idle is actually a bit higher since I haven't adjusted that area of the tune since the thermostat was done.

[kevin87turbot]

When you install a larger than stock cam in an LS engine, you either have to desensitize or disable misfire detection. It really depends on how big the cam is and the overall engine combo as well. If you were to install your cam in an LS7, it would really tame it down considerably. You would have to do very little to desensitize that combo.

However, that cam is considerably more aggressive in your 4.8L. In your application, I would start by multiplying the low rpm misfire mode event times by 2 (Engine Diag>Misfire>Cylinder Mode. You'll need to change both the Idle (Moving) and Idle (Not Moving) tables. I suspect if you'll do that and increase the idle speed to 700 rpm, then you'll eliminate the "misfires." You can try a smaller number and sneak up on it, but it should work nicely for you.

[The Black Sheep]

Hey, thanks for the suggestions, guys. The changes did help and the misfire counts did 0 out, but I still feel the truck is occasionally hunting for the timing still. I multiplied the low RPM moving/not moving by 2 initially and still got misfire counts, so I added 50% more and they zero'd out. Here's a log with the changes. If you watch the spark graph you can see where the loping stops and it "misses" for a bit. The MAP did drop to about 45-46 kpa so that's nice.

750 rpm desensitized.hpl

Update: I'm going to disable those tables and see what happens. I have some tunes from other tuners and it seems it's not uncommon to do, so it's worth a shot in my book. Russ K had some good tips in this thread.

Misfire Cylinder Mode Event