Breaking up/misfiring under boost
Thread Starter
Joined: Apr 2006
Posts: 3,890
Likes: 1,137
From: Grand Rapids, Michigan
Breaking up/misfiring under boost Developed a new problem, every time I get into boost in 2nd-4th gear, my engine is misfiring like crazy. This is what I have done so far to fix/diagnose:
- Swapped all coil packs out for different ones ( D510C coils )
- Pulled plugs to inspect ( BR7ES plugs ), all look good. .020" gap
- Resistance checked the plug wires, all are roughly 150 ohms
- Swapped out the crankshaft position sensor with another GM one
- Replaced the fuel filter
- Compression test, all within 3-4%
I've datalogged numerous times and the Holley software isn't showing any issues with crank signal, but AFR is jumping around a bunch. I have a table in the tune adding .2 milliseconds dwell above 7.5 lbs. of boost, but that didn't cause any problems previously. The only thing that is still the same are the plugs and wires. The wires are Summit house brand, but they've never given me any trouble. The plugs look almost as clean as the day I installed them, just a slight tint to the porcelain. I'm probably going to swap plugs and wires because it's the only thing I haven't swapped out yet. Anyone else have experience with this?
- Swapped all coil packs out for different ones ( D510C coils )
- Pulled plugs to inspect ( BR7ES plugs ), all look good. .020" gap
- Resistance checked the plug wires, all are roughly 150 ohms
- Swapped out the crankshaft position sensor with another GM one
- Replaced the fuel filter
- Compression test, all within 3-4%
I've datalogged numerous times and the Holley software isn't showing any issues with crank signal, but AFR is jumping around a bunch. I have a table in the tune adding .2 milliseconds dwell above 7.5 lbs. of boost, but that didn't cause any problems previously. The only thing that is still the same are the plugs and wires. The wires are Summit house brand, but they've never given me any trouble. The plugs look almost as clean as the day I installed them, just a slight tint to the porcelain. I'm probably going to swap plugs and wires because it's the only thing I haven't swapped out yet. Anyone else have experience with this?
Just a shot in the dark here, but could any wires be getting pinched when the motor torques over under boost? If you blow the tires off in first, that could explain why it only happens in 2nd and up.
TECH Enthusiast
Joined: Feb 2020
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Haven't had any such problems with my FI LS stuff but have had wires cause all sorts of problems in the past 45 yrs or so . I replaced mine after 5yrs just to stay ahead of it - cheap insurance. Recently had a truck with 5.3 that had brand new plugs and a misfire under load - was 1 of the new plugs . For the cost of it I'd swap wires and plugs, might save a lot of time trying to diagnose it. And if it doesn't fix it you've done some maintenance and have some spare parts. If it does fix it , throw the old stuff out unless you can figure out what's good or not
Thread Starter
Joined: Apr 2006
Posts: 3,890
Likes: 1,137
From: Grand Rapids, Michigan
It drives completely fine out of boost. No misfires or rough running. It acts like it's blowing out the spark, but I'm not running enough boost for that. As for plug wires getting pinched, my #8 wire sits against the heater box, but torqueing over would move it away, not pinch it. This was an abrupt failure as well, was good one day, went to drive it the next and had the misfire. I don't think it's anything like a broken valve spring, I was meticulous in setting them up and I only use BTR platinum springs in my rides. Not to mention, it pulls hard in 1st gear up to 8000 RPM. I don't think I'd be able to hit that kind of RPM if there were a mechanical problem.
TECH Enthusiast
Joined: Feb 2020
Posts: 734
Likes: 209
Haven't had any such problems with my FI LS stuff but have had wires cause all sorts of problems in the past 45 yrs or so . I replaced mine after 5yrs just to stay ahead of it - cheap insurance. Recently had a truck with 5.3 that had brand new plugs and a misfire under load - was 1 of the new plugs . For the cost of it I'd swap wires and plugs, might save a lot of time trying to diagnose it. And if it doesn't fix it you've done some maintenance and have some spare parts. If it does fix it , throw the old stuff out unless you can figure out what's good or not
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Verdad Gallardo Thread Starter
Joined: Apr 2006
Posts: 3,890
Likes: 1,137
From: Grand Rapids, Michigan
It seems to be less pronounced at part throttle, but it's hard to tell. Every so often, it'll pull just fine. I ordered a set of Taylor plug wires just to be safe, and I'm going to swap the plugs as well.
99% of the time it's ignition related, change plugs/wires. If that doesn't do it I would be pulling a valve cover and inspecting valve springs to make sure something didn't happen there just because it's easy and quick to put eyes on them. if that's not it, could be a Holley problem.
whats the AFR doing? Check the main ground on coil packs. If you dont have access to a scope, grap a spray bottle and spray the wires if there is a slight crack anywhere that is hiding chances are youll find it. OHMing the wires doesn't wont account for a bad wire. Does it misfire out of boost but under a load? Any of the plugs fouled? When running in a misfire situation a plug can foul causing a misfire under load. Check your plugs first if your firing the parts canon on it. Good looking plugs can and will bite you.
Thread Starter
Joined: Apr 2006
Posts: 3,890
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From: Grand Rapids, Michigan
@Pro Stock John Now you have me worried I may have cracked my intake. When I sprung a fuel injector O-ring leak at the track, I didn't know it and made a pass and at the top of third gear it backfired pretty hard. It sounded like it came from the exhaust, but it's kind of hard to tell. My intake is one of the original FAST intakes from about 20+ years ago, who knows how brittle the plastic may have gotten. The only thing that makes me think it didn't happen is I am still seeing full boost. I would think if there were a crack, it wouldn't make as much boost.
As for AFR, it's fluctuating a bit, not seeing spikes in either direction though. I checked the grounds on the coil packs, they're nice and snug. Doesn't misfire out of boost under a load, it's only when the turbo spools up. All the plugs look great. One could be cracked, though. Since they're cheap, I'm going to replace them for piece of mind.
As for AFR, it's fluctuating a bit, not seeing spikes in either direction though. I checked the grounds on the coil packs, they're nice and snug. Doesn't misfire out of boost under a load, it's only when the turbo spools up. All the plugs look great. One could be cracked, though. Since they're cheap, I'm going to replace them for piece of mind.
Ok so no misfire under load out of boost.. interesting
So something is taxing it under "extreme" load. I hate throwing parts at a car but in this situation plugs might be in order and for piece of mind. Cheap easy. One fouled plug one big headache.
This is all an assumption its an ignition misfire and not fuel. Since your in there anyway put your hands of on the injector plugs make sure nothing is loose.
Hopefully reluctor didn't move on the crank.
So something is taxing it under "extreme" load. I hate throwing parts at a car but in this situation plugs might be in order and for piece of mind. Cheap easy. One fouled plug one big headache.
This is all an assumption its an ignition misfire and not fuel. Since your in there anyway put your hands of on the injector plugs make sure nothing is loose.
Hopefully reluctor didn't move on the crank.
Thread Starter
Joined: Apr 2006
Posts: 3,890
Likes: 1,137
From: Grand Rapids, Michigan
