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Bad misfire under load after warmup

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Old 07-17-2017, 07:39 AM   #1
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Default Bad misfire under load after warmup

I've been chasing a misfire for awhile now and need some more assistance please.

68 Chevelle, LS2, TVS2300, density tuned (no MAF). Only has front O2 sensors and no evap system.

Yesterday I took the car up the road to the store and got on it a bit...about 85% throttle. The car had great power, pulled strong, was pretty smooth. On the way back from the store, I did the same thing but this time it misfired bad under load. Even had a loud backfire out the exhaust. How I describe this misfire is as I punch the gas, the car will launch forward then quickly die down, then pull forward again a few times. After a certain RPM...about 4000-4500 I'd say, it mellows out and the misfire is gone. The computer is not throwing any codes. When I scan the system using my Harbor Freight CanTech scanner, no misfires are indicated.

- I've replaced the fuel filter regulator.
- I have 58 PSI fuel pressure at the fuel rail at idle and when I hit the throttle. I have not monitored my fuel pressure under load as I'm driving yet.
- I have replaced the NGK spark plugs with new.
- I've checked for spark at each plug and coil
- Compression on all cylinders is between 195-210 psi

I have not done any inspecting of knock sensor and harness or crank sensor and harness yet. There is some discussion out there that these may be culprits.

I have not done any troubleshooting of the throttle body. Some info out there suggests an IAC or TPS could cause this as well.

I'd love some additional inputs please.
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Old 07-18-2017, 12:27 AM   #2
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Depending on the computer connecting will be different levels of helpful. Make sure you don't have any valvetrain noise and if you do check for broke springs now because you have limited time before a broke one will cause major issues.
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Old 07-18-2017, 06:58 AM   #3
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Depending on the computer connecting will be different levels of helpful. Make sure you don't have any valvetrain noise and if you do check for broke springs now because you have limited time before a broke one will cause major issues.
Can you explain "computer connecting"?

I don't have any unusual valve train noise and I have pulled the valve covers to visually inspect the springs. No, I did not remove them, but I did at least take a close look at them and couldn't see any visible defects.

I'm trying to pull misfire data using the Torque app but when I pull test results, for each cylinder misfire test it says "test incomplete or dependent test failed" and I have no idea why.
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Old 07-18-2017, 10:04 AM   #4
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Good, now you know it's not going to send a valve through a piston and have time to figure it out. Yes, the torque app is a way to connect and get info usually. I have never had any issues pulling test, so I'm not the guy to help with that. Might try hitting a parts store as most will help for free with a scan.
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Old 07-18-2017, 10:10 AM   #5
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Oh, you might try reinstalling Torque.
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Old 07-18-2017, 10:17 AM   #6
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Sounds like knock sensor pulling timing, it's "Hearing something" my LS1 had a bad harmonic balancer drove me nuts for along time and spent lotsa money chasing it FWIW
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Old 07-18-2017, 12:54 PM   #7
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Good, now you know it's not going to send a valve through a piston and have time to figure it out. Yes, the torque app is a way to connect and get info usually. I have never had any issues pulling test, so I'm not the guy to help with that. Might try hitting a parts store as most will help for free with a scan.
The computer isn't throwing any codes. I have a Harbor Freight CenTech scanner that is pretty capable. It does have misfire data but shows no misfires on any cylinder being recorded.
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Old 07-18-2017, 12:55 PM   #8
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Sounds like knock sensor pulling timing, it's "Hearing something" my LS1 had a bad harmonic balancer drove me nuts for along time and spent lotsa money chasing it FWIW
Is there any way to isolate this? What happens if I disconnect the knock sensor?

How did you come to find a bad harmonic balancer? What were your symptoms?
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Old 07-23-2017, 08:51 AM   #9
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I chased the pulling timing issue over 5 years, replaced knock sensors, plugs, wires, two trips to the dyno and multiple dealerships,anything that could cause a high speed miss, 4000-4400 rpm range. I was setting one night going over in my mind all the mods I had done and remembered my son bought me a SLP underdrive balancer for Christmas on a whim I bought a stock balancer and had it installed and MISS GONE. I was PISSED at SLP for selling junk...
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Old 07-24-2017, 08:58 AM   #10
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I chased the pulling timing issue over 5 years, replaced knock sensors, plugs, wires, two trips to the dyno and multiple dealerships,anything that could cause a high speed miss, 4000-4400 rpm range. I was setting one night going over in my mind all the mods I had done and remembered my son bought me a SLP underdrive balancer for Christmas on a whim I bought a stock balancer and had it installed and MISS GONE. I was PISSED at SLP for selling junk...
Well, it's a stock balancer and it's drilled and pinned as required for the super charger. I'll take a peak at it though, thank you for sharing that.
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Old 07-26-2017, 01:13 AM   #11
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Check plugs and wires.. had it happen to me on saturday..had a multiple misfire and was low on power under load. ...changed plugs and put my taylor mag core race wires and its good as new.. was using ngk tr55 and msd wires and i opted to try ngk tr5ix and it took care of it.. but next option would be the crank sensors imo
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Old 07-26-2017, 07:00 AM   #12
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Originally Posted by xdanny510x View Post
Check plugs and wires.. had it happen to me on saturday..had a multiple misfire and was low on power under load. ...changed plugs and put my taylor mag core race wires and its good as new.. was using ngk tr55 and msd wires and i opted to try ngk tr5ix and it took care of it.. but next option would be the crank sensors imo
I appreciate the suggestion. The plugs are new and the MSG wires have only about 3000 miles on them. No codes to speak of.

I do have a crank sensor and cam sensor on the way.
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Old 07-26-2017, 07:55 AM   #13
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On a blower motor, the spark plug gap should be about .025. How are your plugs gapped?
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Old 07-26-2017, 08:01 AM   #14
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On a blower motor, the spark plug gap should be about .025. How are your plugs gapped?
I will double check before I provide an answer to that. I thought I gapped them .025-.030.
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Old Yesterday, 05:10 AM   #15
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What plugs? My car did this when I swapped to a non projected tip plug (br7ef) I put my tr7ix back in and it was fine. Also, check to see the crank sensor is plugged in all the way.
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Old Yesterday, 06:08 AM   #16
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They are NGK TR6 gapped at .025.

I have inspected the crank sensor and cam sensor. Just waiting for new ones in the mail to swap out one at a time to see if that has any effect.
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Old Yesterday, 04:22 PM   #17
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A couple of folks have suggested my tune may need to be adjusted. It was tuned in utah, about 6000' elevation. I'm not at sea level. I drove the car just fine for a few weeks here before it started acting up so I'm doubtful, but open to the possibility. Getting it in for another tune is the challenge.
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Old Yesterday, 05:49 PM   #18
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What drive system is on the blower and how much boost are you running? If it's just a single belt 6 rib then it could be a belt slip issue maybe.

A tuner would be able to adjust things just to test out different ideas of what it is without changing parts. I would likely be quicker and cheaper than changing parts until you find it, but you won't get to learn as much.
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Old Yesterday, 06:28 PM   #19
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What drive system is on the blower and how much boost are you running? If it's just a single belt 6 rib then it could be a belt slip issue maybe.

A tuner would be able to adjust things just to test out different ideas of what it is without changing parts. I would likely be quicker and cheaper than changing parts until you find it, but you won't get to learn as much.

Great suggestion I hadn't thought of. Yes it's a 6 rib belt system. I believe it's setup for 8 or 9lbs boost. Sadly I don't have a boost gauge.

I wouldn't think it would just start to slip... The belt is fairly new. Is there any dead giveaway to check?
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Old Today, 06:57 AM   #20
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Well, it's a stock balancer and it's drilled and pinned as required for the super charger. I'll take a peak at it though, thank you for sharing that.
Check the polymer band on the balancer, oil etc can cause it to get loose and cause knock
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