Misfire only under load - no codes
Thread Starter
Staging Lane
Joined: Feb 2013
Posts: 99
Likes: 2
Misfire only under load - no codes 2004 GTO motor with magnuson supercharger in 69 Chevelle convertible. Trying to tune for several months now (off and on), but still getting misfires from bank 1 under heavy load according to the tuner. No PCM codes thrown. I replaced O2 Sensor, plugs, injectors, wires and coils already. Still can't get it to pull good under load. It idles well and if I take it easy it will rev up and run smooth. Under load it sputters and then eventually will rev and has good power. Any suggestion? I posted this in the conversions folder but wanted to reach out to the broader audience of experts.
Thread Starter
Staging Lane
Joined: Feb 2013
Posts: 99
Likes: 2
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I had the same issue, I too, was puzzled why it didn't throw a code. I could definitely feel the miss under load at high RPM's! However, I knew what my issue was. I burn the #6 plug wire regularly due to my headers...even with heavy heat shielding on the wire. I replace that one wire I keep burning and it goes away. I know you've already said you did plugs/wires, but I'm just agreeing it's odd a misfire you can feel doesn't throw a code...
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Staging Lane
Joined: Feb 2013
Posts: 99
Likes: 2
I had the same issue, I too, was puzzled why it didn't throw a code. I could definitely feel the miss under load at high RPM's! However, I knew what my issue was. I burn the #6 plug wire regularly due to my headers...even with heavy heat shielding on the wire. I replace that one wire I keep burning and it goes away. I know you've already said you did plugs/wires, but I'm just agreeing it's odd a misfire you can feel doesn't throw a code...
Joined: May 2004
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From: Halfway back on the Highway to Hell...again!
A misfire can go from simple to serious mechanical.
Bad lifter (if hydraulic), bent pushrod, bent valve or valve not sealing, bad spring or incorrect spring install etc.
Electrical question is, What/who's harness are you using? possible coil harness not correct. If using a stock harness, the signal wire for #1 cylinder should be red or purple. If its 1, then the other should be on #8. Red on 1, purple on 8 Or vice-versa.
Bad lifter (if hydraulic), bent pushrod, bent valve or valve not sealing, bad spring or incorrect spring install etc.
Electrical question is, What/who's harness are you using? possible coil harness not correct. If using a stock harness, the signal wire for #1 cylinder should be red or purple. If its 1, then the other should be on #8. Red on 1, purple on 8 Or vice-versa.
Thread Starter
Staging Lane
Joined: Feb 2013
Posts: 99
Likes: 2
A misfire can go from simple to serious mechanical.
Bad lifter (if hydraulic), bent pushrod, bent valve or valve not sealing, bad spring or incorrect spring install etc.
Electrical question is, What/who's harness are you using? possible coil harness not correct. If using a stock harness, the signal wire for #1 cylinder should be red or purple. If its 1, then the other should be on #8. Red on 1, purple on 8 Or vice-versa.
Bad lifter (if hydraulic), bent pushrod, bent valve or valve not sealing, bad spring or incorrect spring install etc.
Electrical question is, What/who's harness are you using? possible coil harness not correct. If using a stock harness, the signal wire for #1 cylinder should be red or purple. If its 1, then the other should be on #8. Red on 1, purple on 8 Or vice-versa.
TECH Fanatic
Joined: Feb 2015
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How did you determine bank 1 is misfiring? All cylinders or just 1? How much boost? Plug gap too wide? Did you swap known good parts from bank 2 to bank 1? Log of issue would be good info. Scope ign coils and injectors will provide some good insight.
Thread Starter
Staging Lane
Joined: Feb 2013
Posts: 99
Likes: 2
the guy doing the tuning said he is seeing misfires on bank 1 where it is cycling from lean to rich whenever I give it too much gas under load. Bank 2 shows steady curve where bank 1 doesn't. He can't tell if it is one cylinder or more than one. Magnuson is supposed to deliver 7 pounds of boost. All plugs are gapped as recommended by magnuson instruction manual. I took plugs out to do compression and leakdown tests and noted that 5 and 7 were starting to foul after about 200 miles while all others looked brand new still. Compression was a little higher in those 2 cylinders but within acceptable range of the others. Leakdown test showed very good results and consistent between cylinders.
Thread Starter
Staging Lane
Joined: Feb 2013
Posts: 99
Likes: 2
follow up/ When doing the compression and leak down test, I noticed that the 2 cylinders that had slightly fouled plugs also had replacement fuel injectors. same p/n but different brand. I went back with all of the same brand after cleaning them all again. Quick test this morning and it seemed to run better. Need to go drive around a bit before I call it fixed. Hate to think that was the whole problem all along, but if it was, at least I can move on finally. Thanks for all the advice.
Thread Starter
Staging Lane
Joined: Feb 2013
Posts: 99
Likes: 2
2004 GTO motor with magnuson supercharger in 69 Chevelle convertible. Trying to tune for several months now (off and on), but still getting misfires from bank 1 under heavy load according to the tuner. No PCM codes thrown. I replaced O2 Sensor, plugs, injectors, wires and coils already. Still can't get it to pull good under load. It idles well and if I take it easy it will rev up and run smooth. Under load it sputters and then eventually will rev and has good power. Any suggestion? I posted this in the conversions folder but wanted to reach out to the broader audience of experts.
finally fixed and posting for others that may be seeing similar issues. it turned out the I had 2 fuel injectors that although they were the same p/n, they were different brand than the others. Once I swapped in 2 new ones to match brand of the others, the problem went away. the cheaper imitations must have had a different flow rate or something.
TECH Senior Member
Joined: Jun 2016
Posts: 22,291
Likes: 3,616
From: Central Cal.
finally fixed and posting for others that may be seeing similar issues. it turned out the I had 2 fuel injectors that although they were the same p/n, they were different brand than the others. Once I swapped in 2 new ones to match brand of the others, the problem went away. the cheaper imitations must have had a different flow rate or something.
