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Old 09-05-2017, 11:36 AM   #1
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misfire under load

I have a 94 Camaro lt1 383 stroker with the 6 speed manual and I can't get this misfire figured out. My TPS is set at .59v at idle. It idles great. But when taking of at a stop sign, it will backfire through the intake. It also does it when the rpms are under 2000 and under a load. I can't use 6 gear because of this. It has all new plugs and wires and a new cap and rotor for the opti. Also if I'm idling and snap the throttle quick, it will pop through the intake. I have checked for vacuum leaks and bad plug wires. Everything looks good. It's driving me nuts! I've done some data logging and that shows me everything is good. So confused
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Old 09-05-2017, 03:41 PM   #2
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Re: misfire under load

Can you share a .csv file of your data log? Glad to take a look at it for you. Second set of eyes might be a help. What data logging software are you using?

Would seem you can't have a backfire through the intake unless there is an intake valve open when a spark plug fires on that cylinder. Plug wires and new distributor cap would seem to have limited the possibility of cross-firing, carbon tracking, etc. How about a mechanical problem, valves adjusted too tight, sticky valve, etc.?
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Old 09-05-2017, 05:49 PM   #3
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Re: misfire under load

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Can you share a .csv file of your data log? Glad to take a look at it for you. Second set of eyes might be a help. What data logging software are you using?

Would seem you can't have a backfire through the intake unless there is an intake valve open when a spark plug fires on that cylinder. Plug wires and new distributor cap would seem to have limited the possibility of cross-firing, carbon tracking, etc. How about a mechanical problem, valves adjusted too tight, sticky valve, etc.?
I use tunerpro. still trying to figure it out. I cant figure out how to upload them to the forum tho. I havent checked under the valve covers yet, but there is no weird noises from the engine like a bent valve. im also dissapointed at the HP of the motor. a fresh built 383 stroker with port and polished heads, electric water pump, no ac, no egr, no emissions. 335hp seams pretty low.
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Old 09-05-2017, 09:57 PM   #4
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Re: misfire under load

To upload a file (.csv) or photo from your PC, click on "Go Advanced" under the message box. When the new screen opens, scroll down to "Manage Attachments". Click on that and follow instructions.

If you are not averse to trying another free data logging program, try Scan9495. I prefer it to others.

https://www.camaroz28.com/forums/com...95-lt1-874306/

Last edited by Injuneer; 09-06-2017 at 09:17 AM.
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Old 09-06-2017, 12:49 AM   #5
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Re: misfire under load

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Originally Posted by Injuneer View Post
To upload a file (.csv) or photo from your PC, click on "Go Advanced) under the message box. When the new screen opens, scroll down to "Manage Attachments". Click on that and follow instructions.

If you are not averse to trying another free data logging program, try Scan9495. I prefer it to others.

https://www.camaroz28.com/forums/com...95-lt1-874306/
I'll give it a go and try to post it.
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Old 09-07-2017, 08:45 AM   #6
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Re: misfire under load

this is my drive home.
Attached Files
File Type: csv DLOG-Save-2017Sep07-073750-2G1FP22P2R2145105.csv (2.04 MB, 9 views)
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Old 09-07-2017, 04:10 PM   #7
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Re: misfire under load

Downloaded the file, converted to Excel. It's on the large side (30 minutes, 10,000+ lines of data) and will take a day or so.

A quick review indicates it took a bit longer than usual to go into closed loop. Typically that happens at 206 seconds, which is set by a timer. Yours did not go into closed loop at 206 seconds because the coolant temp was only 127*F. The system waited until ~250 seconds until the coolant temp reached 140*F. Eventually it warmed up to 185*F, but seems a bit slow to warm up.

How big is the cam? Programmed idle speed is 1,050 RPM. Stock M6 programming is 800 RPM fully warmed up, and I had no problem keeping that with a 230/242 114 LSA. I suspect you have something larger than that, with a narrower LSA (mine is a "nitrous" cam, and was selected to pass NJ tailpipe emissions, hence the 114* LSA). With that idle speed, MAF flow is 17 to 18 grams/second, about 2X to 3X what we would see with a stock idle speed. MAP is in the low 60 kPa range, consistent with a healthy (but not radical) cam. Do you have cam specs?

Idle advance is 28-30*, again, something typical of tuning for a larger cam (vs 19-22* for a stock cam).

The long term fuel corrections are all over the place, but not too bad. Ranges from a split of 134 left/120 right in Cell 14 to 121 left/121 right in Cell 6. I haven't found all the cells yet. Idle at the very end in Cell 16 is 126 left/128 right, which is excellent.

As noted, just a preliminary scroll through the file.

Last edited by Injuneer; 09-07-2017 at 04:12 PM.
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Old 09-07-2017, 10:06 PM   #8
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Re: misfire under load

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Downloaded the file, converted to Excel. It's on the large side (30 minutes, 10,000+ lines of data) and will take a day or so.

A quick review indicates it took a bit longer than usual to go into closed loop. Typically that happens at 206 seconds, which is set by a timer. Yours did not go into closed loop at 206 seconds because the coolant temp was only 127*F. The system waited until ~250 seconds until the coolant temp reached 140*F. Eventually it warmed up to 185*F, but seems a bit slow to warm up.

How big is the cam? Programmed idle speed is 1,050 RPM. Stock M6 programming is 800 RPM fully warmed up, and I had no problem keeping that with a 230/242 114 LSA. I suspect you have something larger than that, with a narrower LSA (mine is a "nitrous" cam, and was selected to pass NJ tailpipe emissions, hence the 114* LSA). With that idle speed, MAF flow is 17 to 18 grams/second, about 2X to 3X what we would see with a stock idle speed. MAP is in the low 60 kPa range, consistent with a healthy (but not radical) cam. Do you have cam specs?

Idle advance is 28-30*, again, something typical of tuning for a larger cam (vs 19-22* for a stock cam).

The long term fuel corrections are all over the place, but not too bad. Ranges from a split of 134 left/120 right in Cell 14 to 121 left/121 right in Cell 6. I haven't found all the cells yet. Idle at the very end in Cell 16 is 126 left/128 right, which is excellent.

As noted, just a preliminary scroll through the file.
Cam specs @ .50"
Duration: 238/246
Lift: .365
Intake center line: 112
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Old 09-08-2017, 01:21 AM   #9
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Re: misfire under load

Sounds like the Erson 298/306.

Duration at 0.050" 238/246

Lobe lift 0.365" = 0.548" valve lift w/1.5X rockers; 0.584" w/ 1.6X rockers.

Except Erson quotes that as "108 Lobe Center" which they appear to use as lobe separation angle. If it was ground w/ a 112 LSA, that would explain the smooth idle and the moderate MAP at idle.

Other than the specs on their website, I'm not familiar with their cams. That appears to be a fairly aggressive cam, and would take a dyno tune to optimize. Who did the tune?

What rear axle are you running? Are EGR and EVAP deleted?

Last edited by Injuneer; 09-08-2017 at 01:24 AM.
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Old 09-08-2017, 04:25 AM   #10
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Re: misfire under load

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Sounds like the Erson 298/306.

Duration at 0.050" 238/246

Lobe lift 0.365" = 0.548" valve lift w/1.5X rockers; 0.584" w/ 1.6X rockers.

Except Erson quotes that as "108 Lobe Center" which they appear to use as lobe separation angle. If it was ground w/ a 112 LSA, that would explain the smooth idle and the moderate MAP at idle.

Other than the specs on their website, I'm not familiar with their cams. That appears to be a fairly aggressive cam, and would take a dyno tune to optimize. Who did the tune?

What rear axle are you running? Are EGR and EVAP deleted?
You are right. It is an Erson cam. TPIS Dyno tuned it and he said I don't have enough compression and that's why my HP is low. Compression is 10.5:1. 3.73 gears, I do not have EGR. I still have EVAP system.
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Old 09-08-2017, 11:11 AM   #11
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Re: misfire under load

What kind of dyno results? Depending on the heads, I would think you would be over 500 HP at the flywheel (close to 450 at the rear wheels).

As a reference point, build as tested after the stroker was initially built - M6/steel Street Twin/3" chrome moly DS/12-bolt/3.73 gears/street tire's I had dyno figures for flywheel (490) and rear wheel (427), showing a 13% drivetrain loss. That was using 93 octane pump fuel. When it was baselined w/ C16 for the nitrous tune, made 496 flywheel.

As noted cam (NJ emissions friendly) 230/242 114LSA 0.590"/0.590" w/ a shade over 10.8:1 static CR. HP does suffer with heavy crown nitrous pistons, dropped ring pack. Lighter, more stable pistons would have netted a bit more. And in 2000 there was a more limited head selection. I got the GM LT4's from CNC Cylinder Heads in Pinellas, FL. Flowed a bit under 300 CFM w/ ported GM LT4 intake in place. Much better heads available now.

Last edited by Injuneer; 09-08-2017 at 11:14 AM.
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Old 09-08-2017, 04:37 PM   #12
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Re: misfire under load

The heads are stock with port and polish. Not sure on the CFM. I only got 337hp at 5100 RPMs and 324 torque. Both are to the wheels.
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Old 09-08-2017, 08:15 PM   #13
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Re: misfire under load

Nothing wrong with ported stock heads. Done correctly they can support 500 HP at the flywheel. The benchmark for reasonably priced stock ported heads is Elliott Port Works:

http://elliottsportworks.com/

Advanced Induction is a bit more expensive, and can produce a bit more HP.

Advanced Induction - LTx Performance Parts
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Old 09-09-2017, 12:21 AM   #14
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Re: misfire under load

Yea. My engine builder ported them himself. He's a performance shop and he drag races in the big money and points. I don't think he measured how much flow they have.
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Old 09-11-2017, 06:36 PM   #15
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Re: misfire under load

There's only one portion of the data log that I found where you actually got it into power enrichment mode (2,439 RPM/53% TPP). Everything looked very good. O2 sensors on both sides were a bit leaner (850 mV) than the stock tune, and that's good. It was in Cell 15 and LTFT's L/R were 129/131. No sign of misfires, very little variation in the O2 sensor readings, no blips in the MAP readings indicating a backfire.

I did find periods when it was just idling, vehicle not moving, and one or both sides of the engine would go lean for several seconds, almost like an injector was sticking closed, or the fuel pressure was low.

Still need to look at the places where you were blipping the throttle at idle.
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