Misfire at WOT
05-01-2010, 08:17 AM
#1
Misfire at WOT Ive been yanking my hair out on this one for 2 months
My main issue is a severe random misfire at heavy throttle (80%-WOT). It sounds like Im hitting a rev limiter, whether its 1500 RPMs or 5000. Misfire is across all 8 randomly so I cant do the swap test. If I WOT at a low speed I can smell how utterly rich its running during the pulls. If I let off the throttle back to around 50% or so the car switches back to running perfectly and will rev till I let off. It seems like a spark issue. Plugs, wires, coils all have about 6,000 miles on them and were brand new at the completion of the swap. TR5's are gapped to .050" and appear to be burning clean. No cracks that Ive found. Resistance across the plug wires is 470 +/- 10ohms. Im very careful to get a positive connection from the wires to the plugs
My second issue is a gradual enrichment at idle in gear. STFTs will slowly creep up from 0 to around 15% where the engine will either stall or backfire and reset the STFTs to 0 and repeat. The hotter the day is the worse this gets so Im guessing its heatsoak related. Bank 1 is usually 5% higher than bank 2 at the worst
Cruising the car runs perfectly
Its been tuned with a wideband and ran great for better than a week. Then something happened and these problems took effect. I know theres something bad in the system, I cant cant figure out what
Diagnostics:
I have a snap-on scanner and have been monitoring the live data from the sensors as the engine runs. I do not have any tuning software otherwise Id post an actual run log
TPS shows a good reading from 0% to 100%
IAC is replaced and holds a steady idle
MAF, IAT, CTS, MAP all show good readings
02s fluctuate properly
Ive tried unplugging the O2s and the problems persist
Unplugged the MAF and the STFTs went to -40% for both banks. Im guessing this is my biggest clue, but I havent the slightest where to look. Im not sure how accurate the unplug test was because the IAT is also a part of the MAF
The only things I really dont know how to test are the crank/cam sensors. Crank seems fine because it always starts and the tach never gives a bad reading
Fuel pressure is rock solid at 55psi at start, idle and under load. Im not sure how accurate the number is because its a cheap gauge. Its definitely not starving for fuel
Compression is 160 +/-5 for all 8. No excessive oil consumption or blow-by to suggest bad rings
Intake has been off several times visually looking for cracks. All vacuum lines have been checked repeatedly, but a vacuum leak shouldnt do anything under WOT
Ive pulled the entire harness and rechecked each individual wire. Everything is fine. Ive even added a few more redundant grounds, no change. No idea where to look
Car is a 92 firebird
LM7 longblock with LS1 Fbody dress
4L60E from a friends TA
My main issue is a severe random misfire at heavy throttle (80%-WOT). It sounds like Im hitting a rev limiter, whether its 1500 RPMs or 5000. Misfire is across all 8 randomly so I cant do the swap test. If I WOT at a low speed I can smell how utterly rich its running during the pulls. If I let off the throttle back to around 50% or so the car switches back to running perfectly and will rev till I let off. It seems like a spark issue. Plugs, wires, coils all have about 6,000 miles on them and were brand new at the completion of the swap. TR5's are gapped to .050" and appear to be burning clean. No cracks that Ive found. Resistance across the plug wires is 470 +/- 10ohms. Im very careful to get a positive connection from the wires to the plugs
My second issue is a gradual enrichment at idle in gear. STFTs will slowly creep up from 0 to around 15% where the engine will either stall or backfire and reset the STFTs to 0 and repeat. The hotter the day is the worse this gets so Im guessing its heatsoak related. Bank 1 is usually 5% higher than bank 2 at the worst
Cruising the car runs perfectly
Its been tuned with a wideband and ran great for better than a week. Then something happened and these problems took effect. I know theres something bad in the system, I cant cant figure out what
Diagnostics:
I have a snap-on scanner and have been monitoring the live data from the sensors as the engine runs. I do not have any tuning software otherwise Id post an actual run log
TPS shows a good reading from 0% to 100%
IAC is replaced and holds a steady idle
MAF, IAT, CTS, MAP all show good readings
02s fluctuate properly
Ive tried unplugging the O2s and the problems persist
Unplugged the MAF and the STFTs went to -40% for both banks. Im guessing this is my biggest clue, but I havent the slightest where to look. Im not sure how accurate the unplug test was because the IAT is also a part of the MAF
The only things I really dont know how to test are the crank/cam sensors. Crank seems fine because it always starts and the tach never gives a bad reading
Fuel pressure is rock solid at 55psi at start, idle and under load. Im not sure how accurate the number is because its a cheap gauge. Its definitely not starving for fuel
Compression is 160 +/-5 for all 8. No excessive oil consumption or blow-by to suggest bad rings
Intake has been off several times visually looking for cracks. All vacuum lines have been checked repeatedly, but a vacuum leak shouldnt do anything under WOT
Ive pulled the entire harness and rechecked each individual wire. Everything is fine. Ive even added a few more redundant grounds, no change. No idea where to look
Car is a 92 firebird
LM7 longblock with LS1 Fbody dress
4L60E from a friends TA
05-02-2010, 09:08 AM
#6
Good deal, fuel delivery is ruled out at this point.
Hook up your laptop and record PCM output during a WOT blast. The following fields should be useful: (add more if you find any others that seem relevant)
Spark Advance
LTFT bank 1
LTFT bank 2
Knock Retard
Throttle Position
RPM
Injector Bank 1 Pulse Width (ms)
Injector Bank 2 " "
Bank 1 O2 sensor 1 output (mV)
Bank 2 O2 sensor 1 output (mV)
Manifold Absolute Pressure (KPa)
edit add: Mass Air Flow sensor output
Export it to text or Excel and I'll be glad to take a look.
Hook up your laptop and record PCM output during a WOT blast. The following fields should be useful: (add more if you find any others that seem relevant)
Spark Advance
LTFT bank 1
LTFT bank 2
Knock Retard
Throttle Position
RPM
Injector Bank 1 Pulse Width (ms)
Injector Bank 2 " "
Bank 1 O2 sensor 1 output (mV)
Bank 2 O2 sensor 1 output (mV)
Manifold Absolute Pressure (KPa)
edit add: Mass Air Flow sensor output
Export it to text or Excel and I'll be glad to take a look.
Last edited by crainholio; 05-02-2010 at 09:35 AM.
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05-02-2010, 09:57 AM
#8
Good luck diagnosing this without frame-by-frame data, dude. Won't be easy.
I'm speculating that either you have a transient knock source, or an airflow glitch either real or sensor malfunction.
You might be able to see long term knock retard if your scanner can pull the Adaptive Spark field. It's a multiplier (100%) that is applied globally to the spark advance map based on the PCM seeing persistent knock counts.
When I run 89 octane gas, I see my AdSpark drop to as low as 90%...which means it's deducting 10% of my advance at all points on the map.
With 93 octane gas, I get AdSpark value of 100% consistently.
So check your AdSpark immediately after a hard run and see what you find.
I'm speculating that either you have a transient knock source, or an airflow glitch either real or sensor malfunction.
You might be able to see long term knock retard if your scanner can pull the Adaptive Spark field. It's a multiplier (100%) that is applied globally to the spark advance map based on the PCM seeing persistent knock counts.
When I run 89 octane gas, I see my AdSpark drop to as low as 90%...which means it's deducting 10% of my advance at all points on the map.
With 93 octane gas, I get AdSpark value of 100% consistently.
So check your AdSpark immediately after a hard run and see what you find.
05-02-2010, 10:02 AM
#9
Here's something else...disconnect your MAF, and reset the PCM so no LTFT corrections exist.
Then run it and watch LTFTs, see how accurate your base VE table really is.
Also, what is your IAC field reading at idle? Great way to rule out vacuum leaks is via IAC steps...a vacuum leak will put that sucker at 0 in a hurry, as the PCM tries to bring RPMs down to the configured speed.
Then run it and watch LTFTs, see how accurate your base VE table really is.
Also, what is your IAC field reading at idle? Great way to rule out vacuum leaks is via IAC steps...a vacuum leak will put that sucker at 0 in a hurry, as the PCM tries to bring RPMs down to the configured speed.
05-03-2010, 04:52 PM
#12
I have the same problem,but only on bank #2. I'm also running dual exhaust and that tail pipe is very,very sooty.Cheapy scanner says low o2 voltage and bank 2 lean.Any help would be,well helpfull. Thanks.
05-03-2010, 05:11 PM
#14
Bank #2 not cyl #2. I pulled all the plugs on bank #2 black/sooty on the rim of the plug tan by the center and electrode,pulled one plug on bank #1 and it looked the same just not as sooty.
05-03-2010, 08:34 PM
#15
Sorry, tired eyes after exams. Trade the coil sets and see if it follows
If it doesnt, the issue is in the wiring somewhere. Black plugs means its getting fuel, so the INJ2 fuse feed is getting power and the PCM is commanding the injectors properly. Check the ground wire (blk) from the coil connector to any good ground spot. Anything under 1.5ohms is pass
If thats good, you need to check all 5 PCM wires to the coils. If those check out you could be looking at a faulty PCM, which may be my issue too
If it doesnt, the issue is in the wiring somewhere. Black plugs means its getting fuel, so the INJ2 fuse feed is getting power and the PCM is commanding the injectors properly. Check the ground wire (blk) from the coil connector to any good ground spot. Anything under 1.5ohms is pass
If thats good, you need to check all 5 PCM wires to the coils. If those check out you could be looking at a faulty PCM, which may be my issue too
05-09-2010, 10:29 PM
#20
TECH Regular
I would love to see the tune on this one if the fuel trims go to 40% if the maf is unplugged. That means that your VE table is probably way off and your tuner just did a maf tune. Is this a known good tuner? Is your motor stock or have you put a cam or any other big mods?
Make sure you don't have any exhaust leaks or vacuum leaks.
Make sure you don't have any exhaust leaks or vacuum leaks.
