94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open and
06-01-2020, 07:35 PM
#16
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
You need to understand the operation of the IAC valve. At idle, it opens and closes to adjust the idle RPM to the “target” RPM programmed in the PCM. On cold start, the target idle speed is elevated based on coolant temp - just like the choke on a carb. The PCM opens up the IAC valve by commanding higher IAC counts.
As the coolant temp warms up, the PCM targets a lower RPM, and closes the IAC down. Once the engine reaches operating temperature the PCM program, with automatic trans, targets 550 RPM in gear and 650 RPM in neutral. Your IAC can’t accomplish that. It reduces air flow at 0 counts, but there's still enough air flow that your engine still idles 50 to 100 RPM above where it's supposed to. This is why you want the IAC counts at idle in the range of 20 to 40, so there is headroom to control the idle RPM to the programmed target value. It may be because of a vacuum leak. The throttle stop screw may need to be adjusted.
When you drive the car, the purpose of the IAC valve changes. As speed/RPM increases, the IAC opens up via increased IAC counts so the engine does not stall when the throttle blades close. As speed/RPM increases the IAC counts can exceed 100. I'll look at what yours did in the first data log, but it isn’t a problem if they increase as you apparently observed.
I want to complete the review of the first log before I start on the new one. It takes a significant amount of time/work to set the log up in a form that I find useful. I add columns, color code items to allow me to look at multiple parameters simultaneously, hide irrelevant columns, convert units as required, etc. It’s not just a case of opening the file and looking at it for a few minutes.
One thing I mentioned in my “lost” response was the fact that you need to increase the scanning rate. The PCM updates at least 9 times per second, and your first log was only averaging about 1.5 frames/second. A lot of the data is hidden. Try getting the data logging rate to at least 5 frames/sec. You may have to ask GaryDoug how to do that.
I'll need to find more free time tomorrow to finish up the review of the first log, and setting up the second file. I am “retired”, but I still provide consulting services to construction companies and municipal engineers, as a Licensed Professional Engineer. If I have a “project” under way, I need to give that priority.
As the coolant temp warms up, the PCM targets a lower RPM, and closes the IAC down. Once the engine reaches operating temperature the PCM program, with automatic trans, targets 550 RPM in gear and 650 RPM in neutral. Your IAC can’t accomplish that. It reduces air flow at 0 counts, but there's still enough air flow that your engine still idles 50 to 100 RPM above where it's supposed to. This is why you want the IAC counts at idle in the range of 20 to 40, so there is headroom to control the idle RPM to the programmed target value. It may be because of a vacuum leak. The throttle stop screw may need to be adjusted.
When you drive the car, the purpose of the IAC valve changes. As speed/RPM increases, the IAC opens up via increased IAC counts so the engine does not stall when the throttle blades close. As speed/RPM increases the IAC counts can exceed 100. I'll look at what yours did in the first data log, but it isn’t a problem if they increase as you apparently observed.
I want to complete the review of the first log before I start on the new one. It takes a significant amount of time/work to set the log up in a form that I find useful. I add columns, color code items to allow me to look at multiple parameters simultaneously, hide irrelevant columns, convert units as required, etc. It’s not just a case of opening the file and looking at it for a few minutes.
One thing I mentioned in my “lost” response was the fact that you need to increase the scanning rate. The PCM updates at least 9 times per second, and your first log was only averaging about 1.5 frames/second. A lot of the data is hidden. Try getting the data logging rate to at least 5 frames/sec. You may have to ask GaryDoug how to do that.
I'll need to find more free time tomorrow to finish up the review of the first log, and setting up the second file. I am “retired”, but I still provide consulting services to construction companies and municipal engineers, as a Licensed Professional Engineer. If I have a “project” under way, I need to give that priority.
06-02-2020, 11:18 AM
#17
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
As noted above, didn’t find a “smoking gun” that would explain your problem. I still need to focus on the EGR and EVAP systems to see if there is a connection.
Scan Rate – comments in above post
IAC – comments in above post
Ignition Volts – seems low at idle as ~11.1 volts. Do you have an underdrive pulley? Low voltage affects the fuel system. U/D pulley is a waste of time and causes more problems than it solves.
A/C – Log shows that at ~417 seconds run time, there was an A/C “request”, but the clutch relay did not activate. Is there a problem with your A/C? Generally, it’s best to leave the A/C off when trying to trouble shoot an engine problem. It’s just another complication.
Coolant/Inlet air temps – the numbers appear rational. The log shows that fan #1 turned on at ~183-degF. Not stock programming. Do you have a 160-deg t’stat?
Barometric Pressure – pressure is high at 103.7 kPa (30.61”Hg). Good thing if it’s accurate. Are you located near sea level?
TPS Volts – closed throttle position sensor voltage is in normal range at 0.59 volts
MAF – readings look rational, no indication of air entering the system after the MAF.
MAP – at idle looks a shade high, but relatively steady, indicating a lack of misfires. BAR – MAP = vacuum. Your vacuum at idle is ~19.4”Hg. Good, maybe a shade low. Stock cam?
Spark Advance – appears typical for a stock tune.
Knock Retard/Knock Count – No problem evident. Retard is zero through entire log and count does not change.
LTFT’s – Bank 1 (left) is adding about 3% fuel at idle, within +/-5% tolerance. Bank 2 (right) is adding 6.3%. Not a major problem, but if there is a vacuum leak, look closer at Bank 2. But if there is a leak, the PCM is compensating for it, and it wouldn’t explain your problems.
Closed Loop – System goes into closed loop at 206 seconds (normal timeout).
As far as EGR and EVAP. I just want to look at whether there is any impact on the O2 readings when the systems operate, indicating a problem related to those systems. Maybe the next log will show up a problem. On the next log, did you actually experience the the backfiring and shuddering when you opened the throttle to accelerate?
Scan Rate – comments in above post
IAC – comments in above post
Ignition Volts – seems low at idle as ~11.1 volts. Do you have an underdrive pulley? Low voltage affects the fuel system. U/D pulley is a waste of time and causes more problems than it solves.
A/C – Log shows that at ~417 seconds run time, there was an A/C “request”, but the clutch relay did not activate. Is there a problem with your A/C? Generally, it’s best to leave the A/C off when trying to trouble shoot an engine problem. It’s just another complication.
Coolant/Inlet air temps – the numbers appear rational. The log shows that fan #1 turned on at ~183-degF. Not stock programming. Do you have a 160-deg t’stat?
Barometric Pressure – pressure is high at 103.7 kPa (30.61”Hg). Good thing if it’s accurate. Are you located near sea level?
TPS Volts – closed throttle position sensor voltage is in normal range at 0.59 volts
MAF – readings look rational, no indication of air entering the system after the MAF.
MAP – at idle looks a shade high, but relatively steady, indicating a lack of misfires. BAR – MAP = vacuum. Your vacuum at idle is ~19.4”Hg. Good, maybe a shade low. Stock cam?
Spark Advance – appears typical for a stock tune.
Knock Retard/Knock Count – No problem evident. Retard is zero through entire log and count does not change.
LTFT’s – Bank 1 (left) is adding about 3% fuel at idle, within +/-5% tolerance. Bank 2 (right) is adding 6.3%. Not a major problem, but if there is a vacuum leak, look closer at Bank 2. But if there is a leak, the PCM is compensating for it, and it wouldn’t explain your problems.
Closed Loop – System goes into closed loop at 206 seconds (normal timeout).
As far as EGR and EVAP. I just want to look at whether there is any impact on the O2 readings when the systems operate, indicating a problem related to those systems. Maybe the next log will show up a problem. On the next log, did you actually experience the the backfiring and shuddering when you opened the throttle to accelerate?
06-02-2020, 04:44 PM
#18
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
As noted above, didn’t find a “smoking gun” that would explain your problem. I still need to focus on the EGR and EVAP systems to see if there is a connection.
Scan Rate – comments in above post
IAC – comments in above post
Ignition Volts – seems low at idle as ~11.1 volts. Do you have an underdrive pulley? Low voltage affects the fuel system. U/D pulley is a waste of time and causes more problems than it solves.
A/C – Log shows that at ~417 seconds run time, there was an A/C “request”, but the clutch relay did not activate. Is there a problem with your A/C? Generally, it’s best to leave the A/C off when trying to trouble shoot an engine problem. It’s just another complication.
Coolant/Inlet air temps – the numbers appear rational. The log shows that fan #1 turned on at ~183-degF. Not stock programming. Do you have a 160-deg t’stat?
Barometric Pressure – pressure is high at 103.7 kPa (30.61”Hg). Good thing if it’s accurate. Are you located near sea level?
TPS Volts – closed throttle position sensor voltage is in normal range at 0.59 volts
MAF – readings look rational, no indication of air entering the system after the MAF.
MAP – at idle looks a shade high, but relatively steady, indicating a lack of misfires. BAR – MAP = vacuum. Your vacuum at idle is ~19.4”Hg. Good, maybe a shade low. Stock cam?
Spark Advance – appears typical for a stock tune.
Knock Retard/Knock Count – No problem evident. Retard is zero through entire log and count does not change.
LTFT’s – Bank 1 (left) is adding about 3% fuel at idle, within +/-5% tolerance. Bank 2 (right) is adding 6.3%. Not a major problem, but if there is a vacuum leak, look closer at Bank 2. But if there is a leak, the PCM is compensating for it, and it wouldn’t explain your problems.
Closed Loop – System goes into closed loop at 206 seconds (normal timeout).
As far as EGR and EVAP. I just want to look at whether there is any impact on the O2 readings when the systems operate, indicating a problem related to those systems. Maybe the next log will show up a problem. On the next log, did you actually experience the the backfiring and shuddering when you opened the throttle to accelerate?
Scan Rate – comments in above post
IAC – comments in above post
Ignition Volts – seems low at idle as ~11.1 volts. Do you have an underdrive pulley? Low voltage affects the fuel system. U/D pulley is a waste of time and causes more problems than it solves.
A/C – Log shows that at ~417 seconds run time, there was an A/C “request”, but the clutch relay did not activate. Is there a problem with your A/C? Generally, it’s best to leave the A/C off when trying to trouble shoot an engine problem. It’s just another complication.
Coolant/Inlet air temps – the numbers appear rational. The log shows that fan #1 turned on at ~183-degF. Not stock programming. Do you have a 160-deg t’stat?
Barometric Pressure – pressure is high at 103.7 kPa (30.61”Hg). Good thing if it’s accurate. Are you located near sea level?
TPS Volts – closed throttle position sensor voltage is in normal range at 0.59 volts
MAF – readings look rational, no indication of air entering the system after the MAF.
MAP – at idle looks a shade high, but relatively steady, indicating a lack of misfires. BAR – MAP = vacuum. Your vacuum at idle is ~19.4”Hg. Good, maybe a shade low. Stock cam?
Spark Advance – appears typical for a stock tune.
Knock Retard/Knock Count – No problem evident. Retard is zero through entire log and count does not change.
LTFT’s – Bank 1 (left) is adding about 3% fuel at idle, within +/-5% tolerance. Bank 2 (right) is adding 6.3%. Not a major problem, but if there is a vacuum leak, look closer at Bank 2. But if there is a leak, the PCM is compensating for it, and it wouldn’t explain your problems.
Closed Loop – System goes into closed loop at 206 seconds (normal timeout).
As far as EGR and EVAP. I just want to look at whether there is any impact on the O2 readings when the systems operate, indicating a problem related to those systems. Maybe the next log will show up a problem. On the next log, did you actually experience the the backfiring and shuddering when you opened the throttle to accelerate?
I didn't know the 1st owner but I do know he made a good deal of mods to the suspension and rear diff. I think it is a stock cam - it seems pretty tame but it would not surprise me if the cam was aftermarket so the car would work better with NOS. When the 2nd owner got it had spray hooked up and no cats back in 2011. Here in NJ anything newer than 25 years old has to pass inspection t and I believe that why the 2nd owner has cats installed. Now @ 26 years old I don't need to get it inspected. I see you are here in the wonderful state of NJ also. I live in Wall / Belmar so I'm right @ sea level. I think the time is stock as the PCM was replaced once before I got it. I did turn on the outside air using the defrost setting while running the fist log - I think this engages the ac compressor.
I'll check for vacuum leaks on the 2nd bank area. . I think I answered all the items in question- if I missed anything or there is any further info needed just lmk.
Thx again. I look forward to seeing if there is anything from the 2nd log when you have the time. Once again its greatly appreciated.
Rick
06-02-2020, 10:06 PM
#19
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
To make the scan faster, there isn't a lot you can do. But first, make sure you have the Wait Time entry in the main screen set to zero. The only time we want to increase that is when the pc is too slow to keep up with the scan and crashes but that is very rare. What type of pc are you using? Operating system?
06-02-2020, 10:50 PM
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
Something's slowing it down even more. The latest log (post #15) is down as low as 0.8 frames/second.
This log is brief and so much different - IAC in the 70’s at idle, Cell 16 Bank 1 LTFT is up to 160. Doesn’t even look like the same engine. But I have to look in more detail to see if all that could be the result of replacing the leaking EVAP purge elbow at the throttle body.
This log is brief and so much different - IAC in the 70’s at idle, Cell 16 Bank 1 LTFT is up to 160. Doesn’t even look like the same engine. But I have to look in more detail to see if all that could be the result of replacing the leaking EVAP purge elbow at the throttle body.
06-03-2020, 04:31 PM
#21
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
To make the scan faster, there isn't a lot you can do. But first, make sure you have the Wait Time entry in the main screen set to zero. The only time we want to increase that is when the pc is too slow to keep up with the scan and crashes but that is very rare. What type of pc are you using? Operating system?
Maybe while unplugged the laptop is I'm some type of a power saving mode. That could account for lagging response times. I'll check on that and let you know.
thx for taking the time to give me a hand with this. Greatly appreciated.
Rick
06-03-2020, 04:34 PM
#22
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
Something's slowing it down even more. The latest log (post #15) is down as low as 0.8 frames/second.
This log is brief and so much different - IAC in the 70’s at idle, Cell 16 Bank 1 LTFT is up to 160. Doesn’t even look like the same engine. But I have to look in more detail to see if all that could be the result of replacing the leaking EVAP purge elbow at the throttle body.
This log is brief and so much different - IAC in the 70’s at idle, Cell 16 Bank 1 LTFT is up to 160. Doesn’t even look like the same engine. But I have to look in more detail to see if all that could be the result of replacing the leaking EVAP purge elbow at the throttle body.
. I'll check to see if my laptop is in power saving mode. If it is I'll take it out of that and run another log if needed. Just LMK if that is needed.
Thx again, Rick
06-03-2020, 08:42 PM
#23
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
Don't run another log until I outline the best way to operate the vehicle. And without a faster scan rate, the results become less meaningful.
06-03-2020, 10:38 PM
#24
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
Sorry, I haven't read all the posts. But if nothing works to speed up the scan, I wonder if a re-programmed PCM could cause the slow-down? Does the pcm have a "tune"?
06-06-2020, 11:07 AM
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
06-06-2020, 11:09 AM
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
DATA LOG GUIDELINES
Do not reset PCM. Do not remove a battery cable. Do not pull the PCM BAT fuse.
Start COLD engine. Do NOT turn on A/C or Defrost. Turn off traction control (ASR / TCS) if vehicle equipped.
Leave the accel pedal alone. Do NOT rev the engine while it is warming up. Leave it alone.
Allow engine to idle for at least 4 minutes. Should be long enough to warm up put PCM in closed loop.
Start driving car for about 10 minutes. Vary the accel pedal position and RPM. Make at least one WOT pull from a roll up to at least 5,000 RPM.
Mentally note (or have a passenger write down) when the engine exhibited the problem you are chasing.
Stop driving, let it idle for about 30 seconds, without touching the accel pedal.
Finished.
Do not reset PCM. Do not remove a battery cable. Do not pull the PCM BAT fuse.
Start COLD engine. Do NOT turn on A/C or Defrost. Turn off traction control (ASR / TCS) if vehicle equipped.
Leave the accel pedal alone. Do NOT rev the engine while it is warming up. Leave it alone.
Allow engine to idle for at least 4 minutes. Should be long enough to warm up put PCM in closed loop.
Start driving car for about 10 minutes. Vary the accel pedal position and RPM. Make at least one WOT pull from a roll up to at least 5,000 RPM.
Mentally note (or have a passenger write down) when the engine exhibited the problem you are chasing.
Stop driving, let it idle for about 30 seconds, without touching the accel pedal.
Finished.
06-06-2020, 11:19 AM
#27
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
06-06-2020, 05:09 PM
#28
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
DATA LOG GUIDELINES
Do not reset PCM. Do not remove a battery cable. Do not pull the PCM BAT fuse.
Start COLD engine. Do NOT turn on A/C or Defrost. Turn off traction control (ASR / TCS) if vehicle equipped.
Leave the accel pedal alone. Do NOT rev the engine while it is warming up. Leave it alone.
Allow engine to idle for at least 4 minutes. Should be long enough to warm up put PCM in closed loop.
Start driving car for about 10 minutes. Vary the accel pedal position and RPM. Make at least one WOT pull from a roll up to at least 5,000 RPM.
Mentally note (or have a passenger write down) when the engine exhibited the problem you are chasing.
Stop driving, let it idle for about 30 seconds, without touching the accel pedal.
Finished.
Do not reset PCM. Do not remove a battery cable. Do not pull the PCM BAT fuse.
Start COLD engine. Do NOT turn on A/C or Defrost. Turn off traction control (ASR / TCS) if vehicle equipped.
Leave the accel pedal alone. Do NOT rev the engine while it is warming up. Leave it alone.
Allow engine to idle for at least 4 minutes. Should be long enough to warm up put PCM in closed loop.
Start driving car for about 10 minutes. Vary the accel pedal position and RPM. Make at least one WOT pull from a roll up to at least 5,000 RPM.
Mentally note (or have a passenger write down) when the engine exhibited the problem you are chasing.
Stop driving, let it idle for about 30 seconds, without touching the accel pedal.
Finished.
I'll run a data log by these guidelines and post it tomorrow.
thx. Rick
06-06-2020, 05:10 PM
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
06-06-2020, 05:16 PM
#30
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Re: 94 Z28 LT1. - rough idle/backfiring and shuddering under acceleration - both open
It is possible it has a tune done by the 1st owner pre-2011. It was set up for the 1/4 mile with heavily modified suspension/ headers/ ELocker and nitrous spray. The NOS was removed by the 2nd owner and cats were added in 2011. Not sure what was done by owner 1.
Thx again , Rick