1993 LT1 Idle surging
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Joined: Jun 2013
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From: Kernersville, NC
Well, I'm no expert at reading LT1 data logs, but I did look at this for a while last night.
I can't say for certain where the problem lies. The PCM is pointing at the MSD opti(or something along the high and low res signal paths). However, the log is not recording any problems with the high and low res signals during the time the RPM is going bananas. There are a couple of anomalies with the signal at a few points though.
For me, the next step would be to start scoping things on the engine to see what is going on....but if you don't own or know how to work an o-scope, that will be a problem.
There was an odd trend of the IAC right around where it went closed loop. The overall IAC trend went down.....as if it had started pulling in air from another source and thus reduced the overall IAC counts to keep the engine steady.
I don't have something for you that I can solidly point at and say "this is the problem".
I can't say for certain where the problem lies. The PCM is pointing at the MSD opti(or something along the high and low res signal paths). However, the log is not recording any problems with the high and low res signals during the time the RPM is going bananas. There are a couple of anomalies with the signal at a few points though.
For me, the next step would be to start scoping things on the engine to see what is going on....but if you don't own or know how to work an o-scope, that will be a problem.
There was an odd trend of the IAC right around where it went closed loop. The overall IAC trend went down.....as if it had started pulling in air from another source and thus reduced the overall IAC counts to keep the engine steady.
I don't have something for you that I can solidly point at and say "this is the problem".
Last edited by ACE1252; Apr 1, 2016 at 02:02 PM.
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From: Maricopa, AZ
I see the single O2 as a problem, but I'm not convinced that it's "the problem" as you mentioned that it's been running fine for three years with just one O2.
I've been looking at the low and high res data. The high res is consistent. The low res shows a couple of odd anomalies.....that does not correlate to the rpm logged at that time. I'm not sure if that is scan data issues or the problem that the PCM is flagging the code 36 for....
However as the PCM is flagging code 36, that is related to the signals coming from the opti.....so it's seeing something it does not like that the scan data is not showing. The scan data is only as small sample of what is really going on with the engine. To properly see what the opti high and low res signals are doing, we would need to put an oscilloscope on it.
Just to clarify, the engine was performing okay before your tune up correct? If so, I think something you changed is causing this issue....maybe the MSD opti or another of those components you changed.
I've been looking at the low and high res data. The high res is consistent. The low res shows a couple of odd anomalies.....that does not correlate to the rpm logged at that time. I'm not sure if that is scan data issues or the problem that the PCM is flagging the code 36 for....
However as the PCM is flagging code 36, that is related to the signals coming from the opti.....so it's seeing something it does not like that the scan data is not showing. The scan data is only as small sample of what is really going on with the engine. To properly see what the opti high and low res signals are doing, we would need to put an oscilloscope on it.
Just to clarify, the engine was performing okay before your tune up correct? If so, I think something you changed is causing this issue....maybe the MSD opti or another of those components you changed.
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From: Maricopa, AZ
Yes, it wasn't having any of these issues prior to the tune up. I replaced spark plugs, wires, opti, ign coil, water pump, water pump sensor, injectors, all vacuum lines. I've already took the opti off and inspected it closely, all seems just fine with it. Could bad plugs/wires cause it? A ground maybe? I'm really glad you guys are able to know what your looking for on that log. And yeah, the thing ran for years with just 1 O2 hooked up without anything wrong. The code 36 hasn't tripped the sel as of the last week or so.
Thinking further into this. Both PCM signal inputs are coming from the same O2, BUT the data log is showing a split BLM. That should NOT be happening. They should read exactly the same, everywhere. But they are not. Then the idle does smooth out once the BLM split maxes out Bank 1 at full lean (PCM commanded) while dumping fuel into the Right bank (bank 2). That means the 4 right cylinders are getting a lot more fuel than the 4 left ones. A lot. BUT, when you average out the 2 banks, you get fueling right on target (127.5 actually, but 128 is the target).
Try this, disconnect the O2. Let it idle for a while, long enough for the problem to duplicate. Datalog it while you do it. After the engine is at running temp, maybe 5-10 minutes after start up, put it in gear (if you have an Auto), this will load the engine and the IAC should increase but the idle should stay smooth (it may change with a load on it, either up or down, but it shouldn't surge). If the surging goes away with the O2 unplugged. You found the issue.
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From: Kernersville, NC
His long terms are high, but I don't think they are split bad at all for running off one O2.
Last edited by ACE1252; Apr 1, 2016 at 09:50 PM.
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From: Maricopa, AZ
I opened the log with Excel. The DTC I referenced is what he mentioned in the thread. You need to open it comma delimited to view it correctly with Excel...from there you can build charts of the data.
His long terms are high, but I don't think they are split bad at all for running off one O2.
His long terms are high, but I don't think they are split bad at all for running off one O2.
But the problem is the BLMs should not split at all if both banks are reading the same O2. They should both go up and down exactly the same since they are receiving the exact same data. This may have something to do with the O2 delays and integers in the tune in relation to the stock physical location of the sensors in distance from the exhaust valves though.
I don't use excel to analyze the data unless I'm making tuning adjustments. I wonder why excel shows the DTC code but Scan9495 doesn't. Weird.
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From: Kernersville, NC
Excel did not show any DTC codes. I'm referencing back to the code 36 that he mentioned earlier in the thread that the thing had thrown repeatedly at one point.
At line 5414, the time stamp on the data jumps from 11:47 to 11:53. he was screwing around with something and either lost the connection on the port or shut off the recording, then turned it back on....so as there is a 6 minute gap in the data, I consider from that point on as invalid.....I suspect he went tinkering with something as it was going up and down pretty badly at the 11:47 timestamp.
You may not be able to see that the way you are looking at it, but the coolant temp will show it...as there is an instant jump of almost 30 degrees....temp went from 153 to 180. That is where you are missing 6 mins of data.
At line 5414, the time stamp on the data jumps from 11:47 to 11:53. he was screwing around with something and either lost the connection on the port or shut off the recording, then turned it back on....so as there is a 6 minute gap in the data, I consider from that point on as invalid.....I suspect he went tinkering with something as it was going up and down pretty badly at the 11:47 timestamp.
You may not be able to see that the way you are looking at it, but the coolant temp will show it...as there is an instant jump of almost 30 degrees....temp went from 153 to 180. That is where you are missing 6 mins of data.
Last edited by ACE1252; Apr 2, 2016 at 02:31 AM.
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From: Maricopa, AZ
Excel did not show any DTC codes. I'm referencing back to the code 36 that he mentioned earlier in the thread that the thing had thrown repeatedly at one point.
At line 5414, the time stamp on the data jumps from 11:47 to 11:53. he was screwing around with something and either lost the connection on the port or shut off the recording, then turned it back on....so as there is a 6 minute gap in the data, I consider from that point on as invalid.....I suspect he went tinkering with something as it was going up and down pretty badly at the 11:47 timestamp.
You may not be able to see that the way you are looking at it, but the coolant temp will show it...as there is an instant jump of almost 30 degrees....temp went from 153 to 180. That is where you are missing 6 mins of data.
At line 5414, the time stamp on the data jumps from 11:47 to 11:53. he was screwing around with something and either lost the connection on the port or shut off the recording, then turned it back on....so as there is a 6 minute gap in the data, I consider from that point on as invalid.....I suspect he went tinkering with something as it was going up and down pretty badly at the 11:47 timestamp.
You may not be able to see that the way you are looking at it, but the coolant temp will show it...as there is an instant jump of almost 30 degrees....temp went from 153 to 180. That is where you are missing 6 mins of data.
I think disconnecting the O2 at this point and seeing if it helps or not would help with narrowing down the cause of the issue.
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From: Kernersville, NC
LoL, how long do you think this log is?
The engine started out with a coolant temp of 56*F, air intake temp was 48*F. If we stick with the time base you are on, the coolant temp went from 56*F to 180*F in 12 seconds. Short of throwing the engine in a lake of lava, I don't think that's right.
I'm pretty sure you have your time bases screwed up by a factor of 10. This appears to be 12 minutes worth of data with around 6 minutes missing close to the end.
For an engine, stone cold, with an ambient temp of 48*F(according to the intake temp), it seems reasonable for it to take around 18 minutes to reach 180* with the engine just idling and doing nothing else.
Gary also has a total scan time column as well. It appears to be in seconds. The last value was 1034.25. Divide that by 60 and you get 17.2375 minutes. That seems to back up what I'm telling you.
The engine started out with a coolant temp of 56*F, air intake temp was 48*F. If we stick with the time base you are on, the coolant temp went from 56*F to 180*F in 12 seconds. Short of throwing the engine in a lake of lava, I don't think that's right.
I'm pretty sure you have your time bases screwed up by a factor of 10. This appears to be 12 minutes worth of data with around 6 minutes missing close to the end.
For an engine, stone cold, with an ambient temp of 48*F(according to the intake temp), it seems reasonable for it to take around 18 minutes to reach 180* with the engine just idling and doing nothing else.
Gary also has a total scan time column as well. It appears to be in seconds. The last value was 1034.25. Divide that by 60 and you get 17.2375 minutes. That seems to back up what I'm telling you.
Last edited by ACE1252; Apr 2, 2016 at 11:12 AM.
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Verdad Gallardo TECH Addict
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From: Maricopa, AZ
LoL, how long do you think this log is?
The engine started out with a coolant temp of 56*F, air intake temp was 48*F. If we stick with the time base you are on, the coolant temp went from 56*F to 180*F in 12 seconds. Short of throwing the engine in a lake of lava, I don't think that's right.
I'm pretty sure you have your time bases screwed up by a factor of 10. This appears to be 12 minutes worth of data with around 6 minutes missing close to the end.
For an engine, stone cold, with an ambient temp of 48*F(according to the intake temp), it seems reasonable for it to take around 18 minutes to reach 180* with the engine just idling and doing nothing else.
The engine started out with a coolant temp of 56*F, air intake temp was 48*F. If we stick with the time base you are on, the coolant temp went from 56*F to 180*F in 12 seconds. Short of throwing the engine in a lake of lava, I don't think that's right.
I'm pretty sure you have your time bases screwed up by a factor of 10. This appears to be 12 minutes worth of data with around 6 minutes missing close to the end.
For an engine, stone cold, with an ambient temp of 48*F(according to the intake temp), it seems reasonable for it to take around 18 minutes to reach 180* with the engine just idling and doing nothing else.
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On The Tree
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Thread Starter
On The Tree
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From: Maricopa, AZ
Thread Starter
On The Tree
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Had the vats removed several years ago, and the wire harness is laid out pretty fool proof as far as left and right. I could attempt and post another data log tomorrow. I'm pretty positive the wires are right.
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From: Maricopa, AZ
Your left and right "O2 ready" boxes in Scan9495 aren't showing anything. But, the O2 millivotage is showing. Did you try disconnecting the O2's and reseting the BLM's yet (in Scan 9495, at the bottom of the "Engine" tab page, there's a "Reset Fuel Trim (BLM)" button) to see if the condition goes away?
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On The Tree
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Your left and right "O2 ready" boxes in Scan9495 aren't showing anything. But, the O2 millivotage is showing. Did you try disconnecting the O2's and reseting the BLM's yet (in Scan 9495, at the bottom of the "Engine" tab page, there's a "Reset Fuel Trim (BLM)" button) to see if the condition goes away?
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It forces Open loop. If the condition goes away, there's something wrong with the O2's. If not, it's something else. But when you disconnect them, you have to reset the BLM's too.
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From: Kernersville, NC
I think the problem lies somewhere with the work you did on it during the tune up.
Are you 100% certain that the opti is indexed correctly? If I'm not mistaken, you have a spline drive and those are easy to get wrong.
Do you still have the old components you took off the car when putting the new stuff on it?
Are you 100% certain that the opti is indexed correctly? If I'm not mistaken, you have a spline drive and those are easy to get wrong.
Do you still have the old components you took off the car when putting the new stuff on it?
Last edited by ACE1252; Apr 9, 2016 at 01:16 PM.
