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Old 06-19-2017, 07:57 AM   #1
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Can't keep 95 Z28 LT1 running

I posted a request for help on this a few days ago, but apparently it didn't make the trip. Hopefully, I can describe this problem intelligently enough that someone may recognize what is going on.

My granddaughter bought this car for her boyfriend and now she has no boyfriend and took the car back. Drama. We bought the car from her. It ran like a scalded dog. A few weeks ago, I dropped the shifter (automatic) down into 2nd and accelerated. All of a sudden, the SES light came on, the car started stumbling and I was barely able to limp home less than a 1/2 mile away. To keep the car running, I have to repeatedly and frantically work the accelerator pedal.

Of course, it is the OBD1 system with the 16-pin OBD2 connector. I found on this site the link to the scan9495 software. I bought the 16 pin to USB ALDL cable, installed the software, and was finally able to scan the PCM. No DTC codes! How is it possible to throw a SES light, and run so rough that it is very difficult to keep running, and not have a DTC code? Revving at 3 or 4 grand, it doesn't even seem like it's missing, but it's hard to keep it there, and I don't want to. It definitely will not idle.

Any ideas, anyone?
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Old 06-19-2017, 02:16 PM   #2
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Re: Can't keep 95 Z28 LT1 running

No idea why the system is flagging your posts for "moderator approval". I'll see it the owner of the site can help me out.

Let me get the author of Scan9495 to answer your question on the codes not showing up. Any chance you pulled the power to the PCM at any point before scanning?
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Old 06-19-2017, 02:53 PM   #3
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Re: Can't keep 95 Z28 LT1 running

I will post here what I already told him:

"The only way you would get no trouble codes even though they exist, is if the “mafl.csv” file is not in the same folder as the “scan9495.exe” file and they are both removed from the original zip file. If the files were not like that, you would get an error message, but could continue by clicking on “Continue” in the error message.
"

Either there are no codes or he has a pc problem. Perhaps he has not extracted the files from the zip file or his pc is unable to access the malf.csv file for some reason. Perhaps there is some unreasonable security level on the folder where the files exist. If that were so, he should get a pc error but he has not stated that such is the case.
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Old 06-19-2017, 02:58 PM   #4
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Re: Can't keep 95 Z28 LT1 running

Thanks Gary. Just found out today that his posts have been being dumped into the "moderator approval" file for some reason, and I'm not sure why, I don't find out a post needs approval unless I notice a small figure of a person in the lower right corner of the post.
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Old 06-20-2017, 09:55 PM   #5
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Re: Can't keep 95 Z28 LT1 running

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I will post here what I already told him:

"The only way you would get no trouble codes even though they exist, is if the “mafl.csv” file is not in the same folder as the “scan9495.exe” file and they are both removed from the original zip file. If the files were not like that, you would get an error message, but could continue by clicking on “Continue” in the error message.
"

Either there are no codes or he has a pc problem. Perhaps he has not extracted the files from the zip file or his pc is unable to access the malf.csv file for some reason. Perhaps there is some unreasonable security level on the folder where the files exist. If that were so, he should get a pc error but he has not stated that such is the case.

Gary, I sent you a screenshot of the files in the folder as well as the content of the mail.csv file. No problem with the PC. The data is also changing on the 9495scan screen when the engine is started. Still no DTF codes, and no change in the way the engine runs. No PC errors. I and going to try and get the PCM to throw a code tomorrow, but as many times as I've tried it already, I am pretty sure it would have already thrown a code if it was going to.

Thanks.
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Old 06-20-2017, 10:24 PM   #6
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Re: Can't keep 95 Z28 LT1 running

I don't believe that anything is wrong with the PC or the Scan9495 software. I am just trying to get some expert opinion on why I wouldn't be getting a DTC code, or even more importantly, what my next steps should be to get the car running properly. I have uploaded the only log file that I recorded. Perhaps someone can tell by running it, what might look abnormal and a possible cause. It will look stagnant for a few seconds, but then you should see changing values as I tried to keep the engine running.

Thanks!
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File Type: csv DLOG-Save-2017Jun16-161718-2G1FP22P8S2179958.csv (53.7 KB, 9 views)
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Old 06-21-2017, 07:14 AM   #7
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Re: Can't keep 95 Z28 LT1 running

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No idea why the system is flagging your posts for "moderator approval". I'll see it the owner of the site can help me out.

Let me get the author of Scan9495 to answer your question on the codes not showing up. Any chance you pulled the power to the PCM at any point before scanning?
By "pulling the power to the PCM at any point before scanning", the car battery had gone completely dead from having left the parking lights on, and then was completely charged before attempting the scan. Would that have made any difference?
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Old 06-21-2017, 08:04 AM   #8
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Re: Can't keep 95 Z28 LT1 running

"Completely dead" could clear the codes and the long term (stored) fuel corrections. And many of the codes will not set unless the engine/vehicle is operated under certain parameters, that can only occur with the engine running and/or the vehicle being driven.

I'll take a look at the data log.
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Old 06-21-2017, 11:46 AM   #9
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Re: Can't keep 95 Z28 LT1 running

Looking briefly at the file, there is a problem with the labels in columns B thru F. Do not match an earlier version of the file setup. I've asked Gary to look at that.

Nothing stands out, given the throttle being worked so aggressively. A couple of thoughts...

Does the car still have a catalytic converter? If so, is it possible it is badly clogged? I looked for signs in the MAP sensor readings, but they are jumping around so much. A problem with the cat would not set a code in OBD-1 or turn on the SES light.

Have you looked at the plug wires in the dark while trying to keep it running, looking for arcing on the wires, coil, Opti? Again, no codes in OBD-1 for problems on the high voltage side of the ignition. Also a possibility the rotor in the Opti came loose, no codes for that.

Appears the PCM did reset when power was lost, long term fuel corrections are back at 128. (only 2 cells showing, 16 & 18, but 18 represents a weighted average of all the cells).

I'll keep looking, but not holding out a lot of hope.
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Old 06-21-2017, 01:46 PM   #10
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Re: Can't keep 95 Z28 LT1 running

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Originally Posted by Brclarke View Post
I don't believe that anything is wrong with the PC or the Scan9495 software. I am just trying to get some expert opinion on why I wouldn't be getting a DTC code, or even more importantly, what my next steps should be to get the car running properly. I have uploaded the only log file that I recorded. Perhaps someone can tell by running it, what might look abnormal and a possible cause. It will look stagnant for a few seconds, but then you should see changing values as I tried to keep the engine running.

Thanks!
I did run a scan again this morning, but called away to hospital visit. A number of codes popped up during the scan, but no DTC codes got stored in DTC tab, so may be doing something wrong. First DTC 96, then 71, then 29 then 25 codes popped up all at once. Have to see what they are when I get home. Know any reason why codes momentarily appear during scan, but aren't recorded on DTC tab?
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Old 06-21-2017, 02:55 PM   #11
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Re: Can't keep 95 Z28 LT1 running

DTC 96 does not even seem to be applcable to the '95 OBD1 codes. Ughh!
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Old 06-21-2017, 05:52 PM   #12
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Re: Can't keep 95 Z28 LT1 running

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Originally Posted by Brclarke View Post
I did run a scan again this morning, but called away to hospital visit. A number of codes popped up during the scan, but no DTC codes got stored in DTC tab, so may be doing something wrong. First DTC 96, then 71, then 29 then 25 codes popped up all at once. Have to see what they are when I get home. Know any reason why codes momentarily appear during scan, but aren't recorded on DTC tab?
Attached is a log file of my last scan, as well as the screenshot of what was loaded in the folder. I have to play back the log file very slow because the DTC codes appear and disappear so fast and are not captured in the DTC tab. I am not sure if that is how it is supposed to happen or not. Maybe someone has experienced this or can tell me whether I am wasting my time with these transient codes or not.

Also, since these are fuel injected, it seems strange that I have to work the accelerator very rapidly to keep it running - as if it had an accelerator pump. I cannot just floor it and keep it running - I actually have to pump the pedal.

I apologize if the posts seem annoying, but there are a lot of members on here, and I am hoping someone has experienced similar symptoms.

Thanks!
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Old 06-22-2017, 08:29 AM   #13
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Re: Can't keep 95 Z28 LT1 running

Can anyone tell me how the PCM stores DTC codes? Will the PCM store a transient fault or code, or does the condition have to be constant for a given time for it to be stored in a register? Do the codes correct themselves over time if the condition improves? Just trying to get a handle on whether to disregard the momentary faults that show up in the log file, but don't get captured as a DTC code in the scan9495 software.

I am leaning toward a clogged cat, although I have only seen this happen once in my lifetime as a mechanic, and that was before ECM's and PCM's. This condition happened abruptly, and has not improved. Anybody have any idea how to narrow it down to a cat without dropping the exhaust - perhaps by analyzing the data in the log file? Is anybody out there?
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Old 06-22-2017, 01:55 PM   #14
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Re: Can't keep 95 Z28 LT1 running

Now I don't believe it is a clogged cat. I think that I have gone as far as I'm going to go on this. I don't particularly like the car. Very little is accessible inside the engine compartment. There doesn't seem to be as much support on here as I thought there was going to be. I think I'm just going to call the garage and have them pick it up. If they fix it, we will sell it. My mechanic experience took me through the mid 70's, and it doesn't seem to be helping me much here. I just retired as a manufacturing engineer with lots of experience with data acquisition with gas and diesel experience, but working on this car from home is apparently going to be more than I bargained for.
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Old 06-22-2017, 09:54 PM   #15
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Re: Can't keep 95 Z28 LT1 running

Some codes will disappear from memory after a certain number of sequences when there is no fault detected. It varies by trouble code. I don't have any experience with the program you are using, but if there is a chance there is something wrong with the setup of it, you might just use a hand held scanner for comparison purposes.

You can't always depend on the internet to give you satisfaction. People give their free time and expertise as they see fit and things in this forum are not as active as they once were. This is only one of many forums out there. If you are truly not liking the car, that is probably the best indicator that you will be getting rid of it and that might be the best thing for you.
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