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Strange timing issue

cali navy

Member
Joined
Dec 7, 2007
Messages
15
Location
California
Corvette
91 non-ZR-1
Background on the car:
ZZ4
Edelbrock heads
TPIS Miniram
TPIS stage 3 tune


I'm not 100% sure if this is even a timing issue, but it sounds like one. Under acceleration, it bumbles, bucks and misses. It isn't pinging. If I lightly hold down the accelerator, it happens for a couple of seconds, but then it seems like the computer figures it out and it jumps back to running perfectly. If I press down further, it starts to backfire. It sounds like the timing is off, but the computer finally figures it out after a bit. It happens thru all RPM ranges and all gears. It also doesn't have anything to do with rain, because I just drove it this morning and San Diego is quite rainy right now.

It started quite suddenly. I decided to do a basic maintenance check on it.
Spark plugs (they were the correct color, but were a bit old and burned)
Vacuum lines (All were good except the brake booster line. Replaced everything anyways and added clamps. All devices tested solid)
Plug wires (tested and inspected good)
Distro (cap and rotor were very VERY bad. I replaced them)
Timing (EST disconnected, 6 degrees BTDC on the dot.)
After I connected EST and reset the computer, I pointed the light at the line again. It stayed at 6 degrees, so good. However, when I revved the engine, it didn't move more than 12 degrees. It seems like the ECM isn't advancing timing. WTF?

So, what would cause this? Some timing advance, but not nearly enough? Could it be the computer? Sensors? Has anyone had this happen before?

When it stops raining, I'm going to reseat the chip and reset the computer. Then, I'll head up to my mechanic and have him plug in his computer. I'll see if it works.
 

Hib Halverson

Technical Writer for Internet & Print Media
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71 04 12 19
Diagnosis via Internet can be difficult but I'm going to take a shot at it. I'll guess that the engine controls' EST is not working and the engine is running on the base or "backup" spark curve in the ignition module inside the distributor.

Is the check engine light on?

If yes, what code is set.
If no, does the check engine light come on during bulb check after you key-up?
 

cali navy

Member
Joined
Dec 7, 2007
Messages
15
Location
California
Corvette
91 non-ZR-1
No check engine light when running and yes, to does come on when starting. That is what is really killing me. I'm guessing that the control voltage that pushes the advance is not getting to the distro. Where is that signal and what should it look like. I can't find this info anywhere, and I mean ANYWHERE. Is the control voltage on the EST line with the connector, or is that just an activation voltage? Should it vary?
 

Hib Halverson

Technical Writer for Internet & Print Media
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71 04 12 19
Do you have a factory service manual for your model year?
 

Antz81

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Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
My guess would be that either the ignition module in the distributor is faulty, or it's not receiving the bypass signal from the ecm.
 

ned pepper

Member
Joined
Sep 17, 2012
Messages
15
Location
Indiana
Corvette
91
you won't get the same smooth timing advance curve with the ecm as you would with a centrifugal advance. ecm uses rpm, load, and engine temp -rpm, MAP, CTS. 6e3-c4-1.
not sure that revving the engine as you did will result in the max advance. may be.
 

xfirez51

Well-known member
Joined
Jun 2, 2007
Messages
280
Location
Chicagoland
Corvette
1992 Black Rose ZR-1
My guess would be that either the ignition module in the distributor is faulty, or it's not receiving the bypass signal from the ecm.

The timing should advance once the EST is reconnected. ECM should take over timing at that point.
i would like at ignition module first.
 

cali navy

Member
Joined
Dec 7, 2007
Messages
15
Location
California
Corvette
91 non-ZR-1
Well, just to ensure proper firing and fuel, I threw a tester in line with all of the plugs and they all look the same. Maybe a bit weak, but spark is definitely there. I also replaced the fuel injectors.

So, here is the strange thing. Every time I do something to it (especially when I unplug the battery) The car runs great... for about 2 minutes. Then the same problem comes back.

I'm thinking it's heat or the ECM "learning" incorrectly. It has a TPIS tune. Those shouldn't go bad, they are just EPROMS. But, I'll pull it and throw the stock chip on.

As far as the ignition module, I thought those are either good or completely gone. I've never heard of one just kinda going out. But, they are cheap and I'll see if I can get one locally. If not. I'll order it tonight.
 

ned pepper

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Joined
Sep 17, 2012
Messages
15
Location
Indiana
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91
Well, just to ensure proper firing and fuel, I threw a tester in line with all of the plugs and they all look the same. Maybe a bit weak, but spark is definitely there. I also replaced the fuel injectors.

So, here is the strange thing. Every time I do something to it (especially when I unplug the battery) The car runs great... for about 2 minutes. Then the same problem comes back.

I'm thinking it's heat or the ECM "learning" incorrectly. It has a TPIS tune. Those shouldn't go bad, they are just EPROMS. But, I'll pull it and throw the stock chip on.

As far as the ignition module, I thought those are either good or completely gone. I've never heard of one just kinda going out. But, they are cheap and I'll see if I can get one locally. If not. I'll order it tonight.

did you test fire/spark with a comparable GM tester? if it was a bit weak, a low output coil can be part of this problem, per 6e3-6 section, "Symptoms." esp backfire. that tester is a severe test for spark, and the spark would be blue
if I set my timing at 6, est out, and rev engine slightly, the timing advances to 10-12, from ICM. then, est connected, increase rpm gives ABOUT the same, maybe flashing higher and back. have not been in there for a while but that's how I recall it. engine runs good as new. I think it must be normal due to ecm calculating inputs from MAP, CTS, RPM, maybe IAT at static load. hell I don't know. but the ICM goes bad and that's all she wrote from my experience -not selectivity changing advance.

does sound like ignition problem though. a method used by the FSM is to disconnect sensors to see how the problem reacts, such as maf/map, IAT. it's just looking for clues.
 

cali navy

Member
Joined
Dec 7, 2007
Messages
15
Location
California
Corvette
91 non-ZR-1
Took out the TPIS chip and ran the car. Still bad. In fact, even worse.

Then... I did the simple check. What I should have done from the beginning. Test the Throttle Position sensor. Open. Completely open. So, the computer thinks that the throttle is always at idle. So, when I hit the gas, it runs WAY lean. It has always seemed like fuel starvation, which is the reason I replaced the fuel filter and injectors. That's why it runs fine when I baby it. It's running a bit rich to begin with, so it has a slightly wider idle band. I'm currently trying to locally source a TPS, so when I throw it in, I'll re-post the results.
 

ned pepper

Member
Joined
Sep 17, 2012
Messages
15
Location
Indiana
Corvette
91
Took out the TPIS chip and ran the car. Still bad. In fact, even worse.

Then... I did the simple check. What I should have done from the beginning. Test the Throttle Position sensor. Open. Completely open. So, the computer thinks that the throttle is always at idle. So, when I hit the gas, it runs WAY lean. It has always seemed like fuel starvation, which is the reason I replaced the fuel filter and injectors. That's why it runs fine when I baby it. It's running a bit rich to begin with, so it has a slightly wider idle band. I'm currently trying to locally source a TPS, so when I throw it in, I'll re-post the results.

good analysis! the 91 is supposed to have the non adjustable TPS, so it may be defective.
 

cali navy

Member
Joined
Dec 7, 2007
Messages
15
Location
California
Corvette
91 non-ZR-1
good analysis! the 91 is supposed to have the non adjustable TPS, so it may be defective.

Well, problem. I was looking at it in standard ohms, not Kohms. It shows a smooth change thru the sweep. It seems to be about 1.5 Kohms below what it should be thru the whole sweep, but since the ECM calibrates it right off the bat, it shouldn't matter. As long as it sees the smooth change. I'll still be putting in a new TPS tomorrow, but I have my doubts.

I hate throwing parts at the problem, but at this point I don't know what to look for. I'll take it back to the mech and plug it up on Monday again. I need to make sure all of the sensors are reporting. I'll also do timing tomorrow just to make sure it's at 6 btdc. Can't hurt to make sure...again.
 

ned pepper

Member
Joined
Sep 17, 2012
Messages
15
Location
Indiana
Corvette
91
Well, problem. I was looking at it in standard ohms, not Kohms. It shows a smooth change thru the sweep. It seems to be about 1.5 Kohms below what it should be thru the whole sweep, but since the ECM calibrates it right off the bat, it shouldn't matter. As long as it sees the smooth change. I'll still be putting in a new TPS tomorrow, but I have my doubts.

I hate throwing parts at the problem, but at this point I don't know what to look for. I'll take it back to the mech and plug it up on Monday again. I need to make sure all of the sensors are reporting. I'll also do timing tomorrow just to make sure it's at 6 btdc. Can't hurt to make sure...again.

been there, been there! anyways, good catch on the k ohms. I can quit wondering how it worked most of the time, since it is not adjustable, can't get loose easily. also should not cause backfire.
still meditating on ignition, coil, coil grnd, whatever. open EGR is an option, if you have it. glance at the symptoms pages in the book again.
ned
 

cali navy

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Joined
Dec 7, 2007
Messages
15
Location
California
Corvette
91 non-ZR-1
You would test VDC if you were testing it while running. However, you can do a bench test with it disconnected and removed from the car. You test A-B or B-C and test it the same way as you test volts, but checking OHMs instead.

TPS Diagnostics

I don't have a factory service manual anymore. It kinda got soaked during my last move and ended up just being a big brick of paper.

If the compression test ends up being good, I'm going to do timing and plug the computer back in. I really need to make sure all of the sensors are reporting. The problem, at that point, will be an "end-user" from the ECM. If it is an input that is bad, I should be throwing a code. I don't know. I need a drink and some time to think. It's been over a week and I don't know where I'm going with this anymore.

Oh, and no EGR and it is removed on the tune. Shouldn't the ignition coil just go out, not just kinda go bad?
 
Last edited:

ned pepper

Member
Joined
Sep 17, 2012
Messages
15
Location
Indiana
Corvette
91
You would test VDC if you were testing it while running. However, you can do a bench test with it disconnected and removed from the car. You test A-B or B-C and test it the same way as you test volts, but checking OHMs instead.

TPS Diagnostics

I don't have a factory service manual anymore. It kinda got soaked during my last move and ended up just being a big brick of paper.

If the compression test ends up being good, I'm going to do timing and plug the computer back in. I really need to make sure all of the sensors are reporting. The problem, at that point, will be an "end-user" from the ECM. If it is an input that is bad, I should be throwing a code. I don't know. I need a drink and some time to think. It's been over a week and I don't know where I'm going with this anymore.

Oh, and no EGR and it is removed on the tune. Shouldn't the ignition coil just go out, not just kinda go bad?

I read the page a while ago, refers to "weak coil," also "proper ignition coil output voltage." I think there are various reasons for weak coils, either pickup or secondary. will have to read it. I USED to know it. long time since I been into that. added: any spark has to be blue/white, yellow is too weak. you probably know that.
 
Last edited:

KANE

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Dark Blue 1982 Trans Am(s): Polo Green 1995 MN6
The problem, at that point, will be an "end-user" from the ECM. If it is an input that is bad, I should be throwing a code. I don't know. I need a drink and some time to think. It's been over a week and I don't know where I'm going with this anymore.

As I read through this thread... this is where some type of datalogging would be really helpful.

I had a TPS go out on me and I found it really fast within the datalog. I really wish I had started using software like that YEARS ago. For the longest time, I had a GM troubleshooting manual and a voltmeter. Hib and I had even exchanged emails about what eventually turned out to be a knock self-test fail because the ECM wasn't able to hear knock (needed to add two more degrees before it did).

Back to your problems... I have had coils go bad and ignition modules go out too. For me, the performance simply dropped and they started to run at a degraded capacity. Usually all of the sudden too. And that's how I identified them- everything else tested out and through the process of elimination, they were what was left. Sometimes having a good, known spare can help for parts that are either hard to test or when they go out they do weird things that don't trip codes.


If you need something that would serve as a decent factory service manual for just the EFI... Chevy TPI Swappers Guide by John Baechtel is pretty good. You can find that book used for around $17 from Amazon or new at a Barnes & Noble. There is a whole section in the book that has GM troubleshooting procedures for each code & condition- in addition to a smorgasbord of GM part number for injectors, rails, intakes, and everything else. When I was retrofitting TPI systems on my C3s I found it incredibly useful for GM troubleshooting alone since I usually used cams/heads that didn't come with TPI systems. I know that doesn't help you now- but may be useful long term for you since you don't have the GM manuals anymore. It's also thin enough that you could stash it behind the seat if you needed to keep it someplace where it wouldn't get lost!
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
Well, problem. I was looking at it in standard ohms, not Kohms. It shows a smooth change thru the sweep. It seems to be about 1.5 Kohms below what it should be thru the whole sweep, but since the ECM calibrates it right off the bat, it shouldn't matter. As long as it sees the smooth change. I'll still be putting in a new TPS tomorrow, but I have my doubts.

I hate throwing parts at the problem, but at this point I don't know what to look for. I'll take it back to the mech and plug it up on Monday again. I need to make sure all of the sensors are reporting. I'll also do timing tomorrow just to make sure it's at 6 btdc. Can't hurt to make sure...again.

You need a scan tester do you can look at the spark timing in the data list and see what the ECM is commanding as far as spark.
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
As I read through this thread... this is where some type of datalogging would be really helpful.

I had a TPS go out on me and I found it really fast within the datalog. I really wish I had started using software like that YEARS ago. For the longest time, I had a GM troubleshooting manual and a voltmeter. Hib and I had even exchanged emails about what eventually turned out to be a knock self-test fail because the ECM wasn't able to hear knock (needed to add two more degrees before it did).

Back to your problems... I have had coils go bad and ignition modules go out too. For me, the performance simply dropped and they started to run at a degraded capacity. Usually all of the sudden too. And that's how I identified them- everything else tested out and through the process of elimination, they were what was left. Sometimes having a good, known spare can help for parts that are either hard to test or when they go out they do weird things that don't trip codes.


If you need something that would serve as a decent factory service manual for just the EFI... Chevy TPI Swappers Guide by John Baechtel is pretty good. You can find that book used for around $17 from Amazon or new at a Barnes & Noble. There is a whole section in the book that has GM troubleshooting procedures for each code & condition- in addition to a smorgasbord of GM part number for injectors, rails, intakes, and everything else. When I was retrofitting TPI systems on my C3s I found it incredibly useful for GM troubleshooting alone since I usually used cams/heads that didn't come with TPI systems. I know that doesn't help you now- but may be useful long term for you since you don't have the GM manuals anymore. It's also thin enough that you could stash it behind the seat if you needed to keep it someplace where it wouldn't get lost!

Baechtel's book, is one source, but what you really need is the Factory Service Manual for your model year. It has 10 times the diagnostic info at that book. You also need to read a couple of articles about engine controls diagnostics which are right here on the CAC. Click here and Click here
 

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