Advance issue
Thread Starter
Team Owner
Joined: Jul 1999
Posts: 65,492
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From: Orange Park Florida
Advance issue
OK, this question is based on my RECOLLECTION of what I have seen over the years....
'87 vette, stock....fixed timing....steady at a well advanced stage...off dial scale on engine....IE around 35* or so...but idled with steady reading...
with chip from NJ.....for sure the idle advance was allways steady....
now with same company doing chip in my '72 here, the idle on my 730 computer was allways steady.....I had an unrelated electrical accident....
and the computer and chip died....long story....
at any rate another 7730 computer and another chip from Ernie here in Florida....and the timing jumps all over the place at idel...one HELL of a jitter....folks say that jitter is NORMAL....but I don't recall ever seeing it before.....so that would make two N.J. chips without jitter, one stock '87 chip that I don't THINK jittered.....and now one from Ernie currently in use that does jitter, which is the normal state....???? any why wold it be jittering unless the knock sensor was taking off, which it isn't..??? I put a scope on the line and it was silent as a lamb....hammered the block and watched it take off.....so I know that's not it.....
this is all at idle....
GENE
'87 vette, stock....fixed timing....steady at a well advanced stage...off dial scale on engine....IE around 35* or so...but idled with steady reading...
with chip from NJ.....for sure the idle advance was allways steady....
now with same company doing chip in my '72 here, the idle on my 730 computer was allways steady.....I had an unrelated electrical accident....
and the computer and chip died....long story....
at any rate another 7730 computer and another chip from Ernie here in Florida....and the timing jumps all over the place at idel...one HELL of a jitter....folks say that jitter is NORMAL....but I don't recall ever seeing it before.....so that would make two N.J. chips without jitter, one stock '87 chip that I don't THINK jittered.....and now one from Ernie currently in use that does jitter, which is the normal state....???? any why wold it be jittering unless the knock sensor was taking off, which it isn't..??? I put a scope on the line and it was silent as a lamb....hammered the block and watched it take off.....so I know that's not it.....
this is all at idle....
GENE
Melting Slicks
Joined: Apr 2001
Posts: 2,094
Likes: 217
From: Ca.
Re: Advance issue (mrvette)
When you say the timing "jitters" do you mean while looking at it on a scan program? Or with a timing light? If it's jumping around at idle under a timing light it could be a stretched timing chain and/or worn timing/cam gear.
Thread Starter
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
Re: Advance issue (drive it)
NO it's not a timing chain or some such, the engine is fresh, double roller chain...and the 'jitter' is all over the place, but the idle is smooth....
the jitter is seen with a timing light....in all cases as above....or not seen as in previous occations, I"ve had this timing light for decades, so I know that's not it.....works fine on other cars....
GENE
the jitter is seen with a timing light....in all cases as above....or not seen as in previous occations, I"ve had this timing light for decades, so I know that's not it.....works fine on other cars....
GENE
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From: Fort Worth Texas
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Re: Advance issue (mrvette)
If there is a big difference in timing between two cells on the timing table, and the engine is running right at that area, it could be jumping between the two settings.
For example, this timing table (Which I've changed up for the sake of argument, so don't anybody use this....!!!) has timing set at 600 rpm to 14 degrees, but at 800 rpm it's 18 degrees at 20 or 30 kpa. If the RPM's are around 750-800 it could be switching between the two cells, setting timing at 14 and 18, back and forth.
But then, if the EST wire is unplugged, the ECM shouldn't be using this table, and my theory is blown out of the water! If the ECM is not supposed to be controlling the timing, and it's jumping around, it seems that it must be mechanical in nature. A little play in the distributor or something? Don't know.
For example, this timing table (Which I've changed up for the sake of argument, so don't anybody use this....!!!) has timing set at 600 rpm to 14 degrees, but at 800 rpm it's 18 degrees at 20 or 30 kpa. If the RPM's are around 750-800 it could be switching between the two cells, setting timing at 14 and 18, back and forth.
But then, if the EST wire is unplugged, the ECM shouldn't be using this table, and my theory is blown out of the water! If the ECM is not supposed to be controlling the timing, and it's jumping around, it seems that it must be mechanical in nature. A little play in the distributor or something? Don't know.
Thread Starter
Team Owner
Joined: Jul 1999
Posts: 65,492
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From: Orange Park Florida
Re: Advance issue (Marcho Polo)
That's just IT Craig, I pull the tan/black wire plug, and it settles down to a steady state.....now for some reason folks say, and I can't remember the source now....so I use the colloquial 'folks'......the computer is allways talking to the dist.....well so what??? if the knock sensor doesn't apparently take off, what does the computer have to say about the matter...??? I even 'scoped' the MAP sensor outout...smooth....
I"d believe a bad harmonica balencer, but why would it settle down when connector is pulled????
GENE
I"d believe a bad harmonica balencer, but why would it settle down when connector is pulled????
GENE
Thread Starter
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
Re: Advance issue (Marcho Polo)
You know, Craig, when I look at that table and think a bit more, you just might be right,....the ZZ9 cam and Ernie at Breathless...set the idle at about 800 or so....and IF there is a knee action in the table there, that is what the 'puter is doing....but I swear I see a 'ghost of several positiins of the mark....one clear at most advance, another clear at initial...and several dimmer less definitive at other points in between......makes me wonder about that harmonica balancer though....at any rate, this is all by memory now...and been ignored for over 2 years now at this rate....so when engine is back in....I will eventually get to it again...;-)))
next millenium.....
GENE
next millenium.....
GENE
Drifting
Joined: Mar 2001
Posts: 1,636
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From: Lafayette IN
Re: Advance issue (mrvette)
When the engine RPM is below 400RPM, the ECM does NOT control the timing. The EST module will control the timing, with an input reference from the pickup coil. When the engine RPM is OVER 400RPM, the ECM controls spark timing.
The ECM controls timing by sending out a signal to the EST. If spark knock is detected through the ESC module, the ECM will decrease timing acordingly.
So in short, when above 400RPM, your timing is being controlled by the ECM and executed by the EST module. I have spark scatter at low RPMs and this spark scatter takes place when the idle jumps between 675-750RPM. Once I am applying throttle (higher RPM and different MAP), the spark becomes more constant. However, when I disconnect the EST bypass wire for setting base timing, I have no spark scatter at all...I have a rock solid 6* or 12* base idle--wherever I feel like setting it that day :D
Craig's reasoning is solid. I don't think it is gonna melt your engine :eek: -Matt-
The ECM controls timing by sending out a signal to the EST. If spark knock is detected through the ESC module, the ECM will decrease timing acordingly.
So in short, when above 400RPM, your timing is being controlled by the ECM and executed by the EST module. I have spark scatter at low RPMs and this spark scatter takes place when the idle jumps between 675-750RPM. Once I am applying throttle (higher RPM and different MAP), the spark becomes more constant. However, when I disconnect the EST bypass wire for setting base timing, I have no spark scatter at all...I have a rock solid 6* or 12* base idle--wherever I feel like setting it that day :D
Craig's reasoning is solid. I don't think it is gonna melt your engine :eek: -Matt-
Burning Brakes
Joined: May 1999
Posts: 999
Likes: 1
From: Gardner KS
Cruise-In II Veteran
Re: Advance issue (mrvette)
I had a very similar problem to this recently on a '730 tune. I would even get some spark spikes up into the 80 degree range. Was very strange and I couldn't come up with any solution. Finally, I burned another chip for the heck of it with the exact same .bin file and the problem was gone. Quit thinking about it after that in fear that it would drive me nuts (even more so than I already am). -Jeff
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