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  • December 03, 2021, 05:04:58 PM

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Author Topic: Low Compression. Missing on cold start  (Read 474 times)

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Offline TheBirdman

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Re: Low Compression. Missing on cold start
« Reply #15 on: November 15, 2021, 02:39:03 PM »
I dont know all what the cec system does with timing, because on my eagle I knew right away that I wasnt going to spend the enormous effort to make all the 40 year old emissions garbage work, so I ripped it all out and stuck an HEI and motorcraft in it. No offense to anyone who wants to put in that effort, you do you, but Ive got too many hobbies to make doing the Sisyphus routine one of them.
« Last Edit: November 15, 2021, 02:41:17 PM by TheBirdman »
83 eagle wagon 4.0

Offline Illeagle1984

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Re: Low Compression. Missing on cold start
« Reply #16 on: November 16, 2021, 06:22:08 PM »
On a normal CEC controlled spark, the signal goes from the distributor to the CEC computer, which adjusts the timing as it sees fit before relaying the signal to the ignition control module.  Non-CEC goes from distributor straight to ignition control module.  I'm nearly 100% sure the ignition boxes are the same.

Edit: the CEC looks at a few different things to decide how to change the timing.  It can also play with the air/fuel ratio as it sees fit.  These are mainly based on engine temperature and oxygen sensor input, but I'm sure there's more than just that.
« Last Edit: November 16, 2021, 06:28:40 PM by Illeagle1984 »
It's getting crowded down here:
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Offline MIPS

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Re: Low Compression. Missing on cold start
« Reply #17 on: November 17, 2021, 04:04:24 PM »
Dangit Birdman, you know my autistic feelings about this car. Taking the computer out isn't illegal around here but my OCD would keep me up at night! ;)

It's a little different for later model Eagles but for the 82 model year the computer has no direct control of the spark timing. It requires the use of a three connector "Yellow grommet" Duraspark II module. The extra connector permitting for electronic spark retard. It cannot advance timing beyond the base timing. This is why stock Eagles have such advanced base timing.
For the '82 Eagle when the CTS and the TAC switches say the coolant and air cleaner temperature is cold the computer electronically leaves the timing alone so for cold running it's "more advanced than normal" and the only thing that can change it is the vacuum and mechanical advance. In that regard this means it can still technically manipulate timing indirectly using the Sol-Vac to bump the idle speed but that's it. It has three levels of idle speed to play with and they are not adjustable once set to spec.
Once the two switches satisfy the CeC retards spark timing and adjusts up and down from there as it needs to. This is why an Eagle with disconnected or faulty temperature switches (or you replace the uncommon yellow grommet with a more common two connector blue grommet module and leave the computer disconnected) can knock so badly on hot days. It doesn't retard the spark timing for the hot engine and acts like its over-advanced.

That I have seen in the 84 and later Eagles I have salvaged parts from the computer intercepts the TACH going to the ignition module directly and plays with spark timing from there, presumably because the CeC now runs double duty for both engine management and the knock sensor, so I cannot say for certain how that later system works. Amusingly if you buy a Carter Engine Knock Eliminator it does the exact same thing and intercepts the TACH before reaching the Duraspark.

Among the countless hours I've been under this hood all the related systems have been tested and are known good. Since the problem is only happening with the engine cold we can immediately rule out computer intervention.

Anyways I inspected the cap and rotor and found them to be fouled. That would certainly cause the peculiar cold miss on a cylinder that had good compression.
« Last Edit: November 17, 2021, 05:05:24 PM by MIPS »

Offline TheBirdman

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Re: Low Compression. Missing on cold start
« Reply #18 on: November 17, 2021, 05:30:06 PM »
I am merely here to tempt you away from the way of the true and righteous.
83 eagle wagon 4.0

Offline Canoe

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Re: Low Compression. Missing on cold start
« Reply #19 on: November 19, 2021, 07:51:58 PM »
... for the 82 model year the computer has no direct control of the spark timing. It requires the use of a three connector "Yellow grommet" Duraspark II module. The extra connector permitting for electronic spark retard. It cannot advance timing beyond the base timing. This is why stock Eagles have such advanced base timing. ...
I'd like to be sure I'm understanding this.
Does the "extra connector permitting for electronic spark retard" provide an adjustment to the timing that the Duraspark module responds to, or is that extra connector delivering the actual adjusted timing & dwell signal that the Duraspark module drives the coil with?

Offline MIPS

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Re: Low Compression. Missing on cold start
« Reply #20 on: November 27, 2021, 11:54:33 AM »
The former, I believe. I'd have to attach an oscilloscope to verify. The 82 model computer has TACH as input-only so my guess (all speculation after this point) is using the known engine RPM from that and all the sensor data the computer is able to pulse the spark retard line extremely fast either immediately after the TACH arrives at the module to fire the coil or before the next pulse arrives.
« Last Edit: November 27, 2021, 12:03:57 PM by MIPS »

 

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