Spark of 3.5 on decel?

American Delco GM ECUs and PCMs, ALDL, OBD 1.5.
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Spark of 3.5 on decel?

Postby Mark_ZZ3 » Mon Jul 23, 2018 2:34 pm

Weird problem. On decel or coast as in when you lift off the gas to come to a stop ... after a few seconds the spark advance drops to exactly 3.5 degrees and stays there. The engine wants to stall but it keeps going most of the time. Is there a decel, or coast setting for spark? I've looked through the tables.

The MAP is in the 20 Kpa range, and RPM is in the 2400-1400 range. The lowest recorded spark for this area is 3.516. Only happens on decel with foot off the gas. The Wideband says super lean ... climbs to 20:1.

It feels like a setting for engine braking ... remove fuel, reduce spark, and coast. While it works ... having a bit more timing would help it stay running when you finally come to a stop.

This is a '165 computer running 12P code on a 4 cylinder.

... I'll continue reading the various tables.

Mark.

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Re: Spark of 3.5 on decel?

Postby Mark_ZZ3 » Mon Jul 23, 2018 2:58 pm

Reading the logs ... when it gets to this state, it also cuts fuel. Inj BPW is zero while the spark is 3.5. Wideband is maxed at 22:1. So we are essentially coasting and using no fuel. When you push the clutch in ... the engine will try to stall ... but the stall saver catches it most of the time.

Now the speed sensor is working and you can see the road speed. So it isn't trying to idle. It's in a coast mode of some sort.

Mark.

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Re: Spark of 3.5 on decel?

Postby Biggvl » Mon Jul 23, 2018 3:11 pm

DFCO???
According to chemistry, alcohol is a solution...

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Re: Spark of 3.5 on decel?

Postby vlad01 » Mon Jul 23, 2018 3:20 pm

ditto ^
I'm the director of VSH (Vlad's Spec Holden), because HSV were doing it ass about.

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Re: Spark of 3.5 on decel?

Postby immortality » Mon Jul 23, 2018 3:28 pm

Agreed but it sounds like DFCO is staying on to long. Check the settings but DFCO should cut out if the RPM drops below 1400rpm (I believe).

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Re: Spark of 3.5 on decel?

Postby Mark_ZZ3 » Tue Jul 24, 2018 3:29 am

Thanks so much. Exactly what I needed to know. Now to adjust it.

I can see where it comes on, and as the RPM's come down to the 1400 range, I can see it ad spark back in, but the engine tries to stall. I'll see if I can increase the RPM to overcome this.

Mark.

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Re: Spark of 3.5 on decel?

Postby immortality » Tue Jul 24, 2018 11:21 am

There are about 5 or 6 settings or DFCO.

Are the IAC steps set correctly at idle?

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