Exhaust wreaks during DFCO

[AlpineCJ5]

Hi all,

Ive got a catless, true dual AMC 304 V8 with a fuel only 1127747 TBI setup inside a cj5. It is working very well, but now that I have the top off for summer, I can smell the exhaust some. It smells fine in most cases, but during DFCO, the exhaust wreaks! Like eye wateringly bad. I know it's going lean, because I can feel DFCO kick in and slow the rig down, and my laptop shows the O2 reading very low voltage.

So why is this happening? Is this a common thing, or is it because I'm running fuel only?

thanks,

[Skinny Pedal]

How long is the exhaust? Whats the timing like in high vacuum on decel?

[AlpineCJ5]

It being a cj5, the exhaust is pretty short. As far as timing, I'm running a DUI unit, so max mechanical is 36* or so and max vac is 7 or so, so the most I'd see is 43*

However, it stinks from high rpm all the way to low rpm just before it turns the mode off, so I guess take a well tuned AMC V8 advance curve and add 7 or so degrees at whatever point and that's my advance.

[Skinny Pedal]

Is it safe to assume you are running true dual without a cross over? Headers? The exhaust run down each side and dump out in front of the tires?

[AlpineCJ5]

Headers, no crossover of any kind, exits behind rear tires out to each respective side of the tub

[Skinny Pedal]

K, can you post up the tune? With exhaust like that you are pulling fresh air back in the exhaust when there isnt enough volume to keep it from doing so. Remember full on super fat or super lean will cause excess emissions, your eye watering condition. So exhaust reversion combined with lean gives you super lean and NOX.

[AlpineCJ5]

I'll try and post my bin tonight.

What you say makes perfect sense, but I though DFCO cut the fuel off completely. If that's true, shouldn't I be making no emissions? Pardon my lack of knowledge on the subject.

[Skinny Pedal]

Its my understanding its only leaned by the factor in the tune, its a multiplier not full cut out unless you tune it that lean.

Correct me if I am wrong guys.

Also, have you tuned that portion of the VE table with DCFO turned off?

[AlpineCJ5]

Yessir, my VE table was built with DFCO turned off. I left the DFCO stuff alone, so it'll be the same as the ASDW bin for 5.7 manual tranny.

[Skinny Pedal]

Bock to the timing stuff..... What is your total mechanical curve? 20 degrees, 15, 10........ I would say try it with DFCO turned off and see if it still does it. If no then trying making it DFCO a little richer. Are you running a speed sensor?

[AlpineCJ5]

My mechanical is 24 overall it so. It doesn't make the smell when decelerating without the DFCO. I'm fine removing it, I like to get the best economy possible, but I don't like being poisoned more.

[JeepsAndGuns]

IDK about the 7747, as I ditched it years ago for a 7427. But on my 7427, when it goes DFCO, it shuts the fuel off completely. My exhaust doesn't really have a smell when in DFCO, except mayby a mild warm engine/oil smell. Basically what you would expect a hot cylinder to smell like.
So the 7747 does not actually cut the fuel off? I thought that's what dfco was, decel fuel cut off, if it is just leaning it out, would that not be decel enleanment?

[Skinny Pedal]

If you look at the settings for DFCO it is a TINY window the rest of the time it would be in enleanment. I should have clarified better in other posts. Because of this tiny window is why I was asking about the speed sensor. DFCO only happens in like 400 rpm and 4 mph range, the rest of the time is in enleanment.

[AlpineCJ5]

The DFCO in the ASDW bin is on a lot more. Pretry much anything above 1200 RPM and mid teens MPH with a high vac and its on. The AMC motors seem to make a bit more vac than the GM ones the system came off of, maybe I need to increase the vac threshold.

[AlpineCJ5]

I turned it off for now. Life is much less stinky. I don't smell anything unusual when coasting now, but it does coast faster :)

[brian617]

I dont think that is correct, at least not on the 7427 PCM. Watching the wide band on my truck shows full lean when in DFCO.

[Skinny Pedal]

Full lean doesn't mean there is no fuel, it just means you are at the limits of your wideband. It would be interesting to know if it fully cuts fuel or if its a multiplier like it describes.

[brian617]

Next time I have the lap top hooked up I'll check the PW and DC% to confirm.

[Skinny Pedal]

Next time I have the lap top hooked up I'll check the PW and DC% to confirm.


Awesome!! Thanks

[JeepsAndGuns]

On my 7427, my injector pulse width goes to 0 when in DFCO. Wideband also goes full lean.
I love it. I have a manual trans and you can really feel it help slow it down. Its like a mini jake brake.

[AlpineCJ5]

That's how mine felt. I already miss it :(