DFCO questions

[kamesama980]

1. DFCO is a lot harder to acheive in an automatic due to the engine revs being able to slip lower than drive rpms through use of the torque converter.

2. Toyota has been using DFCO in FI cars since at least '85 (in the manual there's a way to test it: basically, warm up the car and disconnect the TPS and leave it at idle, then open the throttle and watch the tach. when it reaches 1800 rpm (And thinks the throttle is closed, air measurement via airflow meter) it'll cut fuel till the rpms drop below 1300) and my POS bargain basement 1987 TBI 4 banger chevy S10 has it too. Since someone pointed it out, I can tell every time it goes into "deceleration lean-mode" (as the shop manual calls it) the S10 goes into DFCO any time it's over 1500 rpm and holds it down to about 1300-1400. it takes about 1-2 seconds to engage.

[Ford Man]

Originally Posted by R.I.D.E.

If it doesn't have an air pump from the factory, it probably has FI and DFCO. at least since 1968-69.

My 76 Z had no air pump, no EGR, and no Catalyst. Federal version. California was different.

regards
gary

I thought catalyst was mandated on everything 75 and up.

[Jay2TheRescue]

Originally Posted by theholycow

It could DFCO downhill in OD...you'll find out.

Ok, I took it on the highway (I-66). Had about a 15 mile run before I had to get off. I tried to DFCO the entire way. Not once did SG report open loop. I figured I had totally hosed the mileage on that tank, and decided to fillup even though I still had 1/2 tank because I spied an Exxon station @ $3.749 which is 20 cents cheaper than the cheapest gas in my town. One strange thing though... SG had overestimated my fuel usage by about a gallon, and on a 10.8 gallon fillup that is a considerable percentage. Now this is what I'm wondering... Was the calibration of my SG off because I was running a high concentration of methanol in my tank when I calibrated it, or was I really DFCO'ing all the way down I-66. Of course as discussed earlier SG has no idea the fuel is cutoff in DFCO, so maybe the fuel was cutoff, but SG was reading the RPM's and assuming I was using more fuel?

Anyway, I'm calling this a rise in mileage even though its technically .3 MPG lower than my last tank as this tank was mostly short trip city driving, and the last tank was mostly light highway driving. I just have to figure out why it went up. Lower methanol concentration in the tank? 20 miles of highway DFCO attempts? Combination of the two? Something else I'm not even considering right now? Everyone is welcome to look at Beast's gaslog and contribute their thoughts.

[Jay2TheRescue]

Well everyone, I did confirm DFCO with my Scangauge! Open/Closed loop does not work, at least not on a 98 GM truck. I did email SG's tech support for help, but I figured something out before I got a response from them. This is what I was thinking... I can't confirm DFCO by fuel rate, open/closed loop, or FI duty cycle with the SG. One thing I could monitor was the O2 sensor output!!! I programmed Bank1, Sensor1's output as a programmable gauge and named it o2. I then setup my bank of 4 gauges... MPG, RPM, TPS, and o2.

I left mom's house after lunch and went on my drive back home. Going down the backroads I had the cruise set on 45. I noticed in driving most often the reading from the O2 sensor would be between 50 and 85. I then got on the Prince William Parkway. I set the cruise at 55, tranny in OD. Going downhill where you would ordinarily gain a bit of speed I noticed something... and I wouldn't have even noticed it except the O2 sensor readings were clueing me into something going on.

Right as I crest the hill and start going down the cruise lets off the throttle. TPS = 0. If TPS = 0 and RPM ≥ 1500 after about 3 seconds I will ever so slightly feel the TC lockup. Right when the TC locks up the O2 reading will jump to 80 - 85 for about 3 seconds, then the O2 will read 0 until either the RPM drops below 1,000 or throttle is applied. When DFCO cancels I will feel a bump from the TC, then the O2 readings will go back to the normal range. At highway speeds of 65 MPH in OD it pretty much DFCO's whenever the cruise lays off the throttle for more than 3 or 4 seconds. This explains why I haven't seen the mileage gains I expected when I take it nice and slow and set the cruise at 55 on the highway. @ 55 in OD its borderline whether or not DFCO will kick in unless you are going down a grade.

This explains why I'm not getting the mileage gains I expected when neutral coasting and driving slower. I ended up disabling DFCO with those other hypermiling techniques. Now that I know what's going on I hope to see a bigger MPG gain on my next tank, and can better judge when to neutral glide, and when to leave it in gear and DFCO...

-Jay

[dkjones96]

I bet you are feeling a bump from the TC unlocking and then locking again when you hit the gas. Those larger vehicles tend to do that to reduce engine braking and increase coasting distance.

My Cressida used to do that but the Tracker doesn't. In fact, I can go into neutral on the freeway and the converter stays locked even when I'm going back into drive.

[R.I.D.E.]

Ford Man, I think Honda CVCC engines did not have a catalyst. I know for a fact that my Z did not. California versions of the 76 Z did have a catalyst.

The Z even had the large filler neck hole that allowed you to use leaded fuel, even though none was available.

regards
gary

[Jay2TheRescue]

Originally Posted by dkjones96

I bet you are feeling a bump from the TC unlocking and then locking again when you hit the gas. Those larger vehicles tend to do that to reduce engine braking and increase coasting distance.

My Cressida used to do that but the Tracker doesn't. In fact, I can go into neutral on the freeway and the converter stays locked even when I'm going back into drive.

But if the TC is unlocked you won't be able to turn the vehicle's engine with the vehicle's momentum. My theory is that most of that movement would just be swallowed up by the TC, and not turn the engine. What it feels like is if you were driving a 5 speed and you crested the hill with the clutch depressed, and in 5th gear. It feels like the clutch was brought halfway up. Not a big jerk, but once I knew what I was looking for it was easy to notice.

-Jay

[theholycow]

Originally Posted by Jay2TheRescue

But if the TC is unlocked you won't be able to turn the vehicle's engine with the vehicle's momentum.

Why not? An unlocked TC isn't like neutral transmitting zero power, it just transmits less than full power. It would just turn the engine less fast than if it was locked.

My theory is that most of that movement would just be swallowed up by the TC, and not turn the engine. What it feels like is if you were driving a 5 speed and you crested the hill with the clutch depressed, and in 5th gear. It feels like the clutch was brought halfway up. Not a big jerk, but once I knew what I was looking for it was easy to notice.

I think your TC isn't toggling, you're just feeling DFCO engage and disengage.

[suspendedhatch]

Originally Posted by theholycow

I doubt any car from this century is completely unable to DFCO (unless of course it's been modified). I also doubt any Honda after maybe 1995 entirely lacks DFCO.

I know for a fact that all computerized Hondas utilize DFCO. The earliest Honda ECU that I have personal knowledge of was a 1988.

[DracoFelis]

FWIW: One observation I made about DFCO, that hasn't been mentioned yet, is to make sure your throttle really can go all the way down to zero (and not just fairly close to zero). And it might not be possible to have true "zero throttle" if/when the some mechanic (inadvertently) adjusted the throttle cable just a little too tight.

I know, as that appears to have actually happened to me. When I got the CRX, it didn't seem to want to go into DFCO, even after I learned it should be possible. However, I also noticed (independently of the DFCO goals) that the throttle was very "sensitive" and I accelerated at (high) foot angles/positions I didn't like. So to try to fix the foot position "problem", and allow/require me to press a little harder on the foot pedal before getting acceleration, I (found and) adjusted the throttle cable tension "screw" (it wasn't really a true "screw", but you adjust it a little like it was one) to give the throttle cable a little extra slack (maybe all of 2-4 millimeters extra slack, but still I made the cable slightly more "loose"). And that small amount of extra cable slack was all the throttle needed to make my foot angle/position much more comfortable on my foot. And as a side-effect of that cable tension adjustment, I also noticed that the CRX starting easily going into DFCO (when my foot was off the pedal, and my RPMs were up sufficiently), whereas previously it rarely went into DFCO even when I took my foot all the way off the pedal.

i.e. I'm speculating that some former mechanic, in an effort to remove all the "slack" from the throttle cable, actually tightened the cable just a bit "too far", so that what should have been "zero throttle" (because my foot was off the pedal) was actually a very tiny amount of throttle instead. And this small amount of throttle always being present, while not overly harmful to normal car driving, appears to have been just enough gas/throttle to turn off (and keep off) any attempts at DFCO, because ECUs generally require "zero throttle" to kick in DFCO and I was apparently never getting fully "zero throttle" (due to the slightly "too tight" throttle cable).

So I guess the moral of this story, is to check the tension on your throttle cable, and consider playing with that cable tension adjustment if it seems appropriate. Because if/when the cable is even slightly "too tight", it can prevent you from fully going to zero throttle, which will also have the side-effect of preventing DFCO.