Coasting in gear question
 

Sorry for the long post.

I do fairly well hypermiling, but I have a question about coasting. My 2004 Neon is a 5-spd and depending on the steepness of the hill I am descending, I will either take the transmission out of gear (if I don't need the engine breaking) or leave it in gear so that the fuel will cut off and increase my mileage.

My concern is that I was recently traveling from Asheville, NC to Hickory on I-40. Anyone who has traveled that road knows how long and steep I-40 gets on Old Fort. It is about a 4-5% grade for about 5 miles. I left my Neon in gear and the fuel cut out (I monitor this on my scangauge). For the first time I had the Water Temp on the scangauge because of the heat outside and I noticed something strange. The water temperature of the engine dropped to 196 after about 1/2 mile and stayed there (didn't go any lower).

Now I would think that if the fuel was shut off for the 5 minutes it takes to coast to the bottom that the engine would continue to cool the whole way to the bottom. I am traveling between 55-70 MPH, so air is flowing over the radiator (and the engine) the whole time.

I also would think that the thermostat is still open at this temperature because during the winter my car runs 190-192 and when coasting (engine on) I have seen as low as 185.

The reason I am wondering about this is when I checked my mileage for the tank coming from Asheville there was a BIG difference between what the scangauge read and what the actual calculated mileage was (~10%). I have to wonder if the engine is actually burning fuel but the scangauge doesn't see it.

Any thoughts?

if the scangauge reported 10% more fuel used than the gas pump, then I would speculate that the extra airflow under dfco without fuel was the cause. If the scangauge reported %10 less than the pump then :confused: All the climbing might have resulted in a richer mixture than normal maybe?

Sorry should have been a little clearer on that. The scangauge reported my fuel economy at ~40.5MPG, but the actual calculation showed 37.4MPG (~7.5% less). Which to me points to that big downhill as maybe the car was actually burning fuel (and that is why the temp stayed up). Although, the gauge was reading 9999MPG the whole way down. I am just trying to figure out why the temp stayed high and the MPG was off for that particular tank. Will be repeating the trip this weekend.

For the record, the fuel fill-up was normal. The tank after was back to 40+MPG. If I had overfilled the tank (resulting in the lower MPG on that fill), then I would have expected a 7-8% higher MPG on the tank after.

Well if it does the same thing on the return trip and you are being careful on the refuel procedure then that would be further evidence of it missing fuel. Since it goes to 999 on the downhill then it seems to recognize the dfco condition, and if it didn't then it would report more fuel than actual, and it is reporting less than actual.

AFAIK it uses airflow to determine fuel flow and assumes stoic, and it is not unusual for cars to go into enrichment mode with the pedal near the floor, so that is my best guess. Maybe next time you are climbing, see if your car goes into open loop and if so try to find a throttle position just under open loop (if that setting is sufferable).

I will look at that. I will put loop in the scangauge before the trip so I can watch it. Any other gauge that might be helpful?

Also, to make sure I have this right, I want to see closed loop rather than open loop? Closed loop uses the O2 sensor and the air flow to calculate fuel delivery and Open loop uses a pre-determined algorithm and is much less precise. Correct?

Since I have a 5-spd, I try to avoid WOT by down shifting before that is needed (I don't have cruise control). Especially climbing that grade, I down-shift into 4th to keep rpms in the power band (~2500RPM), I know this isn't the best for fuel economy, but i figure it is better than being WOT in 5th.

You may need to adjust your Cutoff setting on the Scangauge. Using LOOP to verify when DFCO is happening can help fine-tune it. You want closed loop under normal driving. For DFCO it should show Open Loop. No fuel, so no need to monitor the exhaust mix.

I would bet this is the issue. You should be able to feel when the injectors shut off(I can feel it very well on my girlfriends Focus). It's hard to explain the feeling exactly, but it feels similar to turning off the OD on an Automatic while coasting(borrow a friends car that has an auto, take it to an area that has little traffic and try it out) but not as obvious. If you don't feel that little bump, you may not be in DFCO even though your SGII thinks you are, which will skew your SG's readout.

On the flip side, if you go into DFCO and your SG doesn't read it than your result comparison will be opposite.

Are you sure your Neon has DFCO? My wife's 2006 Elantra doesn't.

I'm not qualified to comment on your car's electronic makeup, but couldn't just the sudden drama of that much of an elevation change being sustained w/o any throttle just temporarily weird out the computer as far as the temperature gauge situation?

I lived in that area for 11 years, and for part of that; went up and down that exact mt. daily on my driving job as a courier.

Depending on the time of day and weather; if you are rolling downhill from a cooler, less humid CLIMATE, and suddenly you are entering the opposite climate, could that be enough to offset what we would consider normal plunging temperature gauge readings?
(especially this time of year)

General question to all: Are modern current new cars all the same; or are there still some built in adjustibility still left for cars sold in specific areas.? With old cars w carbs; there were jetting differences between say a Daytona Beach car, and one in Denver.

As far as uphill mountain climbing goes, if safe; I've always gotten better mpgs., if I'm tracking at the speed of a heavily loaded 18-wheeler! We all go up with our flashers on (only when needed myself...acute mirror driving) and for me, my rpms. are as relaxed as if I'm driving around my neighborhood at home!
On a steep mountain, 30-32 mph. in 4th ('87 CRX) seems to work gravy for me.

I realize that not everyone has the patience to go that slow, or maybe feels unsafe w that.
Different areas have varying degrees of traffic volume, and more or less lanes available.

I love the west, and intentionally travel toward non-touristy areas, and pick as quiet an Interstate (or any other kind of road) as I can.

I've been up and down the grades of I-80 across Wyo. many, many times.

I couldn't help but notice your "handle!" Does that mean you are, or were a student at WCU?

Happy travels!

Yes it does! Met my wife there too. Also, I didn't think of the weather conditions changing that much. I looked on the maps and it looks like that grade goes from 2800' at the peak to 1400' at the base. That is quite a difference. For the record, I calculate the grade to be an average of 5.3%.

I can usually feel when it does, at least I think it does. :o

Good question, I assumed it did since I "think" I can feel them cut off, how would I check for sure?

Also, going home yesterday I checked the LOOP on a couple of the short hills I descend on my daily commute. When I "feel" the car go into DFCO the readout will go to 9999MPG and LOOP goes from "closed" to "open". So I am assuming that the scangauge is detecting it.

Going into open loop means you have DFCO.

I'll be going throught there next weekend on my way out to California. I'll be stopping in Rock Springs for a night then driving through Utah and Nevada the following day, all the way to Sac Town.

Filled up yesterday. This was a tank to Asheville and up (and down) the Old Fort Grade. Again the scangauge was off, it was reporting 40.6 and my calculations was 38.25, an almost 6% difference. Normally I don't see much over 0.5 MPG (+/- 1-2%) difference between the scangauge and actual.

I don't think anything is wrong with the scangauge, I just think the car is actually burning fuel when it is reporting that it isn't. I was just wanting to know if anyone else had ran into a similar issue. Thanks to everyone for the read and the advice.:thumbup:

I see that much variation all the time, due, I think, to differences in tank fill levels. I try to fill at low speed until the automatic shut-off kicks in, and repeat three times. Even then, I can occasionally add an extra 0.5 - 1.0 gallon after the first click.

Yep, I agree with that. But that should show up in the next fill. If I overfilled this tank (resulting in an abnormally low MPG), then the next should show as an abnormally high MPG fill. That did not happen last trip to Asheville (7-6 fill). However, I did do that that on the 6-1 fill, so I averaged it with the 6-12 fills to eliminate these as abnormally high and low fills from my statistics. I also fill at the same gas station every time (with a few exceptions that i have noted in my fuel logs) to help minimize this issue.

http://ecomodder.com/forum/fe-graphs/graph5055.gif

Not saying that isn't the problem, but it is strange that it has happened on those two trips. That being said, since 4-3-2011 I am fairly consistent with my MPG. The one notable exception on 5-15 was due to a lot of city driving (~40%)

My car - likely heavier than yours - needs around 5% to sustain about 60 mph while coasting (engine on).

If you keep it in gear, I'd expect it to add fuel on the lesser grades and engine brake on the steeper ones.

Any chance you can run downhill in neutral without reaching too high a speed ?
Maybe by starting the rollercoaster ride slower.

I ran into this problem with my scangauge. The DFCO calculation on the SG uses throttle position. My readings were funky while I was trying to get DFCO, so I just turned that feature off. Now my SG is spot on.

The reason your coolant water temperature didn't drop below 196 while engine braking down the Old Fort grade was because you were continually putting energy into the engine. Just as using the friction brakes heats up your brake pads, using engine braking heats up the engine via friction and compressing air in the cylinders. All of that external rotational energy entering the engine has to go somewhere and it eventually gets converted into heat.

I start the decent at the top at 55mph in gear, the grade is steep enough that, even with engine braking, I have to apply the brakes to slow the car from 70 to 55 at least 3 times before I get to the bottom. In neutral, I would almost have to drag the brakes the whole way.

I guess that makes sense. I just didn't think the internal friction would generate that much heat.

That could be it. I try to do as much DFCO as possible and the scangauge and the actual are always pretty close. Maybe next time I make the trip I will cut DFCO on the scangauge off and see what the results are.

Thanks for all the help and suggestions! Love the forum!

I finally got the hang of coasting in gear and I think that skill alone has added another MPG which in my case is about 4% improvement! I used to push in the cluch to coast max distance and get around 100 mpg or 75 with ac durring that time. Now I wait till I get 9999 fule shutoff mode durring engine breaking and coast almost as far. I just let off the gas or downshift, but I don't pick a gear so low it will send my rpm's up high because that seems to use more fule than clutch in coasting. I an getting the hang of it and getting within striking distance of my 30 mpg goal (36% over EPA).

:thumbup: