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HannahWCU 07-13-2011 10:49 AM

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?

dcb 07-13-2011 12:17 PM

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?

HannahWCU 07-13-2011 01:15 PM

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.

dcb 07-13-2011 02:11 PM

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).

HannahWCU 07-13-2011 02:56 PM

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.

PaleMelanesian 07-13-2011 03:00 PM

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.

Joenavy85 07-13-2011 06:19 PM


Originally Posted by PaleMelanesian (Post 250107)
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.

Joenavy85 07-13-2011 06:21 PM

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

SentraSE-R 07-13-2011 06:52 PM

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

larrybuck 07-13-2011 08:52 PM

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!

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