Idle coasting?
 

Is there an advantage to coasting in neutral with the engine idling as opposed to just leaving the car in gear, in situations where you have to use brakes anyway? On one hand, the rpms are higher, but on the other, you are still not pushing the gas pedal down. I do not have a scangauge so I have no way of knowing, really.

Depends if you are meeting DFCO conditions or not.

With most modern fuel injected cars (including yours), if you want to slow anyway, you're better off leaving it in gear. That way, you use DFCO (Deceleration Fuel Cut Off). Under some conditions, when rolling in gear with your foot entirely off the gas pedal, it cuts fuel entirely.

The behavior differs from one car to the next; my 2008 VW readily does it, while my 2002 GMC has to be engine braking for 8 seconds before DFCO kicks in. Most vehicles resume fuel at ~1000 rpm under normal conditions, and ~1250rpm with A/C or some other accessories in use.

Generally, neutral at idle rpm is better. 2 exceptions to this, where DFCO is generally a good choice:
1) you have to slow to a stop.
2) you're on a long downhill where engine braking is required to keep from overrunning the speed limit.

What about older cars? I've got a 93' diesel and I always doubt if it's worth to leave the car in gear when going downhill. I wonder if those 8 seconds will also apply to my car.

With my '83 Cadillac, I have observed (just in passing, haven't really scientifically tested this) that I tend to get better mileage in neutral rather than coasting in gear. On a freeway near my home, there is a several mile stretch of road which goes downhill enough that you have to watch your speed pretty carefully...this stretch is a real moneymaker for the CHP. I have taken it both in neutral, and coasting in gear. My car goes into DFCO after around 8 seconds coasting...I haven't timed it, but that seems about right. Watching the fuel economy display, the average mpg seems to climb faster when in neutral than it does when in gear and DFCO.

Discovered that this car had DFCO last weekend...I was playing with the car's computer while driving down the freeway...was watching the fuel injection pulse width, and saw it go to 0.0 after several seconds of coasting in gear. Hadn't realized DFCO was implemented on such a vintage car...

Diesels are probably different, and probably not many 1993 models (regardless of fuel) are good at DFCO. I'd suggest you use neutral, though best would be to experiment and find out for sure which is better.

your civic DFCOs and the only time you oughta put it in neutral to save gas is when actually stopped.

I don't see why diesels would have to inject more fuel at high rpm/off throttle (DFCO conditions for gas) than at idle. they don't have the same mixture constraints gas engines do.

for years of operation... My 87 chevy s10 4-banger DFCOs after about 2-3 seconds, you can feel it. My old cressida did according to the manual and the only changes made in the engine since '84 was a knock sensor and upped compression.

You mean, it's more likely that DFCO can be done in a diesel engine? I'm just asking because I'm not able to find accurate information about this.

A couple of years ago I had a problem in the fuel tank. When I was going through a fast corner the engine would shut down the moment the clutch was pressed. The mechanic found something that was obstructing the flux of fuel to the engine. One thing I remember is that the engine would keep running if a gear was selected, even if something was obstructing the flux of fuel at that precise moment. Of course, the car would stop a short distance after, unless going a steep downhill. Does this help clearing my question?

Thank you for your answers.

What he meant was that a diesel engine doesn't need to increase fuel rate just because RPM increased -- instead, it can keep using the same fuel rate and run leaner. You had asked if you should leave your diesel in gear or in neutral while descending, and I answered that diesels may not DFCO (I have no idea), and that older cars mostly wouldn't. He was saying that it may still use fuel at the same rate as idle even though your RPM is up.

I just realized that I failed to give even as complete an answer as I could. Even if it doesn't use more fuel to descend in gear than it does to descend in neutral, it does slow your car (or prevent it from accelerating as much). If you use neutral, you gain speed (or fail to lose speed, or lose less speed), which means you won't have to step on the accelerator again as soon.