Joe N.

Some time back in a post about mileage someone stated that we should take advantage of the coasting ability of our trucks. I am new to diesels and I don't understand why a diesel should coast better than a gas engine. My experience with gas engines is that a high compression engine will decelerate faster than a low compression engine. Since a diesel is a high compression engine what is the difference between gas and diesel engines that allows the diesel to coast farther?

I know that automatics don't decelerate like manuals but do the manual diesels still coast better than gas engines with manual transmissions?

Forrest Nearing

the difference between gas and diesel engines is that the diesel engine has no throttle blade and therefore has no manifold vacuum...

on a gasser, when you let off the throttle, the engine is sucking against that closed throttle blade, and it slows it down...

on a diesel, when you let off the throttle, it simply reduces the amount of fuel that's injected... the engine takes a full gulp of air and there is no vacuum to work against... it has to compress that air, but the compressed air pushes right back down on the piston, so a diesel will coast much farther.

Forrest

Mopar1973man

When you relase the throttle completely back to idle while rolling the IP pump is no longer injecting fuel to any cylinders.

This why EGT will drop quicklly when coasting. If you want the same effect on a gasser run it up to speed and cut the ignition and hold the throttle wide open. You got a diesel engine...

That why they made Jakes for diesels...

Commatoze

Wouldn't the IP pump have to inject the amount of fuel equivalent to idle when rolling?

RustyJC

Nope. The APPS is telling the ECM that the "throttle position" is 0%, but RPM is above the governed idle speed, so no fuel is necessary. When RPM drops down to whatever threshold level is programmed into the ECM (something approaching idle RPM), fueling should resume.

The above assumes a manual transmission. An automatic with an unlocked torque converter could be a different kettle of fish.

Rusty

Mopar1973man

Thanx Rusty...

I'm not sure either on the Auto Trannies either... But there was some discussion about the fact that VP44 were possible taking a beating because of the lack of pumping on a long downhill and/or deceleration. (0% throttle)

This is one of the few reasons to grab a bottle of fuel treatment. Power service or what ever you like. Since the VP44 pumps are lubed by the fuel!

That reminds me... I need to buy another one myself...

Joe N.

Thanks for the answers. I knew there would be lots of folks on this site ready to help educate a rookie.

neveragain

Sorry gent but for what I know is the when decelerating have you noticed that it only takes a slight throttle movment to get the engine to accelerate again. This lets me believe that the fueling , ( fuel that is being injected) is slightly less then what the engine is running at. For the long hills around here this is what I have noticed on most highway trucks?and this 99 dog crap.

RustyJC

When the TPS (APPS) senses any movement off the idle position, the ECM resumes fueling.

Rusty

infidel

The pump is still being lubed by return flow even if none is going to the injectors.

jrs_dodge_diesel

I concur, even if the VP44 is not puttin fuel to the injectors, your lift pump is always pumping regardless of what the VP44 is doing. So as long as you have your lift pump pumping, there will always be fuel moving through the VP44 and back through the return line to the tank.

Mopar1973man

Well I stand corrected... I was told that the VP44 could be damaged by coasting long distances... But You got me striaghten out there...

Oh well nobody is perfect huh...

apache

It would seem even with no throttle plate to create vacume the engine still is drawing air into the cylinders and compressing it which would slow it down the same as a gasser. The 20 psi of vacume created in a gasser with the foot off the pedal would create a huge negative pressure in the cylinders and hamper the engines ability to slow it down the same as if it were charged with a positive pressure.

neveragain

Sorry RustyJc
Don't believe that you are correct. The ECM can't see a change of state if the resistance through the APPS is out of spec this is throught the off idle validation cct pins 1 &2 mine is suposed to be <125 ohms and measures 2.65 (meg) million ohms and of the 7 new ones that I measured they range from 2564 ohms to 3.65 meg ohms. So they fuel accordingly so you get fuel delivered at the rate of idle until the ECM sees a MAPS boost presure and starts the fueling process. This is the reason that mine is a gutless wonder. and can't find a good APPS.

wings

Somewhere along the line I too read that when coasting the VP44 gets no lube fuel. However that really can't be right because you coast as much as accellerate and cruise. It all has to equal out. It would not make much sense to purposely not lube a rotating device when lube is readily available....free no less.
Not being a Bosch eng. I can't say exactly what the theory is but...
If you notice just before coming to a stop while coasting the motor runs about100-150 rpm over idle. Then just as you stop it drops to idle. Therefore any time you are rolling the VP pump is putting some fuel into the motor to keep it running thus lube fuel is also flowing thru it.
Now for what ever reason you would turn the key off and coast down against the motor you may have a different story. The logic being the motor is being turned over by an external source...ie the road dyno if you will. Since there is no elect. power for the computer, the motor would be in a shut down mode. The engine oil pump would be ok obviously but the electronics are off yet the mechanically driven fuel pump is still turning. The lift pump would be off so no fuel there either.
At this point I would say don't try to improve mileage by turning the key off on long downhill grades.
I would think the Bosch eng would think some American would do this and to cover their *** they would incorporate a relief valve within the pump to handle this situation since the pump is driven mechanicaly anyway and can pull some fuel thru the lift pump even it it isn't operating.