Engine/Transmission (1998.5 - 2002) Fueling Question

[JPLB22]

I have read on several different threads about how the motor will NOT use any fuel while coasting down hill while in gear. I figure that this is because the inertia of the truck rolling down hill turns the drivetrain therefore turning the engine, and the VP-44 stops injecting fuel. That is what I get from what I've read.



Now that I have a FP gauge at the schrader I noticed that while coasting down hill in gear, there is still 14 psi from the LP. So the LP function does not change.



I am just wanting an explanation of the opperation of the VP-44 during downhill coasting while in gear with the driveline turning the engine ???



Please !!



Jeff

 

[krs]

The vp44 does not pump fuel whenever the driver is not applying throttle, downhill or not. It hasn't got to do with the engine being turned or the phase of the moon. The liftpump operates seperately to maintain pressure but not necessarily flow to the vp44.

 

[JPLB22]

I'm not contradicting your answer but... if it has nothing to do with downhill or the engine being turned by the drivetrain, than at some point the VP must start to inject fuel again or else the truck would just roll to a stop and die. So at what point is the VP told to inject fuel after it stops injecting for whatever reason, assuming you never reapply throttle until you coast to a stop.

I know for a fact that at idle I'm not applying throttle but the VP is injecting fuel. That is why I thought it had something to do with the engine being forced to turn by drivetrain.

The reason that I ask these questions is... I told my F**D buddy that the diesel does not inject ANY fuel while coasting. He looked at my like I was crazy ( he says simple logic says, If the engine is running it is consuming fuel ) and I had no explanation for him, and I would like to have one for him.



TIA



Jeff

 

[LoneRam]

I don't have an answer for you, but your friend can try this if he wants to answer his own logical question. Next time he's going down hill, turn the key off and see what happens. I can tell you since my truck died yesterday on the freeway, nothing. You just slow down same as if you were going down hill. Diesels have alot of compression, but not enough to lock up the tires(with out the use of an exhaust brake) I know the above suggestion is not safe, but the proof will be in the pudding for him. I don't know for sure the engine doesn't use any fuel but it wouldn't supprise me if it didn't. I would "guess" that the computer takes the position of the pedal and realizes the engine rpm is too high and cuts fuel until the rpm corresponds with the position of the pedal. If you left your foot off the gas and began coasting to a stop, the VP-44 would be told to inject once idle rpm was reached. Just a guess, but that should get him off your back. If you add fuel to a diesel, the rpms will only raise. Look at the Mileage thread that's current right now for some good info on this. Good luck with you friend.



Obviosly the above only applies if you are in gear.

 

[TomGolden]

I have a 2 mile long 6 - 7% grade on the way to & home from work every day. When I am going downhill, I can take my foot off the go pedal, but keep the truck in 6th gear, and going 70 MPH, the EGT gauge will get down to 220*.

 

[rashwor]

I think a little differently...

but that is no surprise to those that know me! I believe that even when you release the go pedal to low idle position, there is still fuel being injected by the VP44. I also believe that the amount of fuel corresponds with the RPM shown on the tach, even if rolling down hill in gear. The fuel is not cut completely off by the VP44.



Here is my logic for believeing this. When I am rolling down a hill @ 60mph and I release the go pedal to idle, the tach comes down as the speed goes down and my fuel pressure gauge goes up slowly as everything else is coming down. When I choose to coast down hill by depressing the clutch while going 60 mph, the lift pump pressure gauge shoots up as fast as the tach drops to zero. Thus fuel is being used and injected at a rate / amount that corresponds with the engine RPM. If this is not true, why doesn't the fuel pressure shoot back up to top pressure as soon as I let off the loud pedal like it does when I truly "coast" down the hill?



I will now put on my fire suit.

 

[Ron Bissett]

Re: I think a little differently...

Originally posted by rashwor

but that is no surprise to those that know me! I believe that even when you release the go pedal to low idle position, there is still fuel being injected by the VP44. I also believe that the amount of fuel corresponds with the RPM shown on the tach, even if rolling down hill in gear. The fuel is not cut completely off by the VP44.



Here is my logic for believeing this. When I am rolling down a hill @ 60mph and I release the go pedal to idle, the tach comes down as the speed goes down and my fuel pressure gauge goes up slowly as everything else is coming down. When I choose to coast down hill by depressing the clutch while going 60 mph, the lift pump pressure gauge shoots up as fast as the tach drops to zero. Thus fuel is being used and injected at a rate / amount that corresponds with the engine RPM. If this is not true, why doesn't the fuel pressure shoot back up to top pressure as soon as I let off the loud pedal like it does when I truly "coast" down the hill?



I will now put on my fire suit.

yep ,there must be a small amount of fuel injected to lube the pistons and keep the injector tip cool or that would be the end of line for sure . Some leaned person might shed some light on this subject, and inform us without knowledge. Ron Bissett in Metro Louisville KY

 

[krs]

Maybe the functional term "pump" is the problem. I got the information about not pumping from a treatise on the causes of failure in the vp44 as ' Because the pump is not pumping during decell it is not receiving lubrication from the passage of fuel through it's rotor vanes'. So is there another circuit or mechanism which provides the fuel needed to keep the engine running at idle? There must be, or the above dying scenario would be the fact. We all know that the engine does not die without throttle pressure but can we say that therefore the pump must pump without such pressure?

 

[klenger]

I don't know beans about diesel injection pumps, but I think some of you make be confusing fuel passing throught he pump for lubrication and back to the fuel tank with fuel being injected into the engine. My understanding is that a diesel consumes no fuel when decelerating with the drive train engaged. You can tell this is happening because the injection pump immediately gets quiet when you decelerate, and starts up again when you put in the clutch as you coast to a stop.

 

[JR2]

Originally posted by KRS

So is there another circuit or mechanism which provides the fuel needed to keep the engine running at idle?

Yes there is... if you ever look at an engine with an ODB scanner you can see that the Throttle position sensor returns to zero but the absolute position is at some very small number... ie idle. So the TPS tells the ECM that the peddle is at 0 percent... but the 0 percents is not absolute zero. . Oh that is confusing... let me say it another way... when you let off the peddle the engine returns to idle. The idle voltage is provided by the TPS.





John

 

[JPLB22]

Does an OBD scanner or a Cummins Quick Check II have a function to monitor fuel consumption? Like as in GPH or something like that. If so it would seem that either software system would be able to tell you if the engine is consuming fuel while coasting down hill in gear. ???????



Jeff

 

[kaffine krazed]

Fuel does not get injected into the engine while the engine is being turned by the drivetrain faster than idle rpm with your foot off the throttle.



The reason the fuel pressure readings are diffrent from coasting in gear an coasting in nuetral is the the fuel injection pump is turning at diffrent speeds. The fuel injection pump has 2 internal pumps a vane pump that increases the fuel pressure from the lift pump to around 300psi then another pump that creates the pressure needed to inject the fuel. When coasting in gear the vane pump is still pumping. The injection pump spins at half of the enigne rpm the vane pump inside the injection pump spins at the same speed the vane pump is puming fuel whenever the engine is turning. So when coasting downhill in gear with the enigne rpm higher than idle the vane pump is pumping more fuel than it does at idle so the fuel pressure between the lift pump and the VP-44 will be lower.



There are 2 switches on the TPS that indicate idle or off idle. When the ecm detects that it is at idle it will maintain the engine speed of idle if it goes high it cuts back on the fuel if it goes lower then it adds more fuel. When coasting in gear with the engine spining faster than idle the ecm will tell the VP-44 to stop injecting fuel because the ecm is trying to lower engine speed to idle.